by IRFAN ULLAH Department of Chemistry, Gomal University ...
Transcript of by IRFAN ULLAH Department of Chemistry, Gomal University ...
FORMULATION AND CHARACTERIZATION OF VARIOUS
DRUG CARRIER SYSTEMS AND INVESTIGATING THEIR
ABILITY FOR SOLUBILIZATION OF POORLY WATER
SOLUBLE DRUGS
by
IRFAN ULLAH
Department of Chemistry, Gomal University
Dera Ismail Khan, Pakistan 2013
FORMULATION AND CHARACTERIZATION OF VARIOUS DRUG
CARRIER SYSTEMS AND INVESTIGATING THEIR ABILITY FOR
SOLUBILIZATION OF POORLY WATER SOLUBLE DRUGS
A Dissertation Submitted to Gomal University Dera Ismail Khan in Partial
Fulfillment of the Requirements for the Degree of Doctor of Philosophy
In
Chemistry
by
IRFAN ULLAH A Ph. D scholar
DEPARTMENT OF CHEMISTRY GOMAL UNIVERSITY, DERA ISMAIL KHAN, PAKISTAN
2013
IN THE NAME OF ALLAH
_________________________ THE MOST MERCIFUL
& THE MOST GRACIOUS
Dedicated to My Ever loving Parents, Sweet Brothers, Sisters, my wife and beloved son Muhammad Abdullah.
i
ACKNOWLEDGEMENTS
All glory be to Almighty ALLAH who has created us the most omnipotent and gave me
the courage and zeal to perform this work successfully. All respects for Holy Prophet
HAZRAT MUHAMMAD (peace be upon him) who enlightened our minds to recognize
our creator.
I wish to express my heartiest gratitude to my honorable supervisor Prof. Dr. Musa
Kaleem Baloch (Izaz-i-Fazilat, Tamgha-i-Imtiaz), Professor of Chemistry and Ex-
Dean, Faculty of Sciences, Gomal University, Dera Ismail Khan for his valuable
guidance and helpful discussion about my work into multi aspects over which I can be
proud of forever.
I would like to give my sincere gratitude to Prof. Dr. Azim Khan Khattak, Chairman
Department of Chemistry, Gomal University, Dera Ismail Khan for providing all the
facilities during this work. I am also heartily grateful to Margrette Jayne Lawrence,
professor of Biopharmaceutics, Pharmaceutical Science Division, King’s College
London, United Kingdom, Prof. Dr. Gulrez Fatima Durrani, and Dr. Muhammad
Adeel assistant professor, Department of Chemistry, Gomal University, Dera Ismail
Khan for their cooperation and useful discussion during my whole research work.
I humbly wish the best of luck to all my Dear Class Fellows, for their kind deed or
thoughts for me. I also appreciate the role of the office, library and laboratory staff in our
achievements. Special thanks are reserved for my parents and other members of my
family for their love, prayers, patience, nice and encouraging attitude throughout my life.
The financial support provided by HEC, in terms of Indigenous Scholarships and IRSIP
Fellowship is highly acknowledged.
Irfan Ullah
ii
CONTENTS
Chapter No PARTICULARS Page No
Acknowledgement i
Table of Contents ii
List of Tables v
List of Figures ix
Abstract
xiv
1 INTRODUCTION 1-6
1.1 Drugs and their classification 1
1.2 Carrier used in drugs delivery 3
1.3 AIMS AND OBJECTIVES 7
2 LITERATURE REVIEW 8-16
3
EXPERIMENTAL 17-29
3.1 Materials 17-24
3.1.1 Hydrotropes 17
3.1.2 Surfactants 18
3.1.3 Menthol 22
3.1.4 Drugs 22
3.1.5 Oils 24
3.2 Methods 25-29
3.2.1 Sample preparation 25
3.2.2 Phase diagram 25
iii
3.2.3 pH measurement 26
3.2.4 Estimation of minimum hydrotropic concentration 26
3.2.5 Estimation of critical micelles concentration 26
3.2.6 Centrifugation 27
3.2.7 Measurement of solubility 27
3.2.8 Differential scanning calorimetry 28
3.2.9 Laser light scattering measurement 28
3.2.10 Refractive index measurement 29
4 RESULTS AND DISCUSSION 30-135
4.1 Estimation of minimum hydrotropic concentration 30
4.2 Solubility of drugs in water 33
4.3 Solubility of drugs in buffer solution 33
4.4 Effect of hydrotropes over the solubility of drugs 34
4.5 Estimation of critical micelles concentration of surfactants 58
4.6 Effect of surfactants over the solubility of drugs 68
4.7 Hydrodynamic radius of micelles loaded with drugs 81
4.8 Effect of hydrotropes on critical micelles concentration of
DDAPS
89
4.9 Effect of hydrotropes on solubility of drugs in DDAPS 96
4.10 Aggregation number of DDAPS/NaSal and DDAPS/NaBen
106
4.11 Hydrodynamic radius of DDAPS/NaSal and DDAPS/NaBen 109
iv
mixture
4.12 Effect of addition of butanol on solubility of drugs 111
4.13 Solubility of drugs in oil-in-water microemulsion 112
4.14 Aggregation number of DDAPS, DDAPS/butanol and
microemulsion
113
4.15 Hydrodynamic radius of DDAPS/butanol mixture and
microemulsions
117
4.16 Microemulsion having Menthol and/or Eutectic mixture as an oil 120
4.17 Aggregation number of microemulsion having Menthol and
Eutectic mixture as oil
127
4.18 Hydrodynamic radius of microemulsion having Menthol and
Eutectic mixture as oil
128
4.19 Refractive index of various systems 130
Conclusion 136-137
References 138-153
v
S.No Title of Table Page No.
1 Biopharmaceutical classification of drugs developed by Amidon and
his coworkers
2
2 Techniques employed for the preparation of carrier systems 6
3 Minimum hydrotropic concentration (MHC) calculated from Figure
1 and the one available in the literature
32
4 Solubility of drugs in water; measured at 25oC 33
5 Solubility of drugs at various pH 34
6 Maximum hydrotropic concentration calculated from Figures 2-17 51
7 Various parameters of Meloxicam in aqueous solution of
hydrotropes; measured at 25oC.
54
8 Various parameters of Celecoxib in aqueous solution of hydrotropes
measured at 25oC.
55
9 Various thermodynamic parameters of Lidocaine in aqueous solution
of hydrotropes; measured at 25oC.
56
10 Various thermodynamic parameters of Ibuprofen in aqueous solution
of hydrotropes; measured at 25oC.
57
11 CMC of the surfactants determined from the surface tension of their
aqueous solution and the one available in the literature. The
aggregation numbers are one obtained from the literature.
66
12 CMC of the surfactants determined from the surface tension of their 67
vi
aqueous solution and the one available in the literature. The
aggregation numbers are one obtained from the literature.
13 Various parameters of Meloxicam in aqueous solution of surfactants;
measured at 25oC.
77
14 Various parameters of Celecoxib in aqueous solution of surfactants;
measured at 25oC.
78
15 Various parameters of Ibuprofen in aqueous solution of surfactants
measured at 25oC.
79
16 Various parameters of Lidocaine in aqueous solution of surfactants;
measured at 25oC.
80
17 Hydrodynamic radius of pure and drug saturated micelles of various
surfactants; measured at 25oC
88
18 Surface tension (mN.m-1) of DDAPS in the presence of NaSal
measured at 25oC
90
19 Surface tension (mN.m-1) of DDAPS in the presence of sodium
benzoate; measured at 25oC
92
20 Various parameters of drugs calculated in the presence of DDAPS at
25oC
103
21 Various parameters of drugs calculated in the presence of
DDAPS/NaSal at 25oC
104
22 Various parameters of drugs calculated in the presence of 105
vii
DDAPS/NaBen at 25oC
23 Aggregation number of DDAPS micelles formed in aqueous solution
of 0.4 molar NaSal; measured at 25oC
107
24 Aggregation number of DDAPS micelles formed in aqueous solution
of 0.4 molar NaBen; measured at 25oC
108
25 Effect of addition of NaSal (0.4 mol.L-1)on hydrodynamic radius of
DDAPS; measured at 25oC
109
26 Effect of addition of NaBen (0.4 mol.L-1)on hydrodynamic radius of
DDAPS measured in water/butanol mixture at 25oC
110
27 Effect of addition of butanol to water on solubility of drugs in 0.5
mol.L-1 DDAPS at 25oC
111
28 Effect of addition of 2% (v/v) hydrocarbons over the apparent
solubility of drugs in DDAPS/butanol (0.5 mol.L-1/1.2 mol.L-1)
mixture at 25oC
112
29 Aggregation numbers of DDAPS Micelles in water; measured at
25oC
114
30 Effect of addition of butanol on aggregation number of 0.5 mol.L-1
DDAPS; measured at 25oC
115
31 Effect of addition of 2% (v/v) hydrocarbons over the aggregation
number of DDAPS/butanol (0.5mol.L-1/1.2mol.L-1) mixture;
measured at 25oC
116
32 Hydrodynamic radius of blank and drugs saturated DDAPS Micelles 117
viii
measured at 25oC
33 Effect of addition of butanol on hydrodynamic radius of drug
saturated (0.5mol.-l) DDAPS at 25 oC
118
34 Effect of addition of 2% (v/v) hydrocarbons over the hydrodynamic
radius of DDAPS/butanol (0.5mol.L-1/1.2mol.L-1)/drug systems at
25oC
119
35 Composition of microemulsions having the capacity of sustaining
maximum oil
125
36 Composition of oil in water microemulsions obtained from phase
diagrams and can be diluted to a very low surfactant concentration
126
37 Aggregation number of microemulsions measured at 25oC 127
38 Hydrodynamic radius of microemulsion measured at 25oC 129
39 Refractive index of DDAPS micelles and its microemulsion
measured at 25oC
130
40 Refractive index of DTAB micelles and its microemulsion measured
at 25oC
131
41 Refractive index of SDS micelles and its microemulsion measured at
25oC
132
ix
S.No List of Figures Page.No
1 Surface tension as a function of hydrotrope concentration;
measured at 25oC
31
2 Solubility of Meloxicam in aqueous solution of sodium benzoate
as a function of concentration and temperature
35
3 Solubility of Meloxicam in aqueous solution of sodium
salicylate as a function of concentration and temperature
36
4 Solubility of Meloxicam in aqueous solution of sodium p-
toluene sulfonate solution as a function of concentration and
temperature
37
5 Solubility of Meloxicam in aqueous solution of sodium xylene
sulfonate as a function of concentration and temperatures
38
6 Solubility of Celecoxib in aqueous solution of sodium benzoate
as a function of concentration and temperatures
39
7 Solubility of Celecoxib in aqueous solution of sodium salicylate
as a function of concentration and temperatures
41
8 Solubility of Celecoxib in aqueous solution of sodium p-toluene
sulfonate as a function of concentration and temperatures
41
9 Solubility of Celecoxib in aqueous solution of sodium xylene
sulfonate as a function of concentration and temperatures
42
10 Solubility of Lidocaine in aqueous solution of sodium benzoate
as a function of concentration and temperatures
43
x
11 Solubility of Lidocaine in aqueous solution of sodium salicylate
as a function of concentration and temperatures
44
12 Solubility of Lidocaine in aqueous solution of sodium p-toluene
as a function of concentration and temperatures
45
13 Solubility of Lidocaine in aqueous solution of sodium xylene
sulfonate as a function of concentration and temperatures
46
14 Solubility of Ibuprofen in aqueous solution of sodium benzoate
as a function of concentration and temperatures
47
15 Solubility of Ibuprofen in aqueous solution of sodium salicylate
as a function of concentration and temperatures
48
16 Solubility of Ibuprofen in aqueous solution of sodium p- toluene
sulfonate as a function of concentration and temperatures
49
17 Solubility of Ibuprofen in aqueous solution of sodium xylene
sulfonate as a function of concentration and temperatures
50
18 Surface tension of aqueous solution of Tween 20 as a function of
its concentration; measured at 25oC
59
19 Surface tension of aqueous solution of Tween 80 as a function of
its concentration; measured at 25oC
60
20 Surface tension of aqueous solution of Triton X-100 as a
function of its concentration; measured at 25oC
61
21 Surface tension of aqueous solution of Triton X-114 as a 62
xi
function of its concentration; measured at 25oC
22 Surface tension of aqueous solution of surfactants as a function
of their concentration and measured at 25oC
63
23 Surface tension of aqueous solution of Brij 30 as a function of its
concentration; measured at 25oC
64
24 Surface tension of aqueous solution of Brij 35 as a function of its
concentration; measured at 25oC
65
25 Solubility of Meloxicam in aqueous solution of nonionic
surfactants; measured at 25oC
70
26 Solubility of Celecoxib in aqueous solution of nonionic
surfactants; measured at 25oC
71
27 Solubility of Lidocaine in aqueous solution of nonionic
surfactants; measured at 25oC
72
28 Solubility of Lidocaine in aqueous solution of various
surfactants; measured at 25oC.
73
29 Solubility of Ibuprofen in aqueous solution of surfactants;
measured at 25oC.
74
30 Autocorrelation function of Tween 20 micelles 81
31 Autocorrelation function of Tween 80 micelles 82
32 Autocorrelation function of Brij 30 micelles 83
xii
33 Autocorrelation function of Brij 35 micelles 84
34 Autocorrelation function of Triton X-100 micelles 85
35 Autocorrelation function of Triton X-114 micelles 86
36 Autocorrelation function of DDAO micelles. 87
37 Variation in surface tension of DDAPS by the addition of
sodium salicylate as a function of DDAPS concentration;
measured at 25oC
91
38 Variation in surface tension of DDAPS by the addition of
sodium benzoate; measured at 25oC
93
39 Effect of sodium salicylate on CMC of DDAPS measured at
25oC
94
40 Effect of sodium benzoate on CMC of DDAPS measured at
25oC
95
41 Solubility of Lidocaine in aqueous solution of DDAPS/
(0.4mol.L-1) NaSal mixture; measured at 25oC
97
42 Solubility of Meloxicam in aqueous solution of DDAPS/
(0.4mol.L-1) NaSal mixture; measured at 25oC
98
43 Solubility of Celecoxib in aqueous solution of DDAPS/
(0.4mol.L-1) NaSal mixture at 25oC
99
44 Solubility of Lidocaine in aqueous solution of DDAPS/
(0.4mol.L-1) NaBen mixture; measured at 25oC
100
45 Solubility of Meloxicam in aqueous solution of 101
xiii
DDAPS/(0.4mol.L-1) NaBen mixture; measured at 25oC
46 Solubility of Celecoxib in aqueous solution of
DDAPS/(0.4mol.L-1) NaBen mixture; measured at 25oC
102
47 DSC curve of Lidocaine/Menthol mixture 120
48 Phase diagrams of microemulsions stabilized by DDAPS at 25oC 122
49 Phase diagrams of microemulsion stabilized by DTAB at 25oC 123
50 Phase diagrams of microemulsion stabilized by SDS at 25oC 124
51 Refractive index of DDAPS micelles and its microemulsion;
measured at 25oC
133
52 Refractive index of DTAB micelles and its microemulsion;
measured at 25oC
134
53 Refractive index of SDS micelles and its microemulsion;
measured at 25oC
135
xiv
ABSTRACT
Poor aqueous solubilities of drug candidates limit their bioavailability. A number of
delivery systems are in use to enhance the bioavailability of the drugs with poor solubility
in water. The self-assemblies of hydrotropes, surfactants and oil/water micro emulsions
may provide a means of enhancing solubility and enhance bioavailability of drugs.
Although these drugs delivery systems are in use but the mechanism through which these
delivery systems solubilize the drugs needs detail investigations. The objective of the
current dissertation was to provide the understanding of the mechanism through which
simple aggregates of hydrotropes, micelle of surfactants and oil in water microemulsions
solubilize the drugs. For the purpose, apparent solubility of drugs namely, Meloxicam,
Celecoxib, Ibuprofen and Lidocaine was determined in aqueous solution of hydrotropes,
surfactants, surfactant/hydrotrope, surfactants/butanol mixtures and in oil/ water
microemulsions. These mediums were tested for their ability to enhance the aqueous
solubility of these water insoluble drugs. The results obtained for molar solubilization
ratio (MSR), partition co-efficient (KM) of the investigated drugs concluded that these
were lower in hydrotropes as compared to the one obtained in other stated systems.
Among the hydrotropes, sodium benzoate showed highest (0.006- 0.0107), whereas
sodium p-toluene sulfonate (0.0014- 0.0052) the lowest MSR values. The negative values
obtained for ∆Go illustrated the spontaneous mixing of these drugs in all the investigated
systems. The CMC, HLB, oxyethylene units and aggregation number of surfactants along
with molecular mass of the drug, polarity of the drug and the group attached to them
showed a great impact over the solubility of two model drugs, Meloxicam and Celecoxib
in nonionic surfactants including Tween 20, Tween 80, Brij 30, Brij 35, Triton X-100,
xv
and Triton X-114. It was noted that the surfactants with high aggregation number
solubilized higher amount of drugs and had higher value of MSR than others. The
solubility was enhanced with the increase in number of oxyethylene units in a surfactant.
The solubility was also increased with the increase in number of carbon atoms in alkyl
chain of surfactants used. Similar results were observed when Lidocaine was solubilized
in ionic, nonionic and zwitterionic surfactants. Among the nonionic, N,N,
Dimethyledodecyle amine-N-Oxide (DDAO) whereas among ionic and zwitterionic
surfactants, N,N, Dimethyldodecyle- amonio propane-sulfonate (DDAPS) surfactants
showed higher ability to solubilize the model drug, Lidocaine. The addition of
hydrotropes and/or butanol to aqueous solution of DDAPS showed a noticeable increase
in solubility of all the investigated drugs. In case of oil/ water microemulsion, the
increase in molecular mass of oil in a homologous series increased the solubility of drugs.
It was also noticed that microemulsions had highest ability to solubilize the drugs among
all the investigated systems. The results obtained by light scattering revealed that the
addition of drugs does not increase the aggregation number and hydrodynamic radius of
the surfactants micelles. However, both the aggregation number and size was increased
by the addition of butanol and hydrotropes. The addition of hydrocarbon to the
DDAPS/butanol mixture resulted a decrease in micellar size as well as the aggregation
number. Similar observations were also made for aggregation number and hydrodynamic
radius in case of Menthol or Eutectic mixtures of Lidocaine/Menthol used as an oil phase.
All these observations concluded that the drugs are solubilized in inner core of
micelles/aggregates of the surfactants/hydrotropes. However in case of oil/water
microemulsions these were solubilized only in oil phase of microemulsions.
1
1. INTRODUCTION
1.1. Drugs and their classification
Drugs are the substances that change the bodily function on absorption into the body of
living organism. In pharmacology, a drug is a substance which may have medicinal and
intoxicating, performance when taken or put into a human body. Drugs are mostly
recommended for a limited time, or on a regular basis for chronic disorders. They can be
administered via different routes like, Oral, by Rectal, Topical, Parenteral, Respiratory,
Nasal, Eye and Ear. Therefore, in almost every methodology; vehicles are required or
have to be solubilized before administration. Further to it the drugs having low solubility
cause various problems with respect to their in vivo absorption. Recognizing the
importance of solubility, Amidon and his coworkers classified the drugs based over their
solubility which is also known as Biopharmaceutical classification system (Table 1) [1].
Recent advances in drug discovery (combinatorial chemistry and high through-put
screening) led to the identification of large numbers of compounds with good therapeutic
potential, however most of these compounds have extremely poor aqueous solubility and
fall into class II or IV [2-4].
2
Table 1 Biopharmaceutical classification of drugs developed by Amidon and his
coworkers.
Class Solubility Permeability Description
I High High Drugs having higher absorption than excretion.
II Low High These drugs have limited Bioavailability by their
solvation rate. A correlation can be found between in
vivo bioavailability and in vitro solvation.
III High Low Although absorption of such drug is limited by the
permeation rate but its solvation is very fast.
IV Low Low These drugs have poor bioavailability. They are
poorly absorbed over the intestinal mucosa and a
high variability is expected.
3
1.2.Carrier used in drugs delivery
It is estimated that more than 40% of the drug candidates (pharmaceutically active
ingredients) are discarded due to their poor solubility, and hence low and variable
bioavailability. Therefore, over the last two decades, lot of research efforts have been
focused over the development or improvement of the drug carriers which are also known
as delivery systems, specifically for the challenging drug candidates, belonging to classes
II and IV of the biopharmaceutical classification system [5-7].
In order to overcome the problems of low aqueous solubility of drugs and drug
candidates, new delivery systems are introduced. It has been concluded that for drug
candidates exhibiting highly lipophilic tendencies, would be beneficial to deliver these to
the patient in the pre-dissolved state and the solvent which can be used for the purpose be
either a lipid solution, a lipid emulsion, a microemulsion (ME), or a self-emulsifying (or
micro-emulsifying) drug delivery system (S(M)EDDS) [8-11]. In this way the energy
associated with a solid–liquid phase transition will be reduced that would be otherwise
required for the dissolution of the drug after administration to the patient. Therefore a
number of (carrier) systems have been developed which can solubilized the drugs. A
carrier system required for the proper delivery of the drugs is described as a tool desired
for enhancing the therapeutic effectiveness of a bioactive agent [12-13]. Generally,
nonreactive matrixes are used as a carrier system for the delivery and targeting of drugs
inside or outside the body and protect its load from attack of oxygen, light and/or
enzymes which can destruct it and also prevent the drugs to react with other reactive
components [14-16]. Polymers, biopolymers, lipids and surfactant-based systems and oils
or their mixture can be used as carrier systems. The most common systems used as drug
4
carriers are micro-spheres, nano-spheres, liposomes, nano-liposomes and vesicular
phosphor-lipid gels (VPG), archaeo-somes, and nio-somes [17-25]. Micelles and nano-
particles are the examples of mixed carrier systems that can be prepared from
polymer/surfactant and/or polymer/lipid conjugates mixtures [14, 25]. The particle size of
carrier systems may vary from micrometric to nano-metric size and should be kept
constant during their preparation and application. The key parameters that play a major
role in designing and formulation of carrier systems are the methods of preparation and
loading of drugs, their particle size distribution and firmness and degradation properties
of the bioactive-carrier complex along with the release kinetics of the loaded drugs [26].
The preferred carrier systems are the one that is biodegradable and bio absorbable, as
these can be degraded inside the body due to hydrolytic or enzymatic actions, without
creating any toxic material. A number of techniques (based over lab scale and large scale
applications) can be employed for the formulation of the carrier systems and loading of
bio active materials. Table 2 comprises some of the examples of such methodologies and
the carriers so formed [10, 20, 23, 27-34]. Nevertheless, for the entrapment of sensitive
substances, most of these methods are not suitable as they cannot sustain against
mechanical stresses, high-shear homogenization, sonication, high pressures etc., or
change in pH during the preparation [35]. Moreover these methods of preparation use the
injurious low boiling solvents like acetone, ether, chloroform, to dissolve or solubilize the
constituents [36] and if these solvents are remained in the final product as a residue then
they may not only modify the chemical structure of the entrapped material but also
affects the stability of the carrier system and add toxicity to the product [37]. These
residual solvents which are organic volatile impurities are dangerous to human and
5
animal health and to the environment. Further to it such compounds do not have any
therapeutic benefits [38]. Certainly, it is not an easy job to remove these solvents up to
zero level and the process is also very expensive. Therefore such preparations are not
only unstable but also very costly and dangerous; hence can be concluded as not suitable
for the formulations [39]. Keeping in view above mentioned factors, we have tried to
enhance the solubility of various drugs in water using hydrotropes and/or surfactants, oils
and to solubilize the drugs in their micelles or microemulsions, as the case may be or by
preparing eutectic mixture of Menthol with drugs. The efforts have also been made to
explore the forces involved for the enhancement of solubility of drugs and to propose the
mechanism of solubilization.
6
Table 2 Techniques employed for the preparation of carrier systems.
Preparation method Carrier system References
Thin film hydration method Niosomes/liposomes [20, 23, 27]
Sonication Nanoliposomes/archaeosomes [10]
Heating method Nanoliposomes/liposomes/archaeosomes
/VPG
[10, 28-30]
Reverse-phase evaporation Liposomes/archaeosomes [10, 30]
Ether-injection technique Liposomes [10, 30]
High-pressure
homogenization
VPG* [31]
Precipitation
polymerization
Nanoparticles [32]
Emulsification- diffusion
method
Nanoparticles [32]
Emulsion-solvent
evaporation technique
Microspheres [33-34]
*Vesicular phospholipid gels
7
1.3. AIMS AND OBJECTIVES
Keeping in view the poor-aqueous solubility and high hydrophobicity of Meloxicam,
Celecoxib, Ibuprofen and local anesthetic Lidocaine were selected to enhance their
aqueous solubility and to increase their bioavailability and to identify a best system
which can dissolve the drug up to large extent and show minimum health risks. For the
purpose, the following carrier aqueous systems were investigated:
1. Various hydrotropes/water system
a. Effect of concentration of hydrotropes over solubility
b. Effect of temperature over solubility
2. Aqueous solution of surfactants
3. Hydrotropes/ surfactants/water
4. Butanol/ surfactant / water system
5. Oil/water microemulsions
a. Hydrocarbon as oil
b. Menthol/Lidocaine eutectic mixture
6. Menthol/water/ surfactant system
The solubility of the drug was investigated using UV/Vis spectroscopy whereas the
CMC and MHC were obtained using surface tension measurement. The
hydrodynamic radius and aggregation number of the micelles/microemulsions was
measured using dynamic light scattering techniques. It was tried to identify the best
system for the delivery of these drugs based over the data obtained.
.
8
2. LITERATURE REVIEW
NSAIDs, the nonsteroidal anti-inflammatory drugs are a class of compounds that poses
antipyretic, analgesic, and anti-inflammatory effects which lessen the pain as these
compounds interacting with the cyclooxygenase (COX) enzyme which manufactures
prost-aglandins, producing inflammation [40]. They reduce or eliminating the pain by
preventing the production of prost-aglandins, [41]. In osteo-arthritis of the knee, the
NSAIDs are good enough in reducing the instant pain than placebo but for this condition
the prolong intake of these drugs is not supportive. Limited use is recommended due to
serious side effects that are related with oral NSAIDs [42]. For the treatment of acute
inflammation in joints, the Cyclo-oxygenase-2 (COX 2) inhibitors can be employed as
these drugs are as effective as that of NSAIDs in relieving pain [43], they decrease the
arthritis like NSAIDs but without the gastro-intestinal side effects that is associated with
traditional NSAIDs. The devastating effects of NSAIDs can be avoided by their Intra-
articular administration which is alternative to their delivery administration. For the
maintenance of therapeutic intra-articular levels, the frequent injections of NSAIDs
would be required. The sustained drug delivery devices offers an excellent alternative to
multiple intra-articular injections and produced good results [44], by local administration
of a new slow release NSAID-carrier formulation, the improved NSAIDs efficacy and
relief of adverse effects can be achieved. Lin et al., prepared water soluble diclofenac
sodium loaded microshere by using three low-molecular weight poly-esters, poly (L-
lactic acid), co-poly (lactic acid/glycolic acid), and (delta-valerolactone) i.e. (PLA),
(PLGA) and poly (PV).The oil/oil emulsification solvent evaporation method was
employed for preparation of microshere. Their dissolution behaviours, degradation,
9
physicochemical and micromeritic properties were determined in vitro. By the oil/oil
emulsification solvent evaporation technique, high encapsulation efficiency and
improved monodispersity was attained depending on the amount of drug loaded. The
powder x-ray diffractometry and differential scanning calorimetry revealed the measured
loss of organic solvent from the system throughout micro-encapsulation which was
responsible for the modification of the crystal structure of drug and polyester in the
micro-spheres. In phosphate buffer solution pH 7.4, the in vitro degradation showed
first-order kinetics, and ranked in the order of PV< PLA < PLGA micro-spheres. In all
micro-spheres, the first-order release rate was also found and ranked in the order of PV<
PLA < PLGA microspheres after an initial drug burst [45]. Tuncay et al., formulated the
diclofenac sodium (DS) for intra-articular administration and studied its controlled-
release. They characterized in vitro for release characteristics, drug loading, particle size,
morphology of the surface and yield. Radioactive isotopes were used for the
demonstration of arthritic injuries by gamma scinti-graphy. The evaluation of post-
therapy of arthritic lesions in rabbits established no difference compared to control
groups, the group treated with PLGA (50:50) (Mw 34 000 g.mol-1) DS microspheres,
[46]. Todd et al., likened the effectiveness of Diclofenac to that of the several fresh and
already developed NSAIDs, in a number of clinical trials and found it equivalent.
Diclofenac has a fast onset and long duration of action when used as an analgesic. When
administered intramuscularly in renal and biliary colic it was found to be analogous to,
and superior to, many narcotic and spasmolytic combinations. Establishing safety profile
with diclofenac, widespread medical knowledge has been added. It is well accepted
associated with other NSAIDs and rarely produces gastrointestinal ulceration or other
10
stern side effects. Thus, in the cure of severe and long-lasting aching and provocative
conditions, diclofenac can be considered as one of the few NSAIDs of 'first choice' [47].
Savanna et al., employed emulsification/crosslinking method to manufacture
microspheres loaded with drug. They used various techniques in order to characterize
these microshere. They studied in vitro release and characterized them by size
distribution, Fourier transform infrared (FTIR) spectroscopy, drug loading, differential
scanning calorimetry (DSC), scanning electron microscopy (SEM), gas chromatography
and X-ray diffraction (XRD). They studied in vivo by testing targeting efficiency of
microspheres in rabbits [48]. Thakkar et al., prepared chitosan micro-spheres by
emulsification cross-linking method and heat linked method and also associated the
features of these micro-spheres of chitosan. Two dissimilar cross-linking agents:
HCOOH and glutaraldehyde. SEM was employed for the characterization of these
microspheres for their ability to load drugs, particle size distribution, and release of drug
in-vitro and morphology of surface. The heat-cross-linked micro-spheres has the lower
entrapment efficiency than that of glutar-aldehyde and HCOOH cross-linked micro-
spheres (P <0.05). The heat cross-linked microspheres showed better performance in
releasing the drug as cpmapre to one cross-linked using glutaraldehyde and exhibited
measured release than one, cross-linked with HCOOH when these were studied In-vitro
for the release of drug [49]. Elron-Gross et al., developed Diclofenac formulation for
slow-release by local administration to enhance its efficacy and lessen the contrary effects
and for the purpose they employed micro-particles prepared from collagen-lipid
conjugates as carriers. A high-efficiency drug encapsulation (85%) was displayed by
collagomers that were stable in simulated synovial fluid and presented slow drug release
11
(τ 1/2 = 11days) as well as high affinity to target cells (Kd = 2.6 nM collagen) [50].
Bhardwaj et al., studied the release of dexamethasone from non-extruded and small
sonicated lipo-somes and evaluated the physicochemical properties of hydrophobic drug.
It was noted that the increase in lipid chain length decreases the encapsulation of
dexamethasone. The decrease in enthalpy and increase in the peak width of the main
transition indicated the destabilization of the liposome membranes by Dexamethasone.
DSC analysis showed the heterogeneously distribution of dexamethasone and cholesterol
in the non-extruded liposomes. The drug encapsulation (approximately 50%) and
diameter (DSPC > DPPC > DMPC) of lipo-some were reduced by sonication and
extrusion. As cholesterol and dexamethasone resemble structurally, hence its
incorporation in both extruded and non-extruded DMPC liposomes decreased the drug
encapsulation. A Phase transition was observed when dexamethasone and cholesterol
were loaded in combination in the same DMPC liposomes. Dexamethasone showed
slower release from non-extruded lipo-somes than from extruded lipo-somes. Similarly
liposomes of high phase transition lipid DSPC showed slowest release as compare to
DMPC liposomes and incorporation of cholesterol did not decrease release from DMPC
liposomes. These results indicated that incorporation of hydrophobic drug dexamethasone
affected the properties of lipo-somes [51]. Tsotas et al., studied dexamethasone
formulated in liposome and other gluco-corticoids. However, these liposomes had
variable physicochemical properties [52]. Swarnlata et al., formulated transdermal
delivery of Lidocaine by employing two different kinds of polymer patches; one
comprises on combination of ethyl cellulose (EC) and hydroxyl-propyl-methyl-cellulose
(HPMC) and second with polyvinyl alcohol (PVA) alone. The polymer solution of
12
HPMC 10% and EC 10% was prepared by mixing Methanol and chloroform in the ration
of 1:1. These two solutions were mixed together in various combinations. PVA matrix
patches were prepared by mixing Polymer (5, 10 and 15%) concentration in water with
glycerin (0.5%) as plasticizer [53]. DentiPatch, Lidocaine patch were Prepared by Noven
Parmaceuticals, Inc. These systems can be applied topically to the oral cavity and
contained Lidocaine a local anesthetic agent. For local anesthesia these system were
applied to the buccal mucosa for the release of Lidocaine, which prevents the ionic fluxes
essential for the commencement and transmission of impulses and hence stabilizes the
neuronal membrane thereby effecting local anesthetic action [54]. Dollo et al.,
investigated the in vitro release of Lidocaine from tube cuffs filled which contained
Lidocaine in various forms i.e alkalinized Lidocaine hydrochloride, base form, or
hydrochloride form. The endotracheal tube cuff filled with Lidocaine solution were
compared with the cuffs inflated only with air for their pharmacokinetic and pharma-
codynamic effects. A preliminary pilot clinical study in anesthesia was carried out for
spine surgery in smoker patients and it was noticed that the Lidocaine in neutral base
hydrophobic form presented greater rate (65.161% released after 6 h) of diffusion as
compared to hydrochloride in charged form for which only a infusion phenomenon was
taken place with reference to only 1% of the total drug, when their in vitro experiment
was carried out. In present study minute (20–40 mg vs. 200–500 mg) quantity of the
Alkalized form of Lidocaine hydrochloride was used as compare to previous available
literature value, and allowed no lag time for dispersion. For Lidocaine such a system
could offer a controlled release pool to adjacent tracheal tissue [55]. Morales, developed
metered-dose aerosol delivery system for the delivery of local anesthetic mixture of
13
Lidocaine and Prilocaine in base form without organic solvents. In order to investigate
the safety and efficiency of this unique delivery system, an open labeled pilot study was
made for topical local anesthetic to the glans penis planned at increasing the time of the
IVELT in patients who reported for having premature ejaculation. The aerosol contained
Lidocaine (7.5 mg) and prilocaine (2.5 mg), in base form, per actuation. [56]. Zhang et
al., studied prolonged, local analgesic action by investigating the rapid (~1 min)
Lidocaine delivery from coated micro needles by means of 3M’s compact micro-
structured transdermal system. Formulations comprising Lidocaine and dextran were
developed for uniform and thick coating on the micro needles. High performance liquid
chromatography (HPLC) was employed for the determination of the amount of Lidocaine
coated onto the microneedles. The Lidocaine-coated microneedles were introduced into
inland swine in order to evaluate drug delivery and dermal pharmacokinetics. Finally
HPLC and mass spectrometry (LC-MS) was used for determination of Lidocaine in the
Skin punch biopsies that were collected for analysis. The rate of dissolution of Lidocaine
was much more rapidly from the microneedles as compared to that of EMLA (Eutectic
Mixture of Local Anesthetic) cream) [57]. Ammon, et al., prepared microemulsion based
system for the delivery of lidocaine. This nano sized dispersion system for topical and
transdermal administration of drug and cosmetic agents has shown most promising
results for the hydrophilic and lipophilic molecules [58]. Singh et al., investigated the
development and characterization of microcapsule for the administration of Flurbiprofen
through colon [59]. Najm-ud-din et al., formulated an oral colon specific, pulsatile device
containing Flurbiprofen and tried to achieve time and/or site specific release of drug
loaded. The elementary design contained eudragit microsphere of Flurbiprofen packed in
14
an unsolvable hard gelatin capsule body sealed with a hydrogel plug. The colon-specific
release was achieved by enteric coating the whole device and overcoming the unevenness
in gastric emptying time. By using 1:1:2 of Drug: Eudragit L-100: Eudragit S-100
microsphere of Flurbiprofen was prepared. For maintaining a suitable lag period different
hydrogel polymers were used as plugs, and their amount thus used controlled release of
drug. Controlled release of Flurbiprofen from microsphere can be achieved by enhancing
the amount of hydrophilic polymer used [60]. Bhaskar et al., used Quasi-emulsion
solvent diffusion method for the preparation of micro-sponges containing Flurbiprofen
and Eudragit RS 100 and tried to achieve formulation of Flurbiprofen (FLB) micro-
sponges for colon specific drug delivery. Using entrapment method Flurbiprofen was
captured into a commercial Micro-sponge 5640 system. They investigated the effects of
various parameters including inner phase solvent amount, stirrer type, and drug: polymer
ratio, time and speed of stirring on the physical properties of micro-sponges. Detailed
study of particle size, surface morphology, thermal behaviours and pore structure of
micro-sponges were made by employing the methods of compression coating and pore
plugging of micro-sponges with pectin: hydroxylpropylmethyl cellulose (HPMC)
mixture, the colon specific formulations were prepared followed by tableting. All the
formulations were studied in vitro and the results were kinetically and statistically
evaluated. The shapes of micro-sponges were spherical having high porosity values (61–
72%) with diameter, between 30.7 and 94.5 μm. The pore shapes of micro-sponges were
tubular holes. By deforming the sponge-like structure of micro-sponges to the plastic, the
mechanically strong tablets can be prepared for colon specific drug delivery [61]. Orlu et
al., devolved the colon specific drug delivery system for the Flurbiprofen micro-sponges
15
and characterized it using different techniques [62]. Verma et al., investigated delivery
systems developed by using HPMC Matrices containing Flurbiprofen for the transdermal
delivery and made its in Vitro and in Vivo Evaluation [63]. Chauhan et al., developed
delivery system for the delivery of Flurbiprofen this system comprised on controlled
porosity osmotic pump [64]. Singh et al., investigated that how to increase the
transdermal delivery of Ketoprofen by using bio-adhesive gels [65]. El-Kamel et al.,
developed the Ketoprofen floating oral delivery system and characterized it by employing
routinely used techniques [66]. Philip et al., developed drug delivery system for
Ketoprofen in order to achieve osmotic, controlled and level an in vitro in vivo
correlation [67]. Saber et al., developed the formulation using polyelectrolyte complex as
a matrix former for Ketoprofen sustained release tablet and investigated the effect of
variables affecting the formulation [68]. Dhamankar, et al., formulated the oil in water
microemulsion for the delivery of ketoprofen and improving its transdermal absorption
[69]. Ganesh et al., formulated the floating drug delivery system of Ketoprofen and made
its evaluation using different techniques that are normally employed for characterization
[70]. Oliveira, investigated the Mefenamic acid performance with pectin [71]. Tang, et
al., developed the strategy for enhancing the oral delivery of poorly water soluble drugs
by employing self-emulsifying drug delivery systems [72]. Sevgi, et al,. formulated
micro-particles of Mefenamic acid and investigated it’s in vitro release, and in situ
studies in rats [73]. Havaldar et al., developed the techniques for the estimation of
Cetirizine Dihydro-chloride, Acetyl Salicylic Acid, Paracetamol and Mefenamic in the
pharmaceutical dosage form [74]. Roy et al., formulated the chitosan microsphere of
Mefenamic acid and investigated the effect of method of preparation on them [75].
16
Ramanathan, et al., formulated the floating tablets of Mefenamic acid using with
different grades of HPMC and investigated its release profiles [76]. Calija , et al.,
developed drug delivery system of Naproxen using Alginate–Chitosan micro-particles
and investigated in vitro release behavior [77]. Sarfraz et al., developed the sustained
release matrix system composed of cellulose derivatives and investigated Naproxen
release from them [78]. Karasulu et al,. formulated the microemulsion systems for
topical delivery of Naproxen and evaluate its physico-chemical properties [79].
Sutradhar et al., formulated the Naproxen matrix tablets based on hydrophilic and
hydrophobic polymer and investigated the formulation by comparing it’s in vitro release
Profile [80].
17
3. EXPERIMENTAL
3.1. Materials
3.1.1. Hydrotropes
a) Sodium salicylate
It was purchased from Sigma (UK), its product No. was S3007 and CAS No was 54-21-7.
Its Molecular Formula was C7H5NaO3, and Molar mass was 160.1 g.mol-1. Whereas its
structural formula is given below:
b) Sodium benzoate,
It was purchased from Fluka (UK), its product No was 71300 and CAS No was 532-32-1.
Its Molecular mass was 144.10 g.mol-1 and Molecular Formula was C6H5COONa.
Whereas its structural formula is as follows:
c) Sodium p-toluene sulfonate
It was obtained from Aldrich (UK), its Product No. was 152536 and CAS No was 657-
84-1. Its Molecular mass was 194.18 g.mol-1 and Molecular formula was C7H7NaO3S.
Whereas its structural formula is given below:
18
d) Sodium xylene sulfonate,
It was purchased from Aldrich (UK), its Product No was 243078 and CAS No was 1300-
72-7. Its Molecular mass was 208.21g.mol-1 and Molecular formula was C8H9NaO3S.
Whereas its structural formula is as follows:
3.1.2. Surfactants
The detail information about the surfactants used for the solubilization of drugs is as
under:
a) Tween 20
It was obtained from E. Merck (Germany), its Product No. was 655205-250 mL and CAS
No was 9005-64-5. Its Molar mass was 1309.7g.mol-1, hydrophile-lipophile balance
(HLB) was 16.7, and Molecular formula was C12S6E2.0 (E20 means having 20 oxyethylene
units). Whereas its structure is given below:
X+Y+W+Z= 20
19
b) Tween 80
It was obtained from Merck (Germany), its Product No. was 9490 and CAS No. was
9005-64-5. Its Molar mass was 1228 g.mol-1, HLB was16.7, and Molecular formula was
C18S6E20. Whereas its structural is given below:
W+X+Y+Z= 20
c) Brij 30
It was obtained of Merck (Germany), its Product No. was P4391 and CAS No. was 9002-
92-0. Its Molar mass was 392 g.mol-1, HLB (hydrophile-lipophile balance) was 9.7, and
Molecular formula was (C20H42O5)n. Whereas its structural formula is given as follows:
d) Brij 35
It was obtained from Merck (Germany), its Product No. was 203724-1L and CAS No.
was 9002-92-0. Its Molar mass was 1198 g.mol-1, HLB (hydrophile-lipophile balance)
wass16.9, and Molecular formula was C12H25 (OCH2CH2) 23OH. Whereas its structural
formula is given below:
20
e) Triton X-100
It was purchased from Sigma-Aldrich (UK), its Product No. was X100 and CAS No. was
9002-93-1. Its Molar mass was 625 g.mol-1, HLB (hydrophile-lipophile balance) was
13.5, and Molecular formula was t-Oct-C6H4-(OCH2CH2)n OH. n=9, 10. Whereas its
structural formula is given below:
n=9,10
f) Triton X-114
It was obtained from Sigma-Aldrich (UK), its Product No. was X114 and CAS No. was
9036-19-6. Its Molar mass was 536 g.mol-1, HLB was 12.4, and Molecular formula was t-
Oct-C6H4-(OCH2CH2)n OH. n=7, 8. Whereas its structural formula is given below:
n=7,8
g) N-dodecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS)
It was purchased from Fluka (UK), its Product No. was 40232 and CAS No. was 14933-
08-5. Its Molar mass was 335.55 g.mol-1, and Molecular formula was
C12H25N(CH3)2(CH2)3SO3. Whereas its structural formula is given below:
21
h) N, N-Dimethyl dodecyl amine N-oxide
It was obtained from Sigma (UK), its Product No. was 40103 and CAS No. was 2605-79-
0. Its Molar mass was 201.35 g.mol-1 and Molecular formula was CH3(CH2)9N(O)(CH3)2.
Whereas its structural formula is as follows:
i) Sodium dodecyl sulphate (SDS)
It was purchased from BDH (UK), its Product No. was 44215. Its Molar mass is 288.35
g.mol-1 and Molecular formula was CH3(CH2)11SO4Na. Whereas as its structural formula
is given below:
j) Dodecyl trimethyl ammonium bromide (DTAB)
It was purchased from Sigma (UK), its Product No. was D8638 and CAS No. was 1119-
94-4. Its Molar mass was 308.34 g.mol-1 and Molecular formula was
CH3(CH2)11N(CH3)3Br. Whereas its structural formula is given below:
22
3.1.3. Menthol
It was the purchased from Alfa Aesar (UK), its Product No. was A10474 and CAS No.
was2216-51-5. Its molar mass 156.27 g.mol-1, and Molecular formula was C10H20O.
Whereas its structural formula is given as below:
3.1.4. Drugs
a) Celecoxib
It was purchased from Alfa Aesar (UK), its Product No. was PZ0008 and CAS No was
169590-42-5. Its molar mass was 381.373g.mol-1 and Molecular formula was
C17H14F3N3O2S. Whereas the structural formula is provided below:
b) Meloxicam
It was the purchased from Sigma (UK). Its Product No. was M3935 and CAS No. was
71125-38-7. Its Molar mass was 351.407g.mol-1 and Molecular formula was C14 H13
N3O4 S2. Whereas its structural formula is provided below:
23
c) Lidocaine
It was the purchased from Sigma (UK). Its Product No. was L-7757-25G. Its Molar mass
was 234.34g.mol-1 and Molecular formula was C14H22N2O. Whereas its structural formula
is given below:
d) Ibuprofen
It was the purchased from Fluka (UK), its Product No. was 77519-1G and CAS No.
15687-27-1. Its Molar mass was 206.28g.mol-1 and Molecular formula was C13H18O2.
Whereas its structural formula is provided below:
24
3.1.5. Oils
a) n-Hexane
It was the purchased from Sigma-Aldrich (UK). Its Product No. was 296090 and CAS
No. was 110-54-3. Its Molar mass was 86.18 g.mol-1 and molecular formula was CH3
(CH2)4CH. whereas its structural formula is given below:
b) n-Decane
It was the purchased from Sigma-Aldrich (UK), Its Product No. was 457116 and CAS
No. was 124-18-5. Its Molar mass was 142.28 g.mol-1 and Molecular formula was
CH3(CH2)8CH3. Whereas its structural formula is given as follows:
c) n-Tetradecane
It was the purchased from Sigma-Aldrich (UK), Its Product No. was 172456 and CAS
No. was 629-59-4. Its Molecular mass was 198.39 g.mol-1 and Molecular formula was
CH3 (CH2)12CH3. Whereas its structural formula is given below:
25
3.2. Methods
3.2.1. Sample preparation
Solutions of surfactants, hydrotropes or microemulsions were prepared in triply distilled
and deionized water (solvent) at 25oC. For the preparation of surfactants and hydrotropes
solutions, required amount of surfactants and hydrotropes were added to water and were
stirred for 30 minutes using magnetic stirrer. Further dilution was made by diluting the
freshly prepared stock solutions with water. All the microemulsion samples were
prepared by adding surfactants necessary to solubilize the required amount of oil and then
different amount of water was added slowly along with stirring till the total mass of
mixture became 1gm. These samples were stirred vigorously, till the formation of clear
solution was obtained while the temperature was kept at 25 0C. The area of the
microemulsion existence was determined by preparing microemulsion samples and
transferring to screw caped glass vials and stored at 25 0C for a period of one month in
order to check their thermodynamic stability. The samples which remained completely
clear and non-birefringent (their refractive index remained constant with the passage of
time) solution when observed through crossed Polaroid were classified as
microemulsions.
3.2.2. Phase diagram
Stable microemulsion formulations were plotted on the triangular phase diagram and
identified as cloudy or clear emulsion. The area of microemulsions existence was
determined twice in order to minimize the error and to ensure the accuracy up to 1%w/w
of oil for each sample.
26
3.2.3. pH measurement
The pH of buffer solution and hydrotropes solutions was measured before and after
addition of drugs using pH meter (Denver Instrument Company England). The pH meter
was calibrated using standard buffer solutions of known pH before the measurements
were made.
3.2.4. Estimation of minimum hydrotropic concentration
The minimum hydrotropic concentration, MHC was determined by surface tension
measurement. The surface tensions of the hydrotropes used was determined by
employing tensiometer, Lauda (TEIII), Germany. The calibration of tensiometer was
made by employing the known weight supplied by the manufacturer and then the surface
tension of pure deionized water was determined. The temperature was kept constant at 25
±0.01oC by using water bath, Lauda (E200), Germany. The surface tension data was
divided into two series. The first series was considered up to concentration where the
effect of concentration over the surface tension became negligible and the series two was
considered as rest of the data. Both were subjected to linear regression and the equations
obtained were solved simultaneously to get their crossing point. The concentration at
which these two lines crossed was considered as MHC.
3.2.5. Estimation of critical micelles concentration
The critical micelles concentration, CMC of surfactants was estimated by surface tension
measurement. The surface tensions of the surfactants used was determined by employing
tensiometer, Lauda (TEIII), Germany. The same procedure was adopted for measurement
of critical micelle concentration as that of minimum hydrotrope concentration
27
3.2.6. Centrifugation
The instrument used for the centrifugation was ultra-centrifuge machine supplied by
Sigma UK. The drug loaded samples of surfactants and microemulsions were taken in
ependrof tube of volume 1.5mL and were centrifuged at 1000/15000 rpm for 30 minutes.
The supernatants were collected for further study.
3.2.7. Measurement of solubility
The solubility of the drugs used during the study was determined by employing IRMECO
UV/Vis spectrophotometer (model U2020), USA. For the purpose 1mL of surfactants
solutions /microemulsion and 5mL of hydrotropic solution was saturated separately with
the drug in order to ensure their maximum solubility. The vials containing excess amount
of drugs and surfactants solutions/microemulsions or hydrotropes solutions were sealed
with screw caps and wrapped with para-film in order to avoid the loss of sample through
evaporation. The drug loaded samples of surfactants/microemulsion and hydrotropes
were continuously stirred by employing magnetic stirrer for 72 hrs. at 25±0.5oC, and the
solubility of the drugs was determined after an interval of one hour and plotted vs time. It
was noted that all the drugs attained the equilibrium around 24 hrs. Therefore the data
was obtained after 24 hrs and reported over here. The excess amount was settled down in
the form of amorphous material. Centrifugation of these samples was then made at 15000
revolutions per minute by using ultra centrifuge machine. The aliquot was then diluted up
to the required concentration by using the same surfactant solution/microemulsion of
same concentrations. The UV/Vis measurements were made at the wavelengths equal to
λmax of each investigated drug. The blanks used in this respect were respective surfactant
28
solution/microemulsion or hydrotrope solution having the same concentration as that of
samples. The amount of drugs was calculated by using Beer-Lambert law.
3.2.8. Differential scanning calorimetry
The DSC study of Menthol, Lidocaine and their mixture was performed using Diamond
DSC supplied by Perkin Elmer (UK), installed with Pyris software to analyze the results.
For the purpose, the temperature was varied from 30oC to 100oC at the rate of 5oC/minute
under the atmosphere of nitrogen. The flow rate was kept at 50 mL/mint for the entire
investigation.
3.2.9. Laser light scattering measurement
For the study of size and shape of aggregates formed by the micelles of surfactants or
microemulsions stabilized by suitable surfactants, the static and dynamic light scattering
techniques were employed. Two different instruments were employed for the purpose.
MALS (multi angle light scattering) DAWN EOS (Enhanced Optical system) for static
light scattering and QELS for quasi elastic light scattering, supplied by Wyatt
Technologies Corporation, USA. The ALV/DLS/SLS-5022F compact goniometer system
(ALV, Langen, Germany) and an ALV-5000/EPP multiple-real time correlator (designed
to perform dynamic (DLS) and static light scattering (SLS) measurements
simultaneously) were used in the present study. A JDS Uniphase helium/neon laser
(vertically polarized beam, 22mW laser power at wavelength 632.8 nm, Manteca, CA,
model 1145P-3083) was used as the light source. The temperature of the sample was
determined using a Pt-100 temperature probe (sensitivity ± 0.2K) inserted in the index
matching fluid (filtered toluene) bath in which the sample cell was housed. ALV-
29
5000/E/WIN software (ALV, Langen, Germany) was used for data analysis. Prior to the
measurements, all the samples were filtered through a 0.2 µm polycarbonate filter
(Millipore, Bedford, MA, USA) into dust-free sample cells. The scattering and diffusion
coefficient of each sample was measured at 30o and 150o scattering angles by a difference
of 10 degrees. Filtered toluene was used as a standard in the present study because its
Rayleigh Ratio was accurately known as 1.406 × 10-5 cm-1 at 633 nm. The data was fitted
using the method of cumulant analysis from the standard ALV software which derived
the apparent hydrodynamic radii of the particles assuming them spherical.
3.2.10. Refractive index measurement
Abbs 60/ED refractometer was employed for the measurement of the refractive index of
surfactants, hydrotropes and microemulsion samples. The Abbe 60/ED refractometer
contained sodium lamp as light source for enlightenment of samples. A wavelength
compensation device was employed for chromatizing to mean sodium wavelength
(589.3nm). Refractive index was read directly from the scale divided up to 0.0001.
Temperature control was achieved by circulating the water of required temperature.
30
4. RESULTS AND DISCUSSION
4.1. Estimation of minimum hydrotropic concentration
The surface tension of NaSal, NaBen, NaPTS and NaXS solution was obtained over a
wide range of concentration at 25oC. The results obtained were plotted as a function of
their concentration. The plot showed an expected trend of variation in surface tension
with the concentration, indicating that the surface tension was decreased with the increase
in concentration and became constant after certain concentration (Figure 1). However, the
extent of decrease in surface tension of water with the addition of hydrotropes was
different for a particular concentration of hydrotropes. Further the concentration of
hydrotropes at which the surface tension became constant, known as minimum
hydrotropic concentration (MHC) was also different. NaBen showed the highest ability to
decrease the surface tension than the others. The detail about its exact measurement is
stated in experimental part. The MHC values of hydrotrope ranged from 0.38 to 0.62
mol.L-1 (Table 3). The results were almost the same, as available in the literature [81-82].
The order of the MHC was NaSal > NaBen > NaPTS > NaXS. The trend in decrease in
surface tension and MHC can be due to variation in structure / molecular mass and
dissociation ability of the material.
31
Figure 1 Surface tension as a function of hydrotrope concentration measured at
25oC
40
50
60
70
80
0 0.2 0.4 0.6 0.8 1
Surface tension (mN.m
‐1)
Concentration of hydrotropes (mol.L‐1)
NaSal NaPTS Naxs NaB
32
Table 3 Minimum hydrotropic concentration (MHC) calculated from Figure 1 and
the one available in the literature
Hydrotrope MHC(Exp) (mol.L-1) MHC(Lit) (mol.L-1)
Sodium salicylate 0.62 0.60 [81]
Sodium benzoate 0.48 0.50 [82]
Sodium p-toluene sulfonate 0.40 0.37 [81]
Sodium xylene sulfonate 0.38 0.40 [81]
33
4.2. Solubility of drugs in water
Saturated solution of Meloxicam, Celecoxib Lidocaine and Ibuprofen drugs was made in
pure water. The absorbed light intensity of the solution was measured by UV
spectroscopic measurements at λmax 363, 252 263 and 272 nm for Meloxicam, Celecoxib,
Lidocaine and Ibuprofen, respectively. The solubility was calculated using standard
curves technique of these drugs and displayed in Table 4. The solubility results indicated
that it was comparable to the ones reported in the literature for the same drugs [83-86].
Table 4 Solubility of drugs in water; measured at 25oC
Drugs Solubility (µg/ml)
Meloxicam 7.15
Celecoxib 5.41
Lidocaine 8.41
Ibuprofen 49.00
4.3. Solubility of drugs in buffer solution
In order to explore the effect of pH over the solubility of drugs in water the buffer
solutions of various pH (1-12) were prepared. These buffer solutions were then saturated
with model drugs and the solubility of these drugs was measured and found that it
increases for ibuprofen, meloxicam and decreases for Lidocaine whereas it almost
remains constant for Celecoxib with the increase in pH (Table 5).
34
Table 5 Solubility of drugs at various pH.
Drug Maximum Solubility of drug(mg.mL-1)
Ibuprofen
pH = 1.00 pH =6.00 pH = 7.00 pH = 9.00 pH = 12.00
0.02 0.23 0.85 7.54 31.53
Meloxicam 0.00057 0.0062 0.00715 3.84 18.20
Lidocaine 7.23 0.0483 0.00841 0.00614 0.0024
Celecoxib 0.0027 0.00332 0.00541 0.2588 0.3413
4.4. Effect of hydrotropes over the solubility of drugs
The effect of hydrotropes addition to water and phosphate buffer (pH 7) over the
solubility of drugs was investigated using sodium benzoate, sodium salicylate, sodium
xylene sulfonate and sodium p-toluene sulfonate hydrotropes. The obtained results
indicated that the solubility was increased significantly beyond the Minimum Hydrotrope
Concentration (MHC) of hydrotropes (Figures 2-17) as observed by others for other
systems [87]. These results further indicated that the solubility of the drugs was due to the
formation of aggregates of the hydrotropes rather than change in pH due to addition of
hydrotropes; it most probably means that the drugs got solubilized only in the interior
part of aggregates of hydrotropes [88-90]. It can also be explained in this way as well that
the drugs being hydrophobic and the interior part of aggregates just like micelles in case
of surfactants is hydrophobic hence the drug can easily interact with it and get
solubilized. However, the solubility of the drugs became constant after certain
concentration of hydrotropes, referred as maximum hydrotrope concentration Cmax and it
remained constant, irrespective of the drug solubilized or concentration of hydrotrope
35
(Figures 2-17). This is due to the facts that the aggregates are saturated with drugs and
cannot hold or sustain the drugs any more [91]. This was the reason that the Cmax value
was different for different hydrotropes and varied from 2.00 to 2.25 mol.L-1 for the
hydrotropes investigated (Table 6).
Figure 2 Solubility of Meloxicam in aqueous solution of sodium benzoate as a
function of concentration and temperature
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Meloxicam (mg.mL‐1)
Concentration of sodium benzoate (mol.L‐1)
298K 303K 308 K 313 K
36
Figure 3 Solubility of Meloxicam in aqueous solution of sodium salicylate as a
function of concentration and temperature
0
2
4
6
8
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Meloxicam (mg.mL‐1)
Concentration of Sodium salicylate (mol.L‐1)
298K 303K 308 K 313 K
37
Figure 4 Solubility of Meloxicam in aqueous solution of sodium p-toluene sulfonate
solution as a function of concentration and temperature
0
1.5
3
4.5
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Meloxicam (mg.mL‐1)
Concentration of sodium p‐toluene sulfonate (mol.L‐1)
298K 303K 308 K 313 K
38
Figure 5 Solubility of Meloxicam in aqueous solution of sodium xylene sulfonate as a
function of concentration and temperature
0
2
4
6
8
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Meloxicam (mg.mL ‐1)
Concentration of sodium xylene sulfonate (mol.L‐1)
298K 303K 308 K 313 K
39
Figure 6 Solubility of Celecoxib in aqueous solution of sodium benzoate as a
function of concentration and temperature
0
2
4
6
8
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Celecoxib (mg.mL‐1)
Concentration of sodium benzoate (mol.L‐1 )
298K 303K 308 K 313 K
40
Figure 7 Solubility of Celecoxib in aqueous solution of sodium salicylate as a
function of concentration and temperature
0
2
4
6
8
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Celecoxib (mg.mL
‐1)
Concentration of sodium salicylate (mol.L‐1)
298K 303K 308 K 313 K
41
Figure 8 Solubility of Celecoxib in aqueous solution of sodium p-toluene sulfonate as
a function of concentration and temperature
0
2
4
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Celocoxib (mg.mL
‐1)
Concentration of sodium p‐toluene sulfonate (mol.L‐1)
298K 303K 308 K 313 K
42
Figure 9 Solubility of Celecoxib in aqueous solution of sodium xylene sulfonate as a
function of concentration and temperature
0
2
4
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Celecoxib (mg.mL
‐1)
Concentration of Sodium xylene sulfonate (mol.L‐1)
298K 303K 308 K 313 K
43
Figure 10 Solubility of Lidocaine in aqueous solution of sodium benzoate as a
function of concentration and temperature
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Lidocaine (mg.mL
‐1)
Concentration of Sodium benzoate (mol.L‐1)
298K 303K 308 K 313 K
44
Figure 11 Solubility of Lidocaine in aqueous solution of sodium salicylate as a
function of concentration and temperature
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Lidocaine (mg.mL
‐1)
Concentration of sodium salicylate (mol.L‐1)
298K 303K 308 K 313 K
45
Figure 12 Solubility of Lidocaine in aqueous solution of sodium p-toluene as a
function of concentration and temperature
0
2
4
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Lidocaine (mg.mL
‐1)
Concentration of sodium p‐toluene sulfonate (mol.L‐1)
298K 303K 308 K 313 K
46
Figure 13 Solubility of Lidocaine in aqueous solution of sodium xylene sulfonate as a
function of concentration and temperature
0
2
4
6
8
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Lidocaine (mg.mL
‐1)
Concentration of sodium xylene sulfonate (mol.L‐1)
298K 303K 308 K 313 K
47
Figure 14 Solubility of Ibuprofen in aqueous solution of sodium benzoate as a
function of concentration and temperature
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Ibuprofen (mg.mL‐1)
Concentration of sodium benzoate (mol.L‐1)
298K 303K 308 K 313 K
48
Figure 15 Solubility of Ibuprofen in aqueous solution of sodium salicylate as a
function of concentration and temperature
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Ibuprofen (mg.mL‐1)
Concentration of sodium salicylate (mol.L‐1)
298K 303K 308 K 313 K
49
Figure 16 Solubility of Ibuprofen in aqueous solution of sodium p-toluene sulfonate
as a function of concentration and temperature
0
1
2
3
4
5
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Ibuprofen (mg.mL‐1)
Concentration of sodium p‐toluen sulfonate (mol.L‐1)
298K 303K 308K 313K
50
Figure 17 Solubility of Ibuprofen in aqueous solution of sodium xylene sulfonate as
a function of concentration and temperature
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5
Solubility of Ibuprofen (mg.mL‐1)
Concentration of sodium xylene sulfonate (mol.L‐1)
298K 303K 308K 313K
51
Table 6 Maximum hydrotropic concentration calculated from Figures 2-17
Hydrotropes CMax (mol.L-1)
Sodium salicylate 2.00
Sodium benzoate 2.25
Sodium p-toluene sulfonate 2.00
Sodium xylene sulfonate 2.00
The solubility of all the investigated drugs in hydrotropes was in the order of Sodium
benzoate > sodium salicylate > sodium xylene sulfonate > sodium p-toluene sulfonate.
The solubilization of drugs in water as well as in hydrotropes is certainly a complicated
phenomenon and hence several mechanisms have been proposed, however, the most
plausible explanation for the purpose is the stacking complexation; chaotropy, i.e.,
breakdown of water structure; and /or the formation of micelles/ aggregates by the
hydrotropes in which the drug is solubilized [92-95]. The stacked arrangement reduces
the exposure of the hydrophobic regions to water and hence enhances the solubilization
[94, 96]. The complexation is encouraged by pi-electron of aromatic group of the drugs
overlapping [97], whereas the aggregation process is due to the electrostatic forces or
hydrogen bonding [94, 96, 98]. The same trend has been observed in our investigated
hydrotropes and drug system.
52
From these observations, it can be concluded that the information about the MHC and
Cmax values for the hydrotrope /solute system has significance in detecting the maximum
and minimum solubility limit of the drugs in a particular system. The solubility of the
drugs in the above mentioned hydrotropes was also obtained at various temperatures and
found to increase with the increase in temperature, irrespective of drugs or hydrotropes,
whereas the Cmax remained almost constant [Figures 2-17]. These plots can be exploited
for the estimation of hydrotrope required to solubilize a particular amount of drugs at a
particular temperature; in other words these plots can help in formulating a drug system
accurately. As the solubility of the drug is very much dependent upon the concentration
of hydrotropes and temperature, hence to estimate the drug that can be solubilized for a
particular system can be estimated by their molar solubilization ratio (MSR), an amount
of drug solubilized by one mole of aggregated hydrotrope [99]
(1)
Where St is total amount of drug solubilized at maximum hydrotrope concentration, SMHC
is the amount of drug solubilized at MHC, CMax is the maximum hydrotrope
concentration in solution and MHC is the minimum hydrotropic concentration. As above
MHC, the hydrotropes monomer concentration is equal to MHC so “CMax – MHC” is
considered to be concentration of hydrotrope. MSR can be defined as
(2)
The hydrotropic aggregate-water partition coefficient, KM can be defined as the amount
of drug solubilized in the aggregates of hydrotropes per amount of drug solubilized in
water for a specific amount of hydrotrope. It can be written as:
53
(3)
Where mole fraction of drug in aggregates can be denoted as Xagg and defined as
(4)
And the mole fraction of drug in aqueous phase (Xaq) can be defined as
(5)
The molar volume of water can be denoted as Vm and is equal to 0.01805 dm3.mol-1. By
putting the value of Xagg and Xaq in Equation (3) we obtained the expression for KM as
=
(6)
The process of solubilization can be better understood by having the information about
the thermodynamic characteristics of the drug that are involved in the process.
Thermodynamically, it can be stated that the solubility of drugs is the distribution of drug
among aggregates of hydrotropes and in aqueous phase and more the change in free
energy is better are interactions or more the solubility will be. The free energy of
solubilization (∆Gs0) can be estimated by employing Equation (7)
∆Gs0 = -RT ln KM (7)
Where R is the gas constant, T is the absolute temperature and KM is the molar partition
co-efficient among the aggregates and aqueous phase. The values calculated for MSR,
KM and ∆Gs0 are reported in Tables 7-10. All the systems investigated have negative
54
value of ∆Gs0, which indicated that the process of solubilization is unprompted and
solubility has direct relation with the change in free energy.
Table 7 Various parameters of Meloxicam in aqueous solution of hydrotropes;
measured at 25oC.
Hydrotropes MSR* Log KM ∆Gso (KJ.mol-1)
Sodium benzoate 0.00600 1.98 -11.30
Sodium salicylate 0.00565 1.92 -10.90
Sodium p-toluene sulfonate
0.00220 1.55 -08.84
Sodium xylene sulfonate
0.00370 1.81 -10.32
* MSR, KM, ∆Gs0 stand for molar solubilization ratio, partition coefficient and equilibrium free energy change, respectively
55
Table 8 Various thermodynamic parameters of Celecoxib in aqueous solution of
hydrotropes measured at 25oC.
Hydrotropes MSR Log KM ∆Gso (KJ.mol-1)
Sodium benzoate 0.0045 1.86 -10.61
Sodium salicylate 0.0046 1.89 -10.78
Sodium p-toluene sulfonate
0.0014 1.39 -7.93
Sodium xylene sulfonate
0.0025 1.62 -9.24
56
Table 9 Various thermodynamic parameters of Lidocaine in aqueous solution of
hydrotropes; measured at 25oC.
Hydrotropes MSR Log KM ∆Gso (KJ.mol-1)
Sodium benzoate 0.0107 1.99 -11.35
Sodium salicylate 0.00975 1.92 -10.95
Sodium p-toluene sulfonate
0.0052 1.75 -9.98
Sodium xylene sulfonate
0.0077 1.91 -10.89
57
Table 10 Various thermodynamic parameters of Ibuprofen in aqueous solution of
hydrotropes; measured at 25oC.
Hydrotropes MSR Log KM ∆Gso (KJ.mol-1)
Sodium benzoate 0.0076 1.84 -10.50
Sodium salicylate 0.0074 1.82 -10.38
Sodium p-toluene sulfonate
0.0036 1.57 -8.95
Sodium xylene sulfonate
0.0054 1.76 -10.04
58
4.5 Estimation of critical micelles concentration of surfactants
The surface tension of Tween 20, Tween 80, Brij 30, Brij 35, Triton X-100, Triton X-
114, DDAO, DDAPS, SDS and DTAB measured as a function of their concentration at
25oC was plotted in Figures 18-24. The results gave nice curves against the variation in
concentration as expected for surfactants. It can be noted from these curves that the
surface tension was decreased with the increase in amount of surfactants in the solution
and becomes constant after certain concentration and this concentration was called as
critical micelles concentration (CMC) of the surfactant. The CMC calculated from the
data as stated in experimental section is depicted in Tables 11-12 and are almost the same
as reported in the literature [99-104].
59
Figure 18 Surface tension of aqueous solution of Tween 20 as a function of its
concentration; measured at 25oC
35
45
55
65
75
85
0 20 40 60 80 100 120
Surface tension (mN.m
‐1)
Concentration of Tween 20 (mg.L‐1)
60
Figure 19 Surface tension of aqueous solution of Tween 80 as a function of its
concentration; measured at 25oC
35
45
55
65
75
85
0 5 10 15 20 25 30
Surface tension (mN.m
‐1)
Concentration of Tween 80 (mg.L‐1)
61
Figure 20 Surface tension of aqueous solution of Triton X-100 as a function of its
concentration; measured at 25oC
32
42
52
62
72
82
0 20 40 60 80 100 120 140 160
Surface tension (mN.m
‐1)
Concentration of Triton X‐100 (mg.L‐1)
62
Figure 21 Surface tension of aqueous solution of Triton X-114 as a function of its
concentration; measured at 25oC
35
45
55
65
75
85
0 20 40 60 80 100 120 140
Surface tension (mN.m
‐1)
Concentration of Triton X‐114 (mg.L‐1)
63
Figure 22 Surface tension of aqueous solution of surfactants as a function of their
concentration and measured at 25oC
30
35
40
45
50
55
0 5 10 15 20 25
Surface tension (mN.m
‐1)
Concentration of surfactnats (mmol.L‐1)
DTAB
DDAO
SDS
64
Figure 23 Surface tension of aqueous solution of Brij 30 as a function of its
concentration; measured at 25oC
35
40
45
50
0 0.02 0.04 0.06
Surface tension (mN.m
‐1)
Concentration of Brij 30 (mmol.L‐1)
65
Figure 24 Surface tension of aqueous solution of Brij 35 as a function of its
concentration; measured at 25oC
44
48
52
56
60
0.02 0.04 0.06 0.08
Surface tension (mN.m
‐1)
Concentration of Brij 35 (mmol.L‐1)
66
Table 11 CMC of the surfactants determined from the surface tension of their
aqueous solution and the one available in the literature. The aggregation number is
obtained from the literature.
Surfactants CMC (mg.L-1) Aggregation
number
Experimental Literature
Tween 20 64.2 60 [99] 52 [99]
Tween 80 15.1 13 [99] 133 [99]
Brij 30 10.3 8.3 [100] 101 [100]
Brij35 77.0 74 [100] 40 [100]
Triton X- 100 132.0 130 [100] 146 [100]
Triton X- 114 115.0 110 [100] 189 [100]
67
Table 12 CMC of the surfactants determined from the surface tension of their
aqueous solution and the one available in the literature. The aggregation number is
obtained from the literature.
Surfactants CMC (mmol.L-1) Aggregation
number Experimental Literature
Brij-35 0.051 0.050 [101] 40 [101]
Brij-30 0.034 0.035 [101] 101 [101]
DDAO 1.820 1.800 [103] 76 [103]
SDS 8.200 8.000 [102] 62 [102]
DTAB 14.700 14.600 [102] 74 [102]
DDAPS 2.500 2.500 [104] 67 [104]
68
4.6. Effect of surfactants over the solubility of drugs
The solubility of Meloxicam, Celecoxib, Ibuprofen and Lidocaine was determined by
adding various amounts of surfactants to water. Though the solubility was increased with
the additions of surfactants to the solution but a noticeable increase was observed when
the concentration of surfactants was increased beyond the CMC (Figures 25-29). The
order of solubility in the investigated non-ionic surfactants for Meloxicam and Celecoxib
was Tween 80> Tween 20> Brij 35>Triton X-114> Triton X-100> Brij 30. For
Lidocaine, the order of solubility in the investigated non-ionic surfactants was Dimethyl
dodecyl amine-N-oxide > Brij 35 > Brij 30. As the solubility of Ibuprofen has already
been investigated in a number of ionic and nonionic surfactants [105-106] hence we
determined it’s solubility only in two surfactants, DDAPS and DDAO and the order of
solubility in these surfactants was DDAPS>DDAO. In case of Tween 20 and Tween 80
though the number of oxyethylene units is present in these surfactants is same but their
HLB value for Tween 80 is lower than Tween 20 which may result micelles of larger size
and hence may solubilize more drug [99]. To get a reason behind the enhancement of
solubility of drugs in micelles, we have compared the solubility with aggregation number
of the micelles available in the literature [99-100]. It was concluded that higher the
aggregation number of micelles more the drug is solubilized indicating that the drug is
entrapped in the inner core of the micelles. This can be the reason for Tween 80 showing
high ability for drug solubilization (Table 11). In case of Brij 30, though its aggregation
number was higher than Brij 35 but drug solubilizing capacity of Brij 35 was higher and
was attributed to oxyethylene units which were high in Brij 35.
69
Triton X-114 showed higher solubilizing ability of drugs as compared to Triton X-100. It
was also attributed to low HLB value, greater number of OE units and high aggregation
number. The solubility of drug in DDAO was high due to interactions of amino oxide
group of DDAO with Lidocaine. From these observations, it can be concluded that the
HLB value and oxyethylene units in the surfactant molecules play important role in
solubilization of drugs. The molecular structure of drugs also played a significant role in
solubilizing them in surfactants; if a group in drugs can interact with the surfactant they
will show high solubility and this is the reason that Lidocaine showed high solubility as
compared to Meloxicam Celecoxib and Ibuprofen [93]. The drug solubilization of
Lidocaine in various ionic and zwitterionic surfactants showed DDAPS > DTAB > SDS
order. It can be noted that the stated surfactants have different head groups but the same
hydrophobic chain length and have difference solubilizing ability for Lidocaine. The
plausible reason can be difference in interactions of Lidocaine with head groups of
surfactants used; whereas DTAB has the ability to solubilize lesser amount of drugs than
DDAPS due to having short chain length. All this discussion concluded that there are
various parameters which control the solubility of drugs in surfactants and impact can
vary from system to system.
70
Figure 25 Solubility of Meloxicam in aqueous solution of nonionic surfactants;
measured at 25oC
0
1.5
3
4.5
6
7.5
9
0.001 0.01 0.1 1 10 100
Solubility
of Meloxicam (mmol.L‐1)
Concentration of surfactants (mmol.L‐1)
Tw20
TW80
BR30
BR35
TRX100
TRX114
71
Figure 26 Solubility of Celecoxib in aqueous solution of nonionic surfactants;
measured at 25oC
0
2
4
6
8
0.001 0.01 0.1 1 10 100
Solubility of Celecoxib (mmol.L‐1)
Concentration of surfactants (mmol.L‐1)
Tw20
TW80
BR30
BR35
TRX100
TRX114
72
Figure 27 Solubility of Lidocaine in aqueous solution of nonionic surfactants;
measured at 25oC
0
3
6
9
12
15
18
0.001 0.01 0.1 1 10
Solubility of Lidocaine (mmol.L‐1)
Concentration of surfactants (mmol.L‐1)
Brij30 Brij35 DDAO
73
Figure 28 Solubility of Lidocaine in aqueous solution of various surfactants;
measured at 25oC.
0
3
6
9
12
15
0.4 4 40
Solubility of Lidocaine (mmol.L‐1)
Concentration of surfactants (mmol.L‐1)
DTAB SDS DDAPS
74
Figure 29 Solubility of Ibuprofen in aqueous solution of surfactants; measured at
25oC.
0
2
4
6
8
10
12
14
0.1 1 10 100
Solubility of Ibuprofen (mmol.L‐1)
Concentration of surfactants (mmol.L‐1)
DDAPS
DDAO
75
It is pertinent to express the solubility of drugs in Rm,s (molar solubilization ratio)
defined in terms of total solubility of drug, Stot, the solubility of drug at CMC, Scmc, the
critical micelles concentration, molar concentration of surfactant in solution, Csurf, and
given as Equation (8) [101,107-108]
, (8)
Since the concentration of monomers in the solution will be equal to total concentration
of surfactant minus the concentration of surfactant in micellar form (Csurf-CMC), hence
the above equation can be written as:
,
(9)
The results obtained in this way for Rm,s of the drugs in all the investigated surfactants
are listed in Tables 13-16.
From the solubility data, the partitioned co-efficient, KM was calculated by employing
Equation (10)
KM = (10)
Here XM is the concentration of drug in micelles and Xa is the amount of dug in aqueous
phase. Mathematically, it can be written as:
= ,
, (11)
= (12)
Where, Vm is the molar volume of water and is equal to 0.01805 dm3.mol-1.By
substituting the value of XM and Xa in Equation (10), KM can be obtained as,
76
= ,
, (13)
The results obtained for partitioned co-efficient for drugs in various surfactant systems
are listed in Tables 13-16. It is worth mentioning that the drugs with intermediate polarity
gets solubilized in palisade layer created between the hydrophilic groups and first few
carbon atoms of hydrophobic groups, known as outer core of the micelles [109-110]. The
other expected phase in which these drugs are solubilized is at micellar-water interface
because of hydrogen bonding between C=O, NH, F and OH group of drugs and OE group
of surfactants [111].
The knowledge about thermodynamic properties that control the solubilization process is
quite helpful in understanding the process of solubilization. The addition of drug to the
system containing surfactant results the partitioning of drug among micelles and the
aqueous phase. However, the value of partition coefficient is highly dependent upon the
variation in the free energy of the system. Therefore the standard free energy of
solubilization ∆G∘ can be calculated from the partition coefficient using Equation (14)
[112]
∆G∘ RTlnK (14)
Here∆ ∘, R and T are the standard free energy, gas constant and absolute temperature,
respectively. The values calculated for ∆ ∘ are reported in Tables 13-16 which is
negative for all the explored system, hence the solubilization of drugs is an unprompted
process and that these two parameters are related directly to each other.
77
Table 13 Various parameters of Meloxicam in aqueous solution of surfactants;
measured at 25oC.
Surfactants Rm,s Log KM ∆Gs0 (KJ.mol-1)
Tw 20 0.403 3.92 -22.36
Tw 80 0.470 3.94 -22.47
Brij 30 0.080 3.57 -20.36
Brij 35 0.281 3.82 -21.79
TR X-100 0.210 3.85 -21.96
TR X-114 0.240 3.87 -22.08
78
Table 14 Various parameters of Celecoxib in aqueous solution of surfactants;
measured at 25oC.
Surfactants Rm,s Log KM ∆Gs0 (KJ.mol-1)
Tw 20 0.37 3.87 -22.07
Tw 80 0.42 3.91 -22.30
Brij 30 0.06 3.54 -20.19
Brij 35 0.26 3.78 -21.56
TR X-100 0.18 3.69 -21.05
TR X-114 0.21 3.72 -21.20
79
Table 15 Various parameters of Ibuprofen in aqueous solution of surfactants
measured at 25oC.
Surfactants Rm,s Log KM ∆Gs0 (KJ.mol-1)
DDAPS 0.27 3.71 -21.16
DDAO 0.26 3.52 -20.08
80
Table 16 Various parameters of Lidocaine in aqueous solution of surfactants;
measured at 25oC.
Surfactants Rm,s Log KM ∆Gs0 (KJ.mol-1)
Brij 30 0.026 3.53 -20.0
Brij 35 0.301 3.62 -20.6
DDAO 0.460 3.75 -21.3
SDS 0.211 3.39 -19.3
DTAB 0.220 3.51 -20.0
DDAPS 0.350 3.68 -21.0
81
4.7. Hydrodynamic radius of micelles loaded with drugs
The autocorrelation functions were obtained for the micelles formed by surfactants
dissolved in pure water and the one saturated with drugs using dynamic laser light
scattering technique (Figures 30-36).The hydrodynamic radii obtained in this way are
listed in Table 17 which indicated that the addition of drugs had almost no effect over the
size of micelles for all the investigated surfactants. These observation support the idea
that the drug is solubilized in the inner core of the micelles, hence the aggregation
number and size of the micelles remained constant [113].
Figure 30 Autocorrelation functions obtained by Tween 20 micelles
82
Figure 31 Autocorrelation functions obtained by Tween 80 micelles.
83
Figure 32 Autocorrelation functions obtained by Brij 30 micelles
84
Figure 33 Autocorrelation functions obtained by Brij 35 micelles.
85
Figure 34 Autocorrelation functions obtained by Triton X-100 micelles.
86
Figure 35 Autocorrelation functions obtained by Triton X-114 micelles.
87
Figure 36 Autocorrelation functions obtained by DDAO micelles.
88
Table 17 Hydrodynamic radius of pure and drug saturated micelles of various
surfactants; measured at 25oC
Surfactant Hydrodynamic radius of
pure micelles (nm)
Hydrodynamic radius of
drug saturated micelles
(nm)
Tween 20 4.2 4.22
Tween 80 5.3 5.29
Brij 30 3.6 3.6
Brij 35 5.6 5.58
Triton X 100 3.8 3.2
Triton X 114 4.1 3.6
DDAO 3.2 3.25
DDAPS 2.62 2.63
89
4.8. Effect of hydrotropes on critical micelles concentration of DDAPS
The surface tension of DDAPS/water, DDAPS/NaSal/water and DDAPS/NaBen/water
systems was measured at 25oC for various concentration of DDAPS and NaSal or NaBen
and is listed in Tables 18-19. The same data has also been plotted against concentration
of DDAPS in Figures 37-39 which showed a typical trend of surfactants. The figures
indicated that the surface tension was decreased with the increase in concentration of
DDAPS and became constant beyond certain concentration of DDAPS, designated as
CMC of DDAPS. The addition of NaSal to DDAPS solution reduced the surface tension
but the trend remained the same throughout the investigated concentration range (Figure
37), however, the impact was not much visible beyond the CMC of DDAPS. The reason
behind it can be that NaSal gets adsorbed over the micelles and hence had less impact
over the interfacial behavior of the surfactant. A similar trend was observed in case of
NaBen (Figure 38). The CMC was obtained from these data/ plots and found to decrease
with the increase in concentration of hydrotropes added (Figure 39- 40). By the addition
of 0.4 mol.L-1 of sodium salicylate and sodium benzoate, the CMC of DDAPS was
reduced from 2.5×10-3 mol.L-1 (in water) to 1.18×10-3 mol.L-1 and 2.1×10-3 mol.L-1,
respectively. It is to be noted that both of these hydrotropes have aromatic ring and
carboxylate group hence can get adsorbed at the surface of the micelles, resulting a
decrease in surface charge and the CMC [114]. The CMC of DDAPS is reduced more in
the presence of sodium salicylate as compared to sodium benzoate for the same molar
concentration of the hydrotropes. This may be due to the reason that sodium salicylate
contains –OH group that donates its electrons to ring making it electron rich which intern
90
increases its interaction with micellar surface, hence reduces the CMC of DDAPS more
than the later.
Table 18 Surface tension (mN.m-1) of DDAPS in the presence of NaSal measured at
25oC
Concentration
of DDAPS
(mol.L-1)
Amount (mol.L-1) of NaSal added
0.00 0.04 0.08 0.2 0.4
0.0010 49.07 45.6 42.6 40.1 35
0.0012 48.30 41.5 37.6 34.2 31.8
0.0014 47.50 34.6 34.2 31.9 30.4
0.0016 46.15 32.2 32.1 31.0 30.2
0.0018 44.80 30.7 31.3 30.2 30.0
0.0020 42.40 30.5 30.7 30.1 29.3
0.0022 40.60 29.9 29.3 29.7 29.0
0.0025 39.8 29.8 28.9 28.6 28.8
0.0030 39.6 29.6 28.6 28.4 28.4
0.0035 39.6 29.6 28.4 28.2 28.0
91
Figure 37 Variation in surface tension of DDAPS by the addition of sodium
salicylate as a function of DDAPS concentration; measured at 25oC
25
30
35
40
45
50
55
0 0.001 0.002 0.003 0.004 0.005
Surface tension (mN.m
‐1)
Concentration of DDAPS (mol.L‐1 )
0.04 M NaSal 0.08 M NaSal 0.2 M NaSal
0.4 M NaSal Series1
92
Table 19 Surface tension (mN.m-1) of DDAPS in the presence of sodium benzoate;
measured at 25oC
Concentration
of DDAPS
(mmol.L-1)
Amount (mol.L-1) of sodium benzoate added
0.00 0.04 0.08 0.2 0.4
0.0010 49.07 47.0 45.4 44.2 41.2
0.0012 48.30 43.5 41.4 40.3 38.5
0.0014 47.50 38.9 37.2 35.6 34.3
0.0016 46.15 36.5 34.5 33.1 32.4
0.0018 44.80 33.2 32.3 31.6 30.9
0.0021 42.40 30.2 30.6 29.8 29.5
0.0022 40.60 30.1 29.6 29.2 29.0
0.0025 39.8 29.6 29.3 29.2 28.8
0.0030 39.6 29.5 29.1 29.1 28.6
0.0035 39.6 29.4 29.0 28.9 28.4
93
Figure 38 Variation in surface tension of DDAPS by the addition of sodium
benzoate; measured at 25oC
27
32
37
42
47
52
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004
Surface tension (mN.m
‐1)
Concentration of DDAPS(mmol.L‐1)
0.00 NaBen 0.04 M NaBen 0.08 M NaBen
0.2 M NaBen 0.4 M NaBen
94
Figure 39 Effect of sodium salicylate on CMC of DDAPS measured at 25oC
0
0.6
1.2
1.8
2.4
0 0.1 0.2 0.3 0.4 0.5
CMC of DDAPS (m
mol.L‐1)
Hydrotrope concentration (mol.L‐1)
95
Figure 40 Effect of sodium benzoate on CMC of DDAPS measured at 25oC
2.07
2.1
2.13
2.16
2.19
2.22
0 0.1 0.2 0.3 0.4 0.5
CMC of DDAPS (m
mol.L‐1)
Hydrotrope concentration (mol.L‐1)
96
4.9. Effect of hydrotropes on solubility of drugs in DDAPS
Number of studies has been performed in order to increase the solubility of substrates by
the addition of surfactants [92-97]. The solubility enhancement of substrates (drugs) was
investigated by using DDAPS and its mixture with hydrotropes. The solubility of drugs
was estimated in DDAPS and in the presence of sodium salicylate and sodium benzoate
hydrotropes. The results obtained have been plotted in Figures 41-46.These figures
illustrated that the addition of hydrotropes leads to increase in solubility of drugs though
the trend of the plots remained the same. This was true for all the drugs and both the
hydrotropes investigated. This increase in solubility was attributed to reduction in CMC
which may lead to high number of micelles for the same concentration of DDPAPS.
These observations once again lead to believe that the drugs are solubilized in the inner
core/ internal part of micelles/ aggregates. From these results, the molar solubilization
ratio (Rm,s), micellar partition co-efficient, KM and free energy of solubilization ∆G0 was
calculated using Equations (8-14). The results obtained are summarized in Tables 20-22.
The results indicated that media has a high effect over these parameters rather than the
drugs.
97
Figure 41 Solubility of Lidocaine in aqueous solution of DDAPS/ (0.4mol.L-1) NaSal
mixture; measured at 25oC
0
2
4
6
8
10
1 10
Solubility of Lidocaine (mmol.L‐1)
Concentration of DDAPS (mmol.L‐1)
98
Figure 42 Solubility of Meloxicam in aqueous solution of DDAPS/ (0.4mol.L-1) NaSal
mixture; measured at 25oC
0
2
4
6
8
1 10
Solubility of Meloxicam (mmol.L‐1)
Concentration of DDAPS (mmol.L‐1)
99
Figure 43 Solubility of Celecoxib in aqueous solution of DDAPS/ (0.4mol.L-1) NaSal
mixture at 25oC
0
2
4
6
8
1 10
Solubility of Celecoxib (mmol.L‐1)
Concentration of DDAPS (mmol.L‐1)
100
Figure 44 Solubility of Lidocaine in aqueous solution of DDAPS/ (0.4mol.L-1) NaBen
mixture; measured at 25oC
0
1
2
3
4
5
6
7
8
1 10
Solubility of Lidocaine (mmol.L‐1)
Concentration of DDAPS (mmol.L‐1)
101
Figure 45 Solubility of Meloxicam in aqueous solution of DDAPS/(0.4mol.L-1)
NaBen mixture; measured at 25oC
0
1
2
3
4
5
6
7
1 10
Solubility of Meloxicam (mmol.L‐1)
Concentration of DDAPS (mmol.L‐1)
102
Figure 46 Solubility of Celecoxib in aqueous solution of DDAPS/(0.4mol.L-1) NaBen
mixture; measured at 25oC
0
2
4
6
8
1 10
Solubility of Celecoxib (mmol.L‐1)
Concentration of surfactant (mmol.L‐1)
103
Table 20 Various parameters of drugs calculated in the presence of DDAPS at 25oC
Drugs MSR Log KM ∆Gso (KJ.mol-1)
Lidocaine 0.37 3.69 -21.0
Meloxicam 0.33 3.68 -20.9
Celecoxib 0.24 3.62 -20.6
Ibuprofen 0.27 3.71 -21.16
104
Table 21 Various parameters of drugs calculated in the presence of DDAPS/NaSal
at 25oC
Drugs MSR Log KM ∆Gso (KJ.mol-1)
Lidocaine 0.54 3.80 -21.68
Meloxicam 0.50 3.78 -21.56
Celecoxib 0.44 3.76 -21.45
Ibuprofen 0.35 3.62 -20.65
105
Table 22 Various parameters of drugs calculated in the presence of DDAPS/NaBen
at 25oC
Drugs MSR Log KM ∆Gso (KJ.mol-1)
Lidocaine 0.43 3.73 -21.28
Meloxicam 0.41 3.73 -21.28
Celecoxib 0.38 3.71 -21.16
Ibuprofen 0.31 3.58 -20.42
106
4.10. Aggregation number of DDAPS/NaSal and DDAPS/NaBen
We have also measured the aggregation number of DDAPS/NaSal and DDAPS/NaBen
mixture by employing static laser light scattering technique. For the purpose following
equations were used [115].
‐
∆ 2B C‐C (15)
Here, C and Ccmc are the total concentration (in mol.L-1) of surfactant. Rθ is the excess
Rayleigh ratio, B is second varial coefficient. K is an optical constant calculated as
K /
(16)
Where n is the refractive index of the solvent; dn/dc is the refractive index increment of
the sample; NA is the Avogadro’s number and λ is the wave length of incident light.
The mass of aggregates (in this case Magg) and aggregation number can be calculated as
M (17)
and
N
(18)
The aggregation number of the system DDAPS/NaSal and DDAPS /NaBen obtained in
this way is reported in Tables 23-24. The results indicated that the addition of
hydrotropes to 1.0 to 10 mmol.L-1 concentration of DDAPS increased the aggregation
107
number of DDAPS. This was the reason that this system was able to solubilise higher
amount of drugs as compared to DDAPS alone.
Table 23 Aggregation number of DDAPS micelles formed in aqueous solution of 0.4
molar NaSal; measured at 25oC
Concentration of DDAPS (mol.L-1) Nagg
1.80 116
2.10 132
5.00 158
10.0 167
108
Table 24 Aggregation number of DDAPS micelles formed in aqueous solution of 0.4
molar NaBen; measured at 25oC
Concentration of DDAPS (mol.L-1) Nagg
1.80 102
2.10 118
5.00 139
10.0 155
109
4.11. Hydrodynamic radius of DDAPS/NaSal and DDAPS/NaBen mixture
The size of DDAPS/NaSal/butanol and DDAPS/NaBen/butanol mixture blank and drugs
loaded in terms of hydrodynamic radius was measured using dynamic light scattering
intensity (Tables 25-26). It was noted that the hydrodynamic radius of DDAPS/NaSal and
DDAPS/NaBen mixture was larger than DDAPS alone. The drug loaded and blank
systems had almost the same hydrodynamic radius. These observations concluded that
the drugs are solubilized in the inner core of aggregates and this is the reason for showing
high solubility in such systems.
Table 25 Effect of addition of NaSal (0.4 mol.L-1) on hydrodynamic radius of
DDAPS; measured at 25oC
Concentration of butanol
(mol.L-1)
RH blank (nm) RH drug loaded micelles
(nm)
1.80 2.83 2.85
2.10 2.97 3.00
5.00 3.29 3.28
10.0 3.91 3.90
110
Table 26 Effect of addition of NaBen (0.4 mol.L-1) on hydrodynamic radius of
DDAPS measured in water/butanol mixture at 25oC
Concentration of butanol
(mol.L-1)
RH blank (nm) RH drug loaded micelles
(nm)
1.80 2.79 2.80
2.10 2.91 2.93
5.00 3.20 3.22
10.0 3.82 3.84
111
4.12. Effect of addition of butanol on solubility of drugs
Alcohols are polar compounds and interact with micelles formed by the surfactants
through their polar groups [98]. Therefore, it is expected that the addition of alcohol to
the system may alter the solubility of the drug dissolved in surfactant/water system. For
the quantitative estimation of this phenomenon, the solubility of the drugs was measured
in DDAPS/butanol mixture as a function of contents of alcohol. The result showed that
the solubility of drugs was increased by the addition of butanol from 0.2 to 1.2 mol.L-1 in
0.5 mol.L-1 of DDAPS (Table 27).
Table 27 Effect of addition of butanol to water on solubility of drugs in 0.5 mol.L-1 of
DDAPS at 25oC
Contents of
butanol
(mol.L-1)
Ibuprofen
(mmol.L-1)
Meloxicam
(mmol.L-1)
Celecoxib
(mmol.L-1)
Lidocaine
(mmol.L-1)
0.2 33.4 37.40 28.20 39.50
0.5 35.90 38.33 33.25 40.25
0.8 44.60 45.30 38.33 46.30
1.2 52.35 53.60 40.60 54.62
112
4.13. Solubility of drugs in oil-in-water microemulsion
Hexane, Decane or Tetradecane (2% v/v) was dispersed in 0.5 mol.L-1 DDAPS/1.2
mol.L-1 aqueous solution of DDAPS and butanol to get oil-in-water microemulsions.
Theses microemulsions were tested for their ability to solubilize drugs. It was noted that
these microemulsions had greater ability to solubilize drugs than DDAPS/butanol mixture
(Table 28). However, this ability varied with the variation in nature of oil and every oil
showed different ability. The reason behind such trend might be that the drugs and oils
are hydrophobic and hence the drug is dissolved in oil which is then dispersed in the
mixture in the form of microemulsion [116].
Table 28 Effect of addition of 2% (v/v) hydrocarbons over the apparent solubility of
drugs in DDAPS/butanol (0.5 mol.L-1/1.2 mol.L-1) mixture at 25oC
Hydrocarbon
oil
Ibuprofen
(mmol.L-1)
Meloxicam
(mmol.L-1)
Celecoxib
(mmol.L-1)
Lidocaine
(mmol.L-1)
Hexane 60.10 62.54 41.25 63.54
Decane 70.30 73.56 44.12 75.01
Tetradecane 73.70 76.23 47.54 77.58
113
4.14. Aggregation number of DDAPS, DDAPS/butanol and microemulsion
By employing Equations (14-17), the aggregation number of blank and drug loaded
DDAPS micelles were calculated which remained constant (Tables 29-31) even with the
increase in concentration from 0.06 to 0.5 mol.L-1 of DDAPS and with the addition of
drugs. The addition of butanol to DDAPS solution increased the solubility of drugs. In
order to explore and identify the causes of increase in solubility by the addition of
butanol, the aggregation number of the system was measured using light scattering
technique. The calculated aggregation numbers of systems under observations are listed
in Tables 29-31. The addition of butanol to fixed concentration (0.5mol.L-1) of DDAPS
brought an increase in the aggregation number. Therefore, the increase in solubility with
the addition of butanol was the increase in aggregation number. Whereas the addition of
2% v/v Hexane, Decane, or Tetradecane to 0.5 mol.L-1 DDAPS / 1.2 mol.L-1 Butanol
mixture resulted a decrease in aggregation number. It was noted that larger the alkyl
chain length of hydrocarbon oils greater the decrease in aggregation number of
micelles/droplet (Table 29-31). However, the solubility continued to increase by the
increase of alkyl chain length of hydrocarbons.
114
Table 29 Aggregation numbers of DDAPS Micelles in water; measured at 25oC
Concentration of DDAPS (mol.L-1) Nagg
0.06 63
0.12 63
0.18 63
0.24 63
0.30 63
0.50 63
115
Table 30 Effect of addition of butanol on aggregation number of 0.5 mol.L-1DDAPS;
measured at 25oC
Concentration of butanol (mol.L-1) Nagg
0.2 74
0.5 90
0.8 298
1.2 387
116
Table 31 Effect of addition of 2% (v/v) hydrocarbons over the aggregation number
of DDAPS/butanol (0.5mol.L-1/1.2mol.L-1) mixture; measured at 25oC
Hydrocarbon Nagg
Hexane 373
Decane 364
Tetradecane 359
117
4.15. Hydrodynamic radius of DDAPS/butanol mixture and microemulsions
The results obtained for hydrodynamic radius of blank and drugs loaded DDAPS,
DDAPS/butanol mixture are listed in Tables 32-34. It had been observed that the
hydrodynamic radius of pure micelles was smaller than DDAPS/Butanol mixture due to
the reason stated earlier. The hydrodynamic radius of microemulsions was smaller than
that of DDAPS/butanol mixture. It was noted that the hydrodynamic radii of micro-
emulsions show a decreasing trend with the increase in chain length of the oil used.
Further the drug loaded and blank systems have nearly the same hydrodynamic radius
(Tables 32-34). These results conclude that the stated mechanism of solubilization is
correct and according to expectations.
Table 32 Hydrodynamic radius of blank and drugs saturated DDAPS Micelles
measured at 25oC
Concentration of DDAPS (mol.L-1)
RH blank (nm) RH drug saturated
micelles (nm)
0.06 2.62 2.63
0.12 2.63 2.64
0.18 2.63 2.63
0.24 2.63 2.63
0.30 2.63 2.63
0.50 2.64 2.64
118
Table 33 Effect of addition of butanol on hydrodynamic radius of drug saturated
(0.5mol.-l) DDAPS at 25 oC
Concentration of butanol
(mol.L-1)
RH of blank (nm) RH of drug saturated
micelles (nm)
0.2 2.94 2.93
0.5 3.05 3.07
0.8 3.45 3.47
1.2 4.12 4.14
119
Table 34 Effect of addition of 2% (v/v) hydrocarbons over the hydrodynamic radius
of DDAPS/butanol (0.5mol.L-1/1.2mol.L-1)/drug systems at 25oC
Hydrocarbon RH of blank (nm) RH of drug saturated
micelles (nm)
Hexane 4.10 4.10
Decane 4.08 4.08
Tetradecane 4.06 4.06
120
4.16. Microemulsion having Menthol and/or Eutectic mixture as an oil
For the estimation of eutectic point of Lidocaine /Menthol mixture the system was
subjected to differential scanning calorimetric (DSC). The melting point of Lidocaine and
Menthol as well as their mixture was determined. The mixture which gave single melting
point was considered as eutectic mixture (Figure 47). The ratio of the eutectic mixture
estimated in this way was 30:70 Lidocaine: Menthol and was fluid at room temperature.
Figure 47 DSC curve of Lidocaine/Menthol mixture
121
For the formation of microemulsions (ME1–ME6) water was added to mixture of
surfactant/oil mixture. It was observed that the addition of water to surfactant/oil mixture
resulted in a continuous transition from cloudy to a transparent phase having low
viscosity systems. Three-component triangular phase diagrams of O/W microemulsions
obtained in this way are displayed in Figures 48-50. The results indicated that 40% w/w
DDAPS formed oil-in-water microemulsions with, 9% w/w Menthol and 13% w/w
eutectic mixture of Menthol/Lidocaine. On the other hand 40% w/w DTAB, formed O/W
microemulsion with 10% w/w Menthol and 11% w/w eutectic mixture. Whereas, 20%
w/w SDS gave O/W microemulsion with 10% w/w Menthol and 34% w/w eutectic
mixture (Table 35). These microemulsions were checked for their stability by adding
water to the system. The microemulsions having lowest surfactant concentration were
selected for further investigation (Table 36).
122
SAA0 10 20 30 40 50 60 70 80 90 100
Menthol
0
10
20
30
40
50
60
70
80
90
100
Water
0
10
20
30
40
50
60
70
80
90
100
SAA0 10 20 30 40 50 60 70 80 90 100
Eutectic
0
10
20
30
40
50
60
70
80
90
100
Water
0
10
20
30
40
50
60
70
80
90
100
Figure 48 Phase diagrams of microemulsion stabilized by DDAPS at 25oC
123
SAA0 10 20 30 40 50 60 70 80 90 100
Menthol
0
10
20
30
40
50
60
70
80
90
100
Water
0
10
20
30
40
50
60
70
80
90
100
SAA0 10 20 30 40 50 60 70 80 90 100
Eutectic
0
10
20
30
40
50
60
70
80
90
100
Water
0
10
20
30
40
50
60
70
80
90
100
Figure 49 Phase diagrams of microemulsion stabilized by DTAB at 25oC
124
SAA0 10 20 30 40 50 60 70 80 90 100
Menthol
0
10
20
30
40
50
60
70
80
90
100
Water
0
10
20
30
40
50
60
70
80
90
100
SAA0 10 20 30 40 50 60 70 80 90 100
Eutectic
0
10
20
30
40
50
60
70
80
90
100
Water
0
10
20
30
40
50
60
70
80
90
100
Figure 50 Phase diagrams of microemulsion stabilized by SDS at 25oC
125
Table 35 Composition of microemulsions having the capacity of sustaining
maximum oil
ME 1 ME 2 ME 3 ME 4 ME 5 ME 6
DDAPS 40 40 ---- ---- ----
DTAB ---- ---- 40 40 ----
SDS ---- ---- ---- ---- 20 20
Menthol 9 ---- 10 ---- 10 ----
Eutectic ---- 13 ----- 11 ---- 34
Water 51 47 52 52 70 66
126
Table 36 Composition of oil in water microemulsions obtained from phase diagrams
and can be diluted to a very low surfactant concentration
ME 1 ME 2 ME 3 ME 4 ME 5 ME 6
Composition % w/w % w/w % w/w % w/w % w/w % w/w
DDAPS 40 40 ---- ---- ---- -----
DTAB ---- ---- 40 40 ---- -----
SDS ---- ---- ---- ---- 20 20
Menthol 4 ---- 8 ---- 7 ----
Eutectic ---- 4 ----- 8 ---- 14
Water 56 56 52 52 73 66
127
4.17. Aggregation number of microemulsion having Menthol and Eutectic mixture
as oil
Table 37 represents the aggregation number, calculated using Equations (14-17) of oil-in
water microemulsion having Menthol and/or eutectic mixture as oil. The table indicated
that on addition of menthol and eutectic mixture to DDAPS/Water, DTAB/Water and
SDS/Water mixture increased the aggregation number of the system from 63,65 to 76,87
and 97 for Menthol and 73,98, and 105 for Eutectic mixture when added to above
mentioned systems separately.
Table 37 Aggregation number of microemulsions measured at 25oC
Microemulsions Aggregation number
DDAPS/Menthol 76
DDAPS/Eutectic 73
DTAB/Menthol 87
DTAB/Eutectic 98
SDS/Menthol 97
SDS/Eutectic 105
128
4.18. Hydrodynamic radius of microemulsion having Menthol and Eutectic mixture
as oil
At infinite dilutions, the hydrodynamic radius of micelles (assuming spherical
aggregates) was found to be 2.54, 1.75, and 1.88 nm for DDAPS, SDS and DTAB
micelles, respectively. The hydrodynamic radius of ionic surfactants was very difficult to
obtain by employing laser light scattering technique due to their head groups repulsion
and the ionic surfactants micelles showed a continuous decrease in their size [117]. In
order to get size/hydrodynamic radius of micelles of ionic surfactants and their
microemulsions 0.002M phosphate buffer of pH 7.4 was used as a solvent instead of
water. The size obtained in this way was 2.87/2.94, 3.14/3.21 and 3.02/3.07 nm for
DDAPS, SDS and DTAB microemulsions with Menthol/ Eutectic, respectively (Table
38). It can be noted that the increase in droplet size/micelle enhanced the solubility of
drugs which certainly support our earlier conclusion.
129
Table 38 Hydrodynamic radius of microemulsion measured at 25oC
Microemulsion system Hydrodynamic radius (nm)
DDAPS/Menthol 2.87
DDAPS/Eutectic 2.94
DTAB/Menthol 3.02
DTAB/Eutectic 3.07
SDS/Menthol 3.14
SDS/Eutectic 3.21
130
4.19. Refractive index of various systems
The results obtained for refractive index of DDAPS and its microemulsions with Menthol
and eutectic mixture in water are listed in Tables 39-41 and plotted in Figures 51-53 as a
function of surfactant concentration. The figures showed that the refractive index was
increased linearly with the concentration of surfactant / microemulsions of menthol or
eutectic mixture, showing that the system was quite homogeneous in the investigated
range. The micellar/ microemulsions systems had different refractive index which
indicated that on addition of Menthol or Eutectic mixture to surfactant changed the dipole
moment of surfactant and hence increased the refractive index. Similar trend was also
noticed for DTAB and SDS systems.
Table 39 Refractive index of DDAPS micelles and its microemulsion measured at
25oC
Surfactant
concentration %
(w/w)
Micelle Menthol Eutectic (30:70)
0.2 1.33114 1.33126 1.33148
0.4 1.33128 1.33151 1.33198
0.6 1.33141 1.33175 1.3325
0.8 1.33156 1.33199 1.33291
1 1.33168 1.33225 1.33339
131
Table 40 Refractive index of DTAB micelles and its microemulsion measured at
25oC
Surfactant
concentration
%(w/w)
Micelle Menthol Eutectic 30:70
0.2 1.33135 1.33148 1.33172
0.4 1.33173 1.33198 1.33246
0.6 1.33209 1.33246 1.33321
0.8 1.33245 1.33296 1.33392
1 1.33283 1.33343 1.33465
132
Table 41 Refractive index of SDS micelles and its microemulsion measured at 25oC
Surfactant
concentration %
(w/w)
Micelle Menthol Eutectic 30:70
0.2 1.33123 1.33141 1.33168
;0.4 1.33149 1.33185 1.33239
0.6 1.33174 1.33225 1.33307
0.8 1.33197 1.33266 1.33375
1 1.33222 1.33308 1.33442
133
Figure 51 Refractive index of DDAPS micelles and its microemulsion; measured at
25oC
1.33
1.331
1.332
1.333
1.334
0 0.2 0.4 0.6 0.8 1 1.2
Refractive index
Surfactant concentration % (w/w)
Micelle Menthol Eutectic 30:70
134
Figure 52 Refractive index of DTAB micelles and its microemulsion; measured at
25oC
1.33
1.331
1.332
1.333
1.334
1.335
0 0.2 0.4 0.6 0.8 1 1.2
Refractive
index
Surfactant concentration % (w/w)
Micelle Menthol Eutectic 30:70
135
Figure 53 Refractive index of SDS micelles and its microemulsion; measured at 25oC
1.33
1.331
1.332
1.333
1.334
1.335
0 0.2 0.4 0.6 0.8 1 1.2
Refractive index
Surfactant concentration % (w/w )
Micelle Menthol Eutectic 30:70
136
CONCLUSION
Solubility in an aqueous media is the prerequisite of the drugs; therefore the efforts were
made to enhancement the solubility of Meloxicam, Celecoxib, Ibuprofen and Lidocaine
drug by trying the nature of the aqueous media. For the purpose a solution of
hydrotropes, surfactants, surfactant/hydrotrope, surfactants/butanol mixtures and oil/
water microemulsions were tested and tensiometry, UV/Vis spectrophotometry and light
scattering techniques were employed to understand the mechanism of solubilization and
the various parameters over it. The results obtained in this way concluded that the order
of solubilization of system investigated was hydrotropes < surfactants <
surfactant/hydrotropes mixture < surfactant/Butanol mixture < oil in water
microemulsion. On the other hand Lidocaine drugs had the highest solubility in these
systems whereas Ibuprofen had the lowest. The results also conclude that NaBen
hydrotrope was having high capacity to solubilize the drugs whereas NaPTS has the
lowest. Overall the solubilizing capacity was noted to in the order of NaBen > NaSal >
NaXS >. NaPTS.
The results obtained in case of role of surfactants concluded that surfactant play
significant role in enhancing the solubility of drugs and the effect can be multiplied by
the addition of hydrotropes etc. However, in case of nonionic surfactants oxyethylene
units (OE), HLB and Aggregation number had noticeable impact over their solubilizing
capacity. The addition of hydrotropes and Butanol to aqueous solution of DDAPS not
only affected the solubility of model drugs in DDAPS micelles but also decreased its
CMC. The results also revealed that the addition of hydrocarbons to DDAP/Butanol
mixture (microemulsions) increased the solubility of tested (Ibuprofen) drug and an
137
increase in alkyl chain length of oil further increased the solubility. On the other hand
hydrodynamic radius of micelles remained the same in spite of addition of drugs;
whereas the addition of hydrotropes, Butanol and hydrocarbons to aqueous solutions of
DDAPS lead to an increase in aggregation number. The Hydrodynamic radius and
aggregation number of microemulsions was increased by the addition of Menthol and/or
Eutectic mixture of Menthol/Lidocain. It has therefore been concluded that the drugs with
even very low solubility can be solubilized up to required level by using various
surfactants/ surfactants-hydrotropes systems. However, the problem is very serious in the
sense that the system to be selected must not be dangerous for health and this is really a
big challenge for the scientists.
138
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