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Chapter-I
General Introduction
Amphiphile is a term describing a chemical compound possessing both
hydrophilic (water-loving) and lipophilic (fat-loving) properties. Such a compound
is called amphiphilic or amphipathic. It is derived from Greek word amphi,
meaning both, and terms relates to the facts that all surfactant molecules consist of
at least two parts, one which is soluble in specific fluid (the lyophilic part) and one
which is insoluble (the lyophobic part). When the fluid is water, one usually talks
about the hydrophobic and hydrophilic parts, respectively. A hydrophilic molecule
or portion of a molecule is one that is typically charge-polarized and capable of
hydrogen bonding, enabling it to dissolve more readily in water than in oil or other
hydrophobic solvents. Hydrophilic and hydrophobic parts of molecules are also
known as polar and non-polar, respectively. Hydrophobic portion of molecules in
water often cluster together forming micelles. Water on hydrophobic surfaces will
exhibit a high contact angle.
Amphiphilic compounds bear an ionic (cationic, anionic, or zwitterionic) or
non-ionic polar head group and a hydrophobic portion. In aqueous medium, they
are able to organize themselves as micelles, bilayers, monolayers, hexagonal or
cubic phases. Aggregation is not, however, just limited to aqueous solution; it is
sometimes observed in non-aqueous polar solvents such as ethylene glycol and
non-polar solvents such as hexane (in the latter case giving rise to inverse
structures) [1]. The extent of the hydrophobic and hydrophilic portions determines
the extent of partitioning. The spatial separation between the polar and non-polar
moieties, as well as molecular shape [2] and hydrophilic and hydrophobic balance
[3], determines their tendency to form the different structures, which eventually
can be interconverted as a function of pH, temperature, ionic strength, and
concentration. Alkyl chain-containing surfactants seem to associate according to
phase separation model. Normally, these micelles exhibit aggregation numbers
between 50 and 200 [4]. The amphiphile with more or less equilibrated
hydrophilic and lipophilic tendencies are likely to migrate to surface or interface.
It doesn‟t happen if the amphiphilic molecule is too hydrophilic or too
hydrophobic, in which case it stays in one of the phases. Because of its dual
affinity, an amphiphilic molecule doesn‟t feel „at ease‟ in any solvent. This is why
amphiphilic molecules exhibit a very strong tendency to migrate to interface or
surfaces and to orientate so that the polar group lies in water and the non-polar or
apolar is placed out of it. Amphiphiles often exhibit other properties besides
lowering of surface tension.
Amphiphiles play a key role in the existence of the life and widely used in
the industry, medicine, pharmacology, etc. [5, 6]. The single features of
amphiphiles that give rise to broad utility is their ability to coexist with and
function as an interface between polar and non-polar phases. This ability is
determined by a balance between ionic and dipolar interactions with polar media
and dispersion interactions with non-polar media.
Solutes showing hydrophobic self-association may be classified into four
categories on the basis of chemical structure: (1) flexible chain compounds
(surfactants, etc.), (2) aromatic or heterocyclic ring or fused ring structures (drugs,
dyes, etc.), (3) alicyclic fused ring compounds (bile salts, etc.), and (4)
macromolecular solutes (proteins, etc.) [7]. The self-association behavior must
relate to the chemical structure of solutes. The simplest type of association, viz.,
dimerization, may take place in all the self-associating systems being considered.
The formation of higher multimers may overshadow it, however, more or less
completely [7].
Surfactants and Their Classification
The term surfactant is a blend of surface active agent [8]. Surfactants are
usually organic compounds that are amphiphilic, meaning they contain both
hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore,
they are soluble in both organic solvents and water. The term surfactant was
coined by Antara products in 1950. In Index Medicus and the United States
National Library of Medicine, surfactant is reserved for the meaning pulmonary
surfactant. The tail part may consist of one or more hydrocarbon chains, usually
with 6-22 carbon atoms. The chains may be linear or branched, may contain
unsaturated portions or aromatic moieties, and may be partly or completely
halogenated (as in fluorocarbon surfactants) while a charged or uncharged
hydrophilic species (group) acts as the head part of the molecule. The hydrocarbon
chain interacts weakly with the water molecules in an aqueous environment,
whereas the polar or ionic head group interacts strongly with water molecules via
dipole or ion–dipole interactions. It is this strong interaction with the water
molecules that renders the surfactant soluble in water.
Surfactants reduce the surface tension of water by adsorbing at the liquid-
gas interface. They also reduce the interfacial tension between oil and water by
adsorbing at the liquid-liquid interface. Many surfactants can also assemble in the
bulk solution into aggregates. Examples of such aggregates are vesicles and
micelles. Thermodynamics of the surfactant systems are of great importance,
theoretically and practically [9]. This is because surfactant systems represent
systems between ordered and disordered states of matter. Surfactant solutions may
contain an ordered phase (micelles) and a disordered phase (free surfactant
molecules and/or ions in the solution).
Surfactants have been widely used in both industrial and domestic fields
since the first surface-active product was prepared commercially by C. Schollar in
Germany in 1930 [10]. Surfactants have impact on almost all aspects of our daily
life, either directly in household detergents and personal care products or
indirectly in the production and processing of materials which surround us [10-
14]. Surfactants have even been the subject of investigation into the origin of life,
meteorites containing lipid- like compounds and may be an interstellar prebiotic
earth source of cell membrane materials [15]. Moreover, surfactants play a major
role in the oil industry, for example in enhanced and tertiary oil recovery. They are
also occasionally used for environmental protection, e.g., in oil slick dispersants.
Some surfactants are known to be toxic to animals, ecosystems and humans, and
can increase the diffusion of other environmental contaminants [16-18]. Despite
this, they are routinely deposited in numerous ways on land and into water
systems, whether as part of an intended process or as industrial and household
waste. Some surfactants have proposed or voluntary restrictions on their use.
A surfactant can be classified by the presence of formally charged groups in
its head. A non-ionic surfactant has no charge groups in its head. The head of an
ionic surfactant carries a net charge. If the charge is negative, the surfactant is
more specifically called anionic; if the charge is positive, it is called cationic. If a
surfactant contains a head with two oppositely charged groups, it is termed
zwitterionic.
(i) Cationic surfactants
The vast majority of cationic surfactants are based on the nitrogen atom
carrying positive charge. Both amine and quaternary ammonium-based products
are common [19, 20]. A common class of cationics is the alkyl trimethyl
ammonium chloride, where R contains 8–18 C atoms. Primary, secondary or
tertiary amines of cationic surfactants are pH dependent. Primary amines become
positively charged at pH < 10, secondary amines become charged at pH < 4. The
prime use of cationic surfactants is their tendency to adsorb at negatively charged
surfaces, e.g., anticorrosive agents for steel, flotation collectors for mineral ores,
dispersants for inorganic pigments, antistatic agents for plastics, other antistatic
agents and fabric softeners, hair conditioners, anticaking agent for fertilizers and
as bactericides.
Examples: Dodecyltrimethylammonium bromide CH3 (CH2)11N+
(CH3) Br–
Dodecylamine hydrochloride CH3 (CH2)11N+H3 Cl
–
(ii) Anionic Surfactants
The surface-active portion of the molecules bears a negative charge.
Carboxylate (CnH2n+1COO–X
+), sulfate (CnH2n+1OSO3
–X
+), and sulfonate
(CnH2n+1SO3–X
+), are the polar groups found in anionic surfactants. These are the
most widely used class of surfactants in industrial applications [21, 22] due to their
relatively low cost of manufacture. Anionics are used in most detergent
formulations and the best detergency is obtained by alkyl and alkylarye chains in
the C12–C18 range. The counterions most commonly used are sodium, potassium,
ammonium, calcium and various protonated alkyl amines.
Examples: Sodium laurate NaCOO)(CHCH 1023
Sodium dodecylbenzene sulfonate NaSOHC)(CHCH 3461123
(iii) Non-ionic surfactants
The surface-active portion of the molecules bears no apparent ionic charge.
Non-ionic surfactants have either a polyether or a polyhydroxy unit as the polar
group. In the vast majority of non-ionics, the polar group is a polyether consisting
of oxyethylene units, made by polymerization of ethylene oxide. Several classes
can be distinguished: alcohol ethoxylates, alkyl phenol ethoxylates, fatty acid
ethoxylates, monoalkanolamide ethoxylates, sorbitan ester ethoxylates, fatty amine
ethoxylates and ethylene oxide–propylene oxide copolymers (sometimes referred
to as polymeric surfactants).
Examples: Polyoxyethylene monohexadecyl ether OH)CH(OCH)(CHCH21221523
Polyoxyethylene octylphenylether 9.5422214
O)HO(CHC
(iv) Zwitterionic surfactants
Zwitterionic surfactants contain two charged groups of different sign.
Whereas the positive charge is almost invariably ammonium, the source of
negative charge may vary, although corboxylate is by far the most common.
Zwitterionics are often referred to as amphoterics. An amphoteric surfactant is one
that changes from net cationic via zwitterionics to net anionic on going from low
to high pH. Neither the acid nor the basic site is permanently charged, i.e.,
compound is only zwitterionic over a certain pH range.
The change in charge with pH of the truly amphoteric surfactants naturally
affects properties such as foaming, wetting, detergency, etc. These will all depend
strongly on solution pH. At the isoelectric point the physico-chemical behavior
often resembles that of non-ionic surfactants. Zwitterionic as a group are
characterized by having excellent dermatological properties. They also exhibit low
eye irritation and are frequently used in shampoo and other cosmetic products.
Common types of zwitterionic surfactants are N-alkyl derivatives of simple amino
acid, such as glycine (NH2CH2COOH), betaine (CH2)2NCH2COOH) and amino
propionoic acid (NH2CH2CH2COOH).
Examples: 3-(Dimethyldodecylammonio)-propane-1-sulfonate
332231123 SO)(CH)(CHN)(CHCH
N-Dodecyl-N,N-dimethyl betaine COOCH)(CHN)(CHCH 2231123
(iv) Polymeric surfactants
There has been considerable recent interest in polymeric surfactants due to
their wide application as stabilizers for suspensions and emulsions. These are
formed by association of one or several macromolecular structures exhibiting
hydrophilic and lipophilic characters.
Example: Polystyrene-block-poly(vinyl acetate)
–CHm
CH2
–
Br
Br
CC
O
)n
(CH2CH2OCH3
(v) Gemini surfactants
A surfactant usually contains only one polar group. Recently, there has
been considerable research intrest in a new class of surfactants, the gemini
surfactants [23-25] have generated interest in colloid chemistry due to their
superior performance over conventional surfactants in various industrial
applications. Gemini surfactants are also referred to as twin surfactants, dimeric
surfactants [8, 26], or bis surfactants. The term „Gemini surfactant‟, coined by
Menger, has become accepted in the surfactant literature for describing dimeric
surfactants, that is, surfactant molecules containing two hydrophobic groups and
two hydrophilic groups, connected by a linkage (spacer) close to hydrophilic
groups. A schematic representation of a gemini surfactant is shown in Fig.1.1.
Most geminis are composed of two identical halves, but unsymmetrical gemini
surfactants have also been synthesized, either having different hydrophobic tail
lengths, or different types of polar groups (heterogemini surfactants), or both. The
first reports on dimeric surfactants concerned bisquaternary ammonium halide
surfactants which were used to catalyze chemical reactions [27].
Tail Spacer TailHead Head
Fig.1.1: Schematic representation of a gemini surfactant.
Gemini surfactants are subject to much evaluation work at the present time
and one may foresee that the attractive features of these surfactants, such as high
efficiency, low cmc (at least one order of magnitude lower than for the
corresponding conventional (monomeric) surfactants, on a weight percent basis)
and surface tension values (10–100 times more efficient at reducing the surface
tension of water and the interfacial tension at an oil/water interface than
conventional surfactants), and a very steep rise in viscosity with concentration,
will find practical use. The high surfactant efficiency and the low cmc values have
triggered much effort into the potential use of gemini surfactants as solubilizers of
various kinds. In model experiments, using hydrocarbons as the compound to be
solubilized, geminis have been found to be better than conventional surfactants,
both on a molar and weight basis. Gemini is also of interest as lubricating agents
as a result of their tight packing at the surfaces. Some anionic gemini surfactants
have low Krafft temperatures, which make them applicable in cold water. Also,
cationic gemini surfactants possess interesting biological properties.
Much effort is also being devoted to exploit the specific geometry of
gemini surfactants to create structures of well-defined geometry. Gemini
surfactants form vesicles and liquid crystalline phase over broad concentration
ranges, a property that can be taken advantage of for a variety of applications. One
example of such work is the preparation of mesoporous molecular sieves. A
number of patents and papers have appeared in scientific literature [24, 25, 28,
29]. All charge types of gemini surfactants cationic [30, 31], anionic [32], non-
ionic [33] and zwitterionic [34] and a variety of structural types; alkylglucoside
based [35], sugar based [36], with unsaturated linkages [37] and almost all types
with flexible, rigid, and heterotype spacers have been synthesized. The search for
the synthesis of new novel types of gemini surfactants is increasing from their
application point of view [38, 39].
(vi) Bolaform Surfactants
Connecting two surfactant moieties towards the end of their hydrophobic
tail results in a so-called „bolaform surfactant‟ (Fig.1.2) and the physico-chemical
properties of such species are very different from those of conventional and
gemini surfactants. Their self-association ability is less, compared to conventional
ionic surfactants. However, they show biological activity [40, 41] and some
special bolaforms are capable giving rise to organized assemblies of peculiar
structure [42].
Fig.1.2: Schematic representation of a bolaform surfactant.
Micelle formation and critical micelle concentration
Micelle formation, or micellization, is an important phenomenon not only
because a number of important interfacial phenomena, such as detergency and
solubilization, depend on the existence of micelles in solutions, but also because it
affects other interfacial phenomena, such as surface or interfacial tension
reduction, that do not directly involve micelles.
The characteristic property of amphiphilic molecules is their capacity to
aggregate in solutions. The aggregation process depends on the amphiphilic
species and the conditions of the system at which they are dissolved. The
concentration at which this phenomenon occurs is called the critical micelle
concentration (cmc) [43, 44]. Micelle formation is primarily controlled by three
forces: the hydrophobic repulsion between the hydrocarbon chains and aqueous
solution, the charge repulsion of ionic head groups, and the van der Waals
attraction between the hydrocarbon tails [45, 46]. The length of the hydrocarbon
tail, size of the head group, and interaction of the hydrocarbon tails, with one
another and with the aqueous solution determines the size and shape of the
micelle, the cmc, and the aggregation number. Shapes of micelles can vary from
rough spheres to prolate ellipsoids, and the diameter of micelles generally ranges
from 3 to 6 nm [46]. This small diameter prevents micelles from being filtered
from solution and form appreciably scattering light. The term cmc was established
by Davis and Bury [47] defining it as a concentration range below which the
surfactant molecules in the solution remain as monomers and above which
practically all additional surfactants added to the solution form micelles. cmc is an
important property of the surfactants which reflects its micellization ability. Below
the cmc value, the physico-chemical properties of ionic surfactants resemble those
of strong electrolytes and, above the cmc, these properties change dramatically,
indicating a highly cooperative association is taking place. This is illustrated by
Preston‟s classic graph [43] in Fig.1.3. Just above the cmc, micellar structure is
considered to be roughly globular or spherical [4, 48].
Fig.1.3: Preston‟s classic graph showing variation in physical properties of
surfactant solutions below and above the cmc value of sodium dodecyl sulphate.
The determination of the value of cmc can be made by use of many
physical properties, but most commonly the breaks in electrical conductivity,
surface tension, light scattering, or fluorescence spectroscopy–concentration
curves have been used for this purpose. The cmcs also have very frequently been
determined from change in spectral characteristics of some dyestuff added to
surfactant solution when the cmc of the latter is reached. However, this method is
open to the serious objection that the presence of dyestuff may affect the value of
cmc. An excellent critical evaluation of the methods determining cmc is included
in a comprehensive compilation by Mukerjee and Mysels [44].
In a micellar solution, there is always a dynamic equilibrium between
surfactants monomers, monolayers and micelles (Fig.1.4).
Fig.1.4: Surfactant existence in different phases, dependent on surfactant
concentration.
Types of micelles
Normal micelle
Normal micelle structure just above the cmc can be considered as roughly
spherical [49-51]. In aqueous solution the hydrocarbon tails are oriented inward
and head groups are positioned into the bulk solvent [52]. The micelles are always
in dynamic equilibrium. The interior of a micelle is viewed as being much like a
liquid hydrocarbon droplet.
The micellar surface appears to be an amphipathic structure which is
supported by the binding of both hydrophobic organic molecules and hydrophilic
ions with micelles. The amphipathicity is a property shared with the surfaces of
proteins and membranes [53, 54]. Menger has proposed that water can penetrate
inside the micelle upto a certain level [55, 56], the idea get support from
fluorescence and 1H–NMR measurements. Partial molar volume determinations
indicate that the alkyl chains in the core are more expanded than those in the
normal liquid state [57].
An ionic micelle formed in polar solvents such as water generally consists
of three regions (Fig.1.5): (i) The interior or core of the micelle which is
hydrocarbon like as it consists of hydrocarbon chains of the ionic surfactant
molecules. (ii) Surrounding the core is an aqueous layer known as the Stern layer.
The Stern layer constitutes the inner part of the electrical double layer. It contains
the regularly spaced charged head groups and 60-90% of the counterions (the
bound counterions). The head groups are hydrated by a number of water
molecules. One or more methylene groups attached to the head group may be wet.
The core and the Stern layer form the kinetic micelle. (iii) The outer layer is a
diffuse layer and contains the remaining counterions and is called the Gouy-
Chapman layer that extends further into aqueous phase. The thickness of this layer
is determined by the (effective) ionic strength of the solution.
Fig.1.5: Schematic representation of the regions of spherical micelle.
Counterions binds a considerable amount to ionic micelles, which can be
evaluated by electrochemical measurement. They are bound to micelles primarily
not only by strong electrical field created by head groups, but also by specific
interactions that depend upon head group and counterion type [4, 51, 58]. A two-
site model has been successfully applied to the distributions of counterions; i.e.,
they are assumed to be either “bound” to the micellar pseudophase or “free” in the
aqueous phase [59-61]. The head group and counterion concentrations in the
interfacial region of an ionic micelle are on the order of 3-5 M, which gives the
micellar surface some of the properties of concentrated salt solutions [61-63].
Although the solution as a whole is electrically neutral, both the micellar and
aqueous pseudophases carry a net charge because thermal forces distribute a
fraction of the counterions radially into the aqueous phase [60, 61].
For non-ionic micelles the structure is essentially the same, except that the
outer region contains no counterions. It arrests water molecules at the palisade
layer (which includes the head groups and first few methylene groups) by
hydrogen bonding of the water with polyethylene oxide groups [64]. Water may
remain trapped in this region.
Reverse micelle
Aggregation of monomers can occur when amphiphiles are dissolved in
non-polar organic solvents. In reversed micelles the hydrocarbon tails are oriented
outward into the bulk solvent while the hydrophilic head groups are oriented
inward. Reverse micelle is more complex than normal micelles but can be used to
solubilize polar solutes in non-polar solvents. Dipole-dipole [65, 66] interactions
hold the hydrophilic head groups together in the core. The water molecules are
strongly associated with the head groups of surfactant. The aggregation properties
of surfactants in non-polar media are often altered markedly by the presence of
traces of water or additives. The size and properties of reverse micelles vary with
amount of water present [67-70]. Water in reverse micelles is expected to behave
very differently from ordinary water because of extensive binding and orientation
effects induced by polar heads forming the water core [71]. The inner cavity of
reverse micelles has been compared with active site of enzymes [69, 70]. Enzymes
have been encapsulated inside the water pool of reverse micelles without affecting
their activity. In recent years, the field of reverse micelles has witnessed a
significant growth of interest, partly due to the finding that proteins, other
biopolymers, and even bacterial cell can be solubilized in the reverse micellar
system: in fact, this has permitted the extension of area of interest to new domains,
i.e., biocatalysis and chemical biotechnology.
Mixed micelle
The formation of micelles from more than one chemical species gives rise
to what are known as mixed micelles. As usually used, however, the mixed
micelles means a micelle composed of amphiphiles capable themselves of forming
micelles. Thus mixed micellization is a special case of solubilization. The physico-
chemical properties of mixed micelles are quite different from those of pure
micelles of individual components. From the application point of view, mixed
micelles are of great importance in biological, technological, pharmaceutical and
medicinal formulation, enhanced oil recovery process for the purpose of
solubilization, suspension, dispersion, etc. [72]. Clint [73] developed analytical
description which contained both micelle composition and monomer concentration
above the mixed cmc for mixtures of non-ionic surfactants. Clint‟s treatment
assumed ideal mixing in the micelle. The properties of mixtures of ionic and non-
ionic amphiphiles [74-76] have been interpreted with the aid of the mixed micelle
formation. It was pointed out that the cmc of mixed surfactants was lower than
either of the single surfactants [73, 77].
Mixed micelles may also form when low molecular weight solutes are
solubilized by micelles of amphiphiles containing a relatively larger non-polar side
chain. The solubilized substances, also called as the penetrating additives [78],
may be located in both the hydrocarbon core [79] and in the hydrophilic mantle
[80-82].
Aggregation number
One of the most fundamental parameters defining a micelle is the
aggregation number (N), which provides direct information about the general size
and shape of aggregates formed by amphiphiles in the solution, and how these
properties are related to the molecular structure of amphiphile. The average
number of monomers in a micelle in a given population distribution or simply the
numbers of monomers making up a micelles, is known as the aggregation number
and is typically 30-200 in water. It is affected by the different factors such as
nature of amphiphile, temperature [2, 83-85], type and concentration of added
electrolyte [83, 86-88], organic additives [89-92], etc. Generally, in aqueous
medium greater the dissimilarity between amphiphile and solvent, the greater the
aggregation number. Hence, aggregation number appears to increase with increase
in hydrophobic character of the amphiphile. An increase in the temperature
appears to cause a small decrease in the aggregation number in aqueous medium
of ionics. For non-ionic surfactants, it increases markedly [93-95].
Micellar aggregation number decreases continuously with increase in
pressure for non-ionic surfactants [96, 97], although the number for ionic
surfactants passes through a minimum at around 1000 atm. Aggregation number of
ionic micelles is reported to increase [98-101] by the addition of electrolytes.
Various experimental techniques, like dynamic light scattering (DLS),
small angle neutron scattering (SANS), steady-state fluorescence quenching
(SSFQ), time-resolved fluorescence quenching (TRFQ), etc., have been used for
determination of the aggregation number [102-109].
Molecular shape
The extent of interaction between water and amphiphilic molecules can be
expressed by molecular shape and it is mainly determined by a balance between
hydrophobic interactions of the hydrocarbon tails, electrostatic repulsion and
hydration of head groups [14]. The shape of micelle produced in aqueous media
determines various amphiphilic solution properties such as, viscosity,
solubilization, and cloud point. Amphiphiles, which form spherical micelles in
water, have a conical shape in this aggregate type. Cylindrically formed molecules
have a polar region that is equal to non-polar, whereas wedge-shaped molecules
have a large non-polar region thus forming, for example, reversed micelles.
Substances with one hydrocarbon chain often belong to the conical group whereas
substances with two chains or one chain with unsaturations, giving kinks, belong
to cylinders and wedges.
A theory of micellar structure, based upon the geometry of various micellar
shapes and space occupied by the hydrophilic and hydrophobic groups of the
amphiphile molecules, has been developed by Israelachvilli, Mitchell, and Ninham
[110, 111]. The volume VH occupied by the hydrophobic groups in the micellar
core, the length of hydrophobic group in the core lc, and the cross-sectional area a0
occupied by the hydrophilic group at the micelle-solution interface are used to
calculate a packing parameter (Rp), which determines the shape of the micelle
Rp = VH / a0lc (1.1)
The optimal cross-sectional area per amphiphile molecule is observed
experimentally by X-ray diffraction of bilayer systems while the volume and
length of hydrocarbon tail may be calculated by Tanford [112] equations:
VH = (27.4 + 26.9 n) Å3 (1.2)
lc = (1.5 + 1.26 n) Å (1.3)
(n is the number of carbon atoms in the hydrocarbon chain).
As shown in Table 1.1, spherical micelles are formed when Rp is lower than
1/3; wormlike micelles are formed when Rp has a value in between 1/3 to 1/2;
vesicles or bilayers are formed when 1/2 < Rp < 1. When the volume of the
hydrocarbon part is large relative to the head group area (Rp > 1), reverse micelles
are formed.
Table 1.1: Aggregate structures with their corresponding packing parameters.
Effective shape of the
surfactant molecule
Packing parameter
(Rp)
Type of aggregation
cone
<1/3
spherical micelles
truncated cone
1/3–1/2
wormlike micelles
cylinder
1/2–1
bilayers
vesicles
inverted cone
>1
reverse micelles
However, it is to be noted that the solution parameters such as
concentration, pH, temperature and solvent polarity may heavily modify the
specific structures formed.
Factors affecting the value of critical micelle concentration
Since the properties of solutions of amphiphiles change markedly when
micelle formation commences, a great deal of work has been done on elucidating
the various factors that determine the concentration at which micelle formation
becomes significant (i.e., cmc), especially in aqueous media.
Among the factors known to effect the cmc markedly in aqueous solutions
are: (i) structure of amphiphiles, (ii) presence of various additives in the solution,
(iii) experimental conditions such as temperature, pressure, pH, solvent, etc.
(i) Structure of amphiphiles
(a) The hydrophobic group: The large majority of amphiphiles, whether
ionic or non-ionic have hydrophobic regions composed of hydrocarbon chains. For
ionic amphiphiles, increase in the number of carbon atoms in the unbranched
hydrocarbon chains leads to a decrease in the cmc. A generally used rule for
amphiphiles is that the cmc is halved by the addition of one methylene group to a
straight chain hydrophobic group attached to a single terminal hydrophilic group.
The presence of branched chains or double bond hinders micelle formation and
thus increases the cmc. When the number of carbon atoms in a straight-chain
hydrophobic group exceeds 16, however, the cmc no longer decreases so rapidly
with increase in the length of the chain and when the chain exceeds 18 carbon
atoms it may remain substantially unchanged with further increase in the chain
length. This may be due to the coiling of these long chains in water [14]. A phenyl
group that is a part of a hydrophobic group with terminal hydrophilic group is
equivalent to about three and one-half methylene groups. The replacement of
hydrocarbon-based hydrophobic group by a fluorocarbon-based one with the same
number of carbon atoms appears to cause a decrease in cmc. This is explained in
term of the enhanced hydrophobicity.
(b) The hydrophilic group: There is pronounced difference between the
cmcs of ionic and non-ionic surfactants with identical hydrophobic moieties. The
lower cmcs of non-ionic surfactants are a consequence of the lack of electrical
work necessary in forming the micelles.
Effect of nature of polar group of ionic surfactants on the micellar
properties has been reported by Anacker and coworkers and they concluded that
an important factor controlling the micellar size was mean distance of closest
approach of a counterion to the charge center of the surfactant [113]. Thus, for
example, decylammonium bromide forms very much larger micelles than
decyltrimethylammonium bromide because the Br- counterions are able to
approach more closely the charged nitrogen atom of decylammonium thus
effectively shielding the repulsive electrical forces and allowing larger micelle to
form. The more charged groups in the surfactants, the higher the cmc due to
increased electrical work required to form micelles [114, 115]. Zwitterionics
appear to have about the same cmc as ionics with the same number of carbon
atoms in the hydrophobic group. As the hydrophilic group is moved from the
terminal position to a more central position, however, the cmc increases. It is
because the hydrophobic group seems to act as if it had become branched at the
position of the hydrophilic group.
(ii) Presence of various additives in the solution
(a) Effect of electrolytes: Addition of electrolyte causes a reduction in the
thickness of the ionic atmosphere surrounding the polar head groups and a
consequent decreased repulsion between head groups of ionic micelles. These
effects are manifest as a reduction in cmc and an increase in aggregation number,
the effect being more pronounced for anionic and cationic than for zwitterionic
surfactants, and more pronounced for zwitterionics than for non-ionics. The effect
of the concentration of electrolyte on the cmc of ionics is given by the following
relation
log cmc = a log c1 + b (1.4)
where a and b are constants for a particular ionic group and c1 denotes the total
counterion concentration in molar unit [116].
For non-ionics and zwitterionics, Eq. (1.4) does not hold. Instead, the effect
is given by equation [117]
log cmc = – k c1 + constant (c1 < 1) (1.5)
where k is the constant for a particular surfactant, electrolyte and temperature and
c1 is concentration of electrolyte.
The size of counterion is also a determining factor for the cmc value. As the
size of counterion increases, counterion binding also increases due to decrease in
hydrated radius of ion, and hence decrease in cmc occurs [14]. This is the reason
why N)HC( 473 is more efficient in reducing the cmc than N)HC( 452 , which is
more efficient than N)(CH 43 .
There have been attempts to examine the salts effect on micelle formation
in the light of Hofmeister (lyotropic) series [118, 119]. The series plays a notable
role in a wide range of biological and physico-chemical phenomena. The change
in cmc of non-ionics and zwitterionics on the addition of electrolyte has been
attributed [120, 121] mainly to salting-out or salting-in (i.e., the effects of ion size
and decrease in dielectric constant) of the hydrophobic groups in the aqueous
solvent by the electrolyte, rather than to effect of the latter on the hydrophilic
groups of the amphiphile. Electrolytes capable of salting-out reduce the cmc of
non-ionic surfactants while salting-in electrolytes increase the cmc. The effect of
anion and cation in the electrolyte is additive and appear to depend on the radius
of the hydrated ion, that is, the lyotropic number; the smaller the radius of the
hydrated ion, the greater the effect. A very recent study carried out by Moulik and
coworkers [122] shows that, for a given anionic surfactant, the order of
effectiveness in reducing the cmc decreases in the order: 2Mg > Cs > K >
4NH > Na > Li . For a given non-ionic surfactant, the effect of anions on the
cmc follows the order: F > Cl > 2
4SO > Br > 3
4PO > 3
353 O(COO)HC > I >
SCN , and the effect of cations follows the order: K > Na > Rb > Li > 2Ca >
3Al [123].
(b) Effect of organic additives: Organic materials may produce marked
changes in the cmc in aqueous media, even if it is present in small amount. Since
some of these materials may be present as impurities or byproducts of the
amphiphiles, their presence may alter the properties of amphiphiles. A knowledge
of the effects of organic materials on the cmc of amphiphiles is therefore of great
importance for both theoretical and practical purposes. The main factor which
causes a decrease in cmc is likely to be the reduction of the free energy of the
micelle due to diluted surface charge density on micelle.
Organic compounds affect the cmc either by penetrating into the micellar
region, or by modifying solvent-micelle or solvent-monomer interactions. Both
increase and decrease of cmc are observed on addition of non-electrolytes like
urea, amides, amino acids, esters, carbohydrates, alcohols, etc. [124-131].
Methanol and ethanol cause a cmc increase, the higher alcohols butanol and
pentanol cause a decrease in this property. Propanol exhibits an intermediate
effect, low concentrations causing a decrease in cmc, higher concentrations (> 1
M) causing an increase. The cmc increasing effect of the lower alcohols has been
attributed to a weakening of the hydrophobic bonding. The cmc decreasing effects
are thought to be a consequence of the penetration of the alcohols into the palisade
layer of micelle, forming a mixed micelle.
Compounds that affect the cmc by modifying solvent-micelle or solvent-
surfactant interactions do so by modifying the structure of the water, its dielectric
constant, or its solubility parameter (cohesive energy density). Water structure
breakers like urea, formamide, and guinidinium salts are believed to increase the
cmc of surfactants in aqueous solutions (the increase of cmc of ionics by urea is
small), because of their disruption of water structure. Materials that promote water
structure such as xylose or fructose decrease the cmc of surfactants [132].
(iii) Effect of experimental conditions
(a) Temperature: The cmc value at a particular temperature is affected by
two different ways: (i) dehydration of hydrophilic group and (ii) disruption of
structured water around the hydrophobic group. Dehydration of hydrophilic part
favors the micellization while disruption of structured water around the
hydrophobic part disfavors the micellization. The relative magnitude of these two
opposing effects, therefore, determines whether the cmc increases or decreases
over a particular temperature range.
The micellization of amphiphile (ionic or non-ionic) is affected by
temperature due to change in interactions between hydrophobic tails and polar
head groups. cmc vs. temperature studies have been performed to obtain
information on these interactions [133]. For ionic systems, cmc first decreases to a
minimum value with temperature and then increases showing a U-shaped behavior
[134, 135]. However, in some cases continuous increase in cmc is observed with
increasing temperature [136, 137]. For non-ionic systems, cmc decreases
continuously with an increase in temperature due to an increase in the
hydrophobicity caused by destruction of hydrogen bonds between water molecules
and hydrophilic group [94]. From the data available, the minimum in the cmc-
temperature curve appears to be around 25 C for ionics [134] and around 50 C
for non-ionics [138, 139]. Data on the effect of temperature on zwitterionics are
limited. They appear to indicate a steady decrease in the cmc of alkylbetains with
increase in the temperature in the range of 6–60 C [140, 141].
(b) Effect of pressure: The effect of pressure on micelle formation of ionic
[142] and non-ionic surfactants [143] has been studied. The cmc increases upto
pressure of about 1000 atmosphere and decreases with further increase of pressure.
It has been suggested that the surfactant molecules when present in the micelle are
in a more expanded condition than when present as the monomer in solution, so
that the initial effects of pressure tend to compress the micelle and mitigate against
the increased freedom of the monomer in the micelle, thus giving a rise in cmc.
The decrease in cmc on increasing the pressure above 1000 atmospheres may be
due to an increase in the dielectric constant of water, making less electrical work
necessary to bring the monomer into a micelle. For non-ionic amphiphiles, the
cmc values increase monotonously and then level off with increasing pressure.
(c) Effect of pH: When amphiphile molecules contain ionizable groups such
as –NH2, –(CH3)2NO and –COOH, the degree of dissociation of the polar group
will be dependent on pH [144]. In general, the cmc will be high at pH values
where the group is charged (low pH for –NH2 and –(CH3)2NO, high pH for
-COOH) and low when uncharged. Some zwitterionic surfactants become cationic
at low pH, a change that can be accompanied by a rapid rise in the cmc [145], or a
more modest rise [146], depending on the structure and hence hydrophilicity of the
zwitterionic form.
(d) Solvent medium: In ethylene glycol, the cmc of surfactants decrease as
the length of the hydrophobic chain increases, but the change is much smaller than
that in water [147]. For polyoxyethylenated non-ionic solutions in benzene and
carbon tetrachloride, cmc decreases with increase in the length of the
polyoxyethylene group at constant hydrophobic chain length.
The cmc in benzene for alkylammonium carboxylates increases with
increase in the length of the alkyl chain of the anion but decreases with increase in
the length of the alkyl chain of the cation; in carbon tetrachloride, there is no
significant change in the value of the cmc with these structural changes. The cmc
is lower in D2O than H2O for different amphiphiles [148, 149]. The hydrophobic
bonds are expected to be stronger in D2O than H2O [150]. Also, micelles in D2O
are larger than H2O [151].
Thermodynamics of micellization
The process of micellization is one of the most important characteristics of
surfactant solution from theoretical as well as practical purposes and hence it is
essential to understand its mechanism (the driving force for micelle formation).
This requires analysis of the dynamics of the process (i.e., the kinetic aspects) as
well as the equilibrium aspects whereby the laws of thermodynamics may be
applied to obtain the free energy, enthalpy and entropy of micellization.
Two general approaches have been employed to tackle micelle formation.
The first and simplest approach treats micelles as a single phase, and is referred to
as the phase separation model [152-154]. Here, micelle formation is considered as
a phase separation phenomenon and the cmc is then the saturation concentration of
the amphiphile in the monomeric state whereas the micelles constitute the
separated pseudophase. Above the cmc, phase equilibrium exists with a constant
activity of the surfactant in the micellar phase. The Krafft point is viewed as the
temperature at which solid hydrated surfactant, micelles and a solution saturated
with undissociated surfactant molecules are in equilibrium at a given pressure. In
the second approach, micelles and single surfactant molecules or ions are
considered to be in association–dissociation equilibrium. In its simplest form, a
single equilibrium constant is used to treat the process. The cmc is merely a
concentration range above which any added surfactant appears in solution in a
micellar form. Since the solubility of the associated surfactant is much greater than
that of the monomeric surfactant, the solubility of the surfactant as a whole will
not increase markedly with temperature until it reaches the cmc region. Thus, in
the mass action [155-157] approach, the Krafft point represents the temperature at
which the surfactant solubility equals the cmc.
(1) The phase separation model
According to this model, micelles and counterions are treated as separate
phase. However, the micelles do not constitute a “phase” according to the true
definition of this concept since they are not homogeneous and uniform throughout.
Similarly, there are problems associated with the application of the phase rule
[158] while considering micelles as separate phase.
(i) Application of the phase separation model to non-ionic surfactants
To evaluate the thermodynamic parameters for the process of micellization
a primary requisite is to define the standard state. The hypothetical standard state
for the surfactant in the aqueous phase is taken to be the solvated monomer at unit
mole fraction with the properties of the infinitely dilute solution. For the surfactant
in the micellar state, the micellar state itself is considered to be the standard state.
If µs and µm are the chemical potentials per mole of the unassociated surfactant in
the aqueous phase and associated surfactant in the micellar phase, respectively,
then since these two phases are in equilibrium
µs = µm (1.6)
For non-ionized amphiphile,
µs = µ0
s + RT ln as (1.7)
where µ0
s is the chemical potential of standard state.
Since the micellar state is in its standard state
µm = µ0
m (1.8)
At low concentration of free monomers, the activity as is replaced by mole fraction
Xs.
If the 0
mΔG is the standard free energy for the transfer of one mole of
amphiphile from the solution to micellar phase, then
0
mΔG = µ0
m – µ0
s = µm – (µs – RT lnXs) = RT lnXs (1.9)
Assuming that the concentration of free surfactant in the presence of micelle is
constant and equal to the cmc value, Xcmc, then
0
mΔG = RT lnXcmc (1.10)
Xcmc is the cmc expressed as a mole fraction, therefore,
Xcmc = ns /(ns + nH2O) (1.11)
Since the number of moles of free surfactant, ns, is small compared to number of
moles of water, nH2O, therefore, Eq. (1.11) can be written as
Xcmc = ns /nH2O (1.12)
Substituting Eq. (1.12) into Eq. (1.10) and applying logarithm we get
0
mΔG = 2.303RT (logcmc – logw) (1.13)
where w = 55.40 M at 20 C.
Application of Gibbs–Helmholtz equation to Eq. (1.10) gives
∂/∂T( 0
mΔG /T)p = – R(∂lnXcmc /∂T)P = 0
mΔH /T2 (1.14)
Hence the standard enthalpy of micellization per mole of monomer, 0
mΔH , is
0
mΔH = – RT2 (∂lnXcmc /∂T)P = R (∂ lnXcmc /∂(1/T))P (1.15)
Also, standard entropy of micellization per mole of monomer, 0
mSΔ , is given by
0
mSΔ = ( 0
mΔH – 0
mΔG ) /T (1.16)
(ii) Application of phase separation model to ionic surfactants
Consider an anionic surfactant, in which n surfactant anions, S–, and n
counter ions M+ associate to form a micelle, i.e.,
nS– + nM
+ Sn
The micelle is simply a charged aggregate of surfactant ions plus an equivalent
number of counterions in the surrounding atmosphere and is treated as a separate
phase.
In the calculation of 0
mΔG , it is necessary to consider the transfer of (1 – g)
moles of counterions from its standard state to micellar state in addition to transfer
of surfactant molecules from the aqueous phase (g is degree of dissociation).
Therefore, Eq. (1.9) can be written as
0
mΔG = RT lnXs + (1 – g) RT lnXx (1.17)
where Xs and Xx are the mole fractions of surfactant ions and counterions,
respectively.
The analogous equations to Eqs. (1.10) and (1.13) for an ionic surfactant in the
absence of added electrolyte are
0
mΔG = (2 – g) RT lnXcmc (1.18)
0
mΔG = (2 – g) 2.303RT (logcmc – logw) (1.19)
It is assumed that micellar phase is composed of the charged aggregates together
with an equivalent number of counterions, and Eqs. (1.18) and (1.19) are
approximated to
0
mΔG = 2RT lnXcmc (1.20)
and
0
mΔG = 4.606RT (logcmc – logw) (1.21)
The enthalpy of micellization, 0
mΔH , for ionic surfactants is given by
0
mΔH = – 2 RT2
(∂ln Xcmc /∂T)P (1.22)
The phase separation model has been questioned for two main reasons. Firstly,
according to this model a clear discontinuity in the physical property of a
surfactant solution, such as surface tension, turbidity, etc., should be observed at
the cmc. This is not always found experimentally and the cmc is not a sharp break
point. Secondly, if two phases actually exist at the cmc, then equating the chemical
potential of the surfactant molecule in the two phases would imply that the activity
of the surfactant in the aqueous phase would be constant above the cmc. If this
was the case, the surface tension of a surfactant solution should remain constant
above the cmc. However, careful measurements have shown that the surface
tension of a surfactant solution decreases slowly above the cmc, particularly when
using purified surfactants.
(2) The mass-action model
This model assumes dissociation-association equilibrium between
surfactant monomers and micelles–thus equilibrium constant can be calculated.
The mass-action model was originally applied to ionic surfactants but latter it was
applied to non-ionic surfactants also.
(i) Application of the mass-action model to non-ionic surfactants
Micelles, M, are considered to be formed by a single step reaction from n
monomers, D, according to
nD M (1.23)
The equilibrium constant for micelle formation, Km, is then given by
Km = am/ (as)n (1.24)
Corkill et al. [159] have shown that at the cmc the standard free energy of
micellization, 0
mΔG , is given by
0
mΔG = RT [(1–1/ n) lnXcmc + f(n)] (1.25)
where
f(n) = 1/ n [ln n2(2n–1/ n –2) + (n –1) ln n(2n –1)/ 2(n
2 –1)] (1.26)
If n is large, Eq. (1.25) reduces to
0
mΔG = RT lnXcmc (1.27)
Applying the Gibbs–Helmohltz equation and assuming the aggregation number, n,
to be large and independent of temperature
0
mΔH = – RT2 (∂lnXcmc /∂T)P = R (∂lnXcmc /∂(1/T))P (1.28)
The aggregation numbers of many non-ionic surfactants vary with temperature and
in some cases a concentration dependence of n has been reported. In such cases
Eq. (1.28) is not applicable.
(ii) Application of mass-action model to ionic surfactants
The ionic micelles, M+p
, is considered to be formed by the association of n
surfactant ions, A+, and (n – p) firmly bound counterions, X
–
nD+ + (n – p) X
– M
+p (1.29)
The equilibrium constant for micelle formation assuming ideality, is thus,
Km = Xm/ (Xs)n (Xx)
n–p (1.30)
where Xx is the mole fraction of counterion.
The standard free energy of micellization per mole of monomeric surfactant is
given by
0
mΔG = – RT/ n lnKm = – RT/ n lnXm/ (Xs)n (Xx)
n-p (1.31)
When n is large, and when data in the region of the cmc are considered, Eq. (1.31)
becomes
0
mΔG = (2 – p /n) RT lnXcmc (1.32)
Eq. (1.32) is of the same form as Eq. (1.18) from the phase-separation model,
since g = p /n. The two equations differ slightly because of differences in the way
in which the mole fractions are calculated. In the phase-separation model the total
number of moles present at the cmc is equal to the sum of the moles of water and
surfactant whereas the total number of moles in the mass-action model is equal to
the sum of the moles of water, surfactant ions, micelles, and free counterions.
The standard enthalpy of micellization (per mole of monomer) is given by
0
mΔH = – (2 – g) RT2 (∂lnXcmc /∂T)P
= (2 – g) R [(∂lnXcmc/∂(1/T)]P (1.33)
The mass action model is more realistic model than the phase separation
model in describing the variation of monomer concentration with total
concentration above cmc. However, it suffers a serious limitation in that it
considers monodispersity of micelle size inspite of polydispersity. The phase
separation model assumes constant surfactant activity and hence surface tension
above cmc although neither of them remains constant. If aggregation number n is
infinite then mass action and phase separation models are equivalent.
Both the mass action and phase separation models, despite their limitations,
are useful representations of the micellar process and may be used to derive
equations relating the cmc to the various factors that determine it. Neither mass
action nor phase separation models are enough to explain the thermodynamics of
micellization completely. From the practical point of view a comprehensive
approach was developed, known as multiple equilibrium model [160], which
corrects the flaws of mass action model. The thermodynamic parameters are
determined either by calorimetry or by measuring cmc at different temperatures
but the results don‟t agree well. So it is clear from the above discussion that more
reliable data are necessary to overcome the present difficulties in the quantitative
interpretation of micellization and also should be aware of limitations and analyze
findings in terms of appropriate models.
Nevertheless, because of the simplicity of its application, the pseudophase
model is widely used to model thermodynamic data, particularly for long chain
surfactants having low cmc values. The mass action model allows for modeling of
thermodynamic properties over a broader concentration range, i.e., premicellar
range as opposed to the pseudo-phase model, which is applicable only in the post-
micellar range. As well, prediction of aggregation numbers can be made from the
mass action model, and it has been more successfully applied to short chain
surfactants.
(3) Other thermodynamic models
The thermodynamics of small systems developed by Hill [161] has been
applied to non-ionized, non-interacting surfactant systems by Hall and Pethica
[162]. In this approach, aggregation number is treated as thermodynamic variable,
thereby enabling variations in the thermodynamic functions of micelle formation
with the mean aggregation number, <n>, to be examined. The thermodynamic
functions of micellization, assuming solution ideality, are as under
0
mΔG = RT [lnX s– (lnXm /<n>)] (1.34)
0
mΔH = – RT2 [(dlnXs /dT)P – 1/ <n>(dlnXm /dT)P] (1.35)
0
mSΔ = – RT (dlnXs /dT)P + RT/ <n>(dlnXm /dT)P – R lnXs + (R/ <n>)lnXm (1.36)
For systems where <n> is large and changes little with temperature, Eqs.
(1.34) to (1.36) are reduced to the corresponding equations of mass-action or
phase-separation models.
Another approach for that of small systems was formulated by Corkill and
coworkers and applied to systems of non-ionic surfactants [163, 164]. This
multiple equilibrium model considers equilibria between all micellar species
present in solution rather than a single micellar species, as was considered by
mass-action theory. The standard free energy and enthalpy of micellization are
given by equations of similar form to equations (1.32) and (1.33) and are shown to
approximate satisfactorily to the appropriate mass-action equations for systems in
which the mean aggregation number exceeds 20.
An interesting model of micelle formation based on geometrical
considerations of micelle shape has been proposed by Tanford [165]. Equations
are presented which relate the micelle size and cmc to a size-dependent free
energy of micellization. The calculations are based on the assumptions of an
ellipsoidal shape. The hydrophobic component of the free energy is estimated in
terms of the area of contact between the hydrophobic core and the solvent. The
hydrophilic component of 0
mΔG , i.e., the free energy of repulsion between the head
groups, is assumed to be inversely proportional to the surface area per head group.
This approach has been further developed by Ruckenstein and Nagarajan [166]
and used in the prediction of the properties of sodium octanoate micellar solutions
[167].
Drugs and Their Classification
The term drug is derived from French word „Drogue‟ which means a dry
herb. A very broad definition of a drug would include „all chemicals other than
food that affect living processes‟. If the affect helps the body, the drug is a
medicine. However, if a drug causes a harmful effect on the body, the drug is a
poison. The same chemical can be a medicine and a poison depending on
conditions of use and the person using it.
Another definition would be “medicinal agents used for diagnosis,
prevention, treatment of symptoms, and cure of diseases”. Contraceptives would
be outside of this definition unless pregnancy was considered a disease.
It is to be noted that drugs are to be used for the benefit of recipient and it is
presumed that this refers to total benefit–physical, mental as well as economical.
Drugs are regarded as biologically active chemical compounds mostly with
a therapeutic purpose which can be broadly classified according to various criteria
including chemical structure or pharmacological action into:
(A) Biological classification
(B) Chemical classification
(C) Classification of drugs according to commercial consideration
(D) Classification by the lay public
(A) Biological classification
Biological classification is based on pharmacotherapeutic and
chemotherapeutic agent, i.e., this is as disease oriented classification of medicinal
agents used in various diseases and hence this classification is based on medicinal
agents which are overall descriptive but not very accurate on scientific grounds.
The broad classification based on gross overall biological effects is as follows:
(a) Drugs acting on central nervous system and peripheral nervous
systems: The central nervous system (CNS) which consists of brain and spinal
cord, and controls and regularizes the special functions like circulation, digestion
and respiration and it also modifies the psychic reactions such as feeling, attitude,
thoughts, and memory. It directs the functions of all tissues of the body. Chemical
influences are capable of producing a myriad of effects on the activity and
function of the central nervous system. Stimulants are drugs that exert their action
through excitation of the central nervous system. Psychic stimulants include
caffeine, cocaine, and various amphetamines. These drugs are used to enhance
mental alertness and reduce drowsiness and fatigue. However, increasing the
dosage of caffeine above 200 mg (about 2 cups of coffee) does not increase mental
performance but may increase nervousness, irritability, tremors, and headache.
(b) Chemotherapeutic drugs: Chemotherapy, in its most general sense, is
the treatment of disease by chemicals especially by killing micro-organisms or
cancerous cells. Various drugs used in chemotherapy are known as
chemotherapeutic drugs, which have important therapeutic use in the treatment of
parasitic infections due to insects, worms, protozoa, viruses, bacteria, etc. These
drugs destroy offending parasites or organisms without damaging the host tissues.
First modern chemotherapeutic agent was Paul Ehrlich's arsphenamine, an arsenic
compound discovered in 1909 and used to treat syphilis. This was later followed
by sulfonamides discovered by Domagk and penicillin discovered by Alexander
Fleming. Most commonly, chemotherapy acts by killing cells that divide rapidly,
one of the main properties of most cancer cells. This means that it also harms cells
that divide rapidly under normal circumstances: cells in the bone marrow,
digestive tract and hair follicles.
(c) Pharmacodynamic agents: These drugs affect the normal processes of
the body like blood circulation, hemodynamic process, i.e., blood pressure, cardiac
outputs, etc. Phamacodynamic agents include mainly the drugs which affect the
heart and blood circulation. It also includes a wide variety of the drugs used in
allergic and gastrointestinal diseases.
(d) Metabolic diseases and endocrine function: It includes variety of drugs
which are not conveniently classified in the other groups, like:
(i) Antirheumatic and autoimmuno disease drugs: Diseases modifying
antirheumatic drugs are the focus of treatment that addresses constant pain and
inflammations of the lung, heart and eye and autoimmune diseases arise from an
overactive immune response of the body against substances and tissues normally
present in the body. In other words, the body actually attacks its own cells. The
treatment of autoimmune diseases is typically with immunosuppression-
medication which decreases the immune response.
(ii) Dermatologicals: Dermatology is the branch of medicine dealing with
the skin and its diseases, a unique specialty with both medical and surgical
aspects. A dermatologist takes care of diseases, in the widest sense, and some
cosmetic problems of the skin, scalp, hair, and nails.
(iii) Anti-inflammatory drugs: Anti-inflammatory drugs are drugs with
analgesic and antipyretic (fever-reducing) effects and which have, in higher doses,
anti-inflammatory effects (reducing inflammation). The most prominent members
of this group of drugs are aspirin, ibuprofen, and naproxen.
(B) Chemical classification
Chemical classification is based on their chemical structure. According to
this classification drugs may come under one or more of these categories such as
quinines, semicarbazides, phenols, lactones, azo compounds, amides, alcohols,
acetals, ketones, hydrocarbons, halogenated compounds, guanides, enols, esters,
etc.
The implication of this classification is that similar chemical structure
should yield similar biological activities. Thus, structurally close analogous
compounds are grouped together with some overall activity but sometimes this
classification is not very accurate as a few compounds with similar structure lack
the activity test. Chemical structural classification of drugs is of advantage for
study of methodology and structure-activity relationship.
(C) Classification of drugs according to commercial consideration
The manufacturers and distributors of therapeutic agents classify the drugs
according to their operational expenses, research investment and profit margins.
Medicines for relatively rare diseases are called orphan drugs and such drugs lack
patient protection as production costs are high and demand is less.
Another classification of commercial consideration depends on the way of
administration of drug, i.e., drugs are given orally, parenterally (meaning
introduced subcutaneously, intravenously or by any route other than by way of the
digestive tract) by inhalation, sublingually, rectally, etc. On the whole, orally
active drugs are preferred by physicians and patients.
(D) Classification by the lay public
The few broad public classification depends upon the action of the drug, for
example, antiseptic and disinfectant, anthelmintics, expectorants, cough mixtures,
laxative and purgative, analgesics, tonics, ointments for skin diseases, etc., but this
classification is scientifically inadequate and public should be well informed about
all aspects of chemistry, biological action and fate of medicinal agent by
experimental biologists and medicinal chemists.
Many pharmacologically active compounds are amphiphilic or hydrophobic
molecules, which may undergo different kinds of association, and whose site of
action in the organism frequently is the plasma membrane. Even if their target is
intracellular, the interaction with this first barrier plays a fundamental role [168].
Classes of amphiphilic drugs include phenothiazine [169-175] and
benzodiazepine [176] tranquillizers [177-179], analgesics [179], peptide [180] and
nonpeptide [182, 183], antibiotics [183, 184], tricyclic antidepressants [185-188],
antihistamines [189], anticholinergics [190], -blockers [191], local anesthetics
(LA) [192-195], non-steroidal anti-inflammatory drugs [196], anticancer drugs
[197], etc. Many of these drugs contain one ore more (condensed or not) aromatic
nuclei, while others are of peptide nature. A great deal of data on the surface
active properties of the amphiphilic drugs can be found in the book by Attwood
and Florence [198] and other reviews [199-201].
Surface active drugs of quite different chemical structure are reported to
self-associate and bind to membranes, causing disruption and solubilization, in a
surfactant-like manner [168]. Depending on the kind of drug, the self-association
of these drugs is classified into two modes: micellar and nonmicellar aggregations.
Here the micellar aggregation means a single multimer (micelle) forms above the
cmc, and nonmicellar (stepwise) aggregation means that i-mer is successively
formed by aggregation of (i-1)-mer and monomer [198].
Mode of drug action
It is important to distinguish between actions of drugs and their effects.
Actions of drugs are the biochemical physiological mechanisms by which the
chemical produces a response in living organisms. The effect is the observable
consequence of a drug action. For example, the action of penicillin is to interfere
with cell wall synthesis in bacteria and the effect is the death of the bacteria.
One major problem of pharmacology is that no drug produces a single
effect. The primary effect is the desired therapeutic effect. Secondary effects are
all other effects beside the desired effect which may be either beneficial or
harmful. Drugs are chosen to exploit differences between normal metabolic
processes and any abnormalities which may be present. Since the differences may
not be very great, drugs may be nonspecific in action and alter normal functions as
well as the undesirable ones. This leads to undesirable side effects.
The biological effects observed after a drug has been administered are the
result of an interaction between that chemical and some part of the organism.
Mechanisms of drug action can be viewed from different perspectives, namely, the
site of action and the general nature of the drug-cell interaction.
Sites of drug action
(i) Enzyme inhibition
Drugs act within the cell by modifying normal biochemical reactions.
Enzyme inhibition may be reversible or non-reversible; competitive or non-
competitive. Antimetabolites may be used which mimic natural metabolites. Gene
functions may be suppressed.
(ii) Drug-receptor interaction
Drugs act on the cell membrane by physical and/or chemical interactions.
This is usually through specific drug receptor sites known to be located on the
membrane. A receptor is the specific chemical constituent of the cell with which a
drug interacts to produce its pharmacological effects. Some receptor sites have
been identified with specific parts of proteins and nucleic acids. In most cases, the
chemical nature of the receptor site remains obscure.
(iii) Non-specific interactions
Drugs act exclusively by physical means outside of cells. These sites
include external surfaces of skin and gastrointestinal tract. Drugs also act outside
of cell membranes by chemical interactions. Neutralization of stomach acid by
antacids is a good example.
Theories of Mixed Micellization
A mixed micelle consists of surfactant molecules of more than one type.
Interest in mixed micelles has largely been driven by industry, in search of
properties that may improve the performance of surfactant systems. Such a
synergistic effect greatly improves many technological applications in areas such
as emulsion formulations, interfacial tension reduction, cosmetic products,
pharmaceuticals, and petroleum recovery, etc. A synergistic interaction display
between the components of a mixture makes its physico-chemical properties better
than individual formulation. Many theoretical models have been put forward for
dealing with the mixed binary system to evaluate the composition and interaction
parameter among the components at the air/water interface and in the micellar
phase.
The first model given by Lange [77, 202] and used by Clint [73], is based
on phase separation model that relates the mole fraction and the critical micellar
concentration (cmc) of the component in an ideal mixture, which is applicable to
systems of mixed surfactants of similar structure, but hardly applicable to
dissimilar combinations. This model is an idealization which neglects the
interaction among different surfactants in the aggregated state. Rubingh [203] and
Rosen [204, 205] have made an attempt to explain the composition and specific
interaction parameters between two surfactants of nonideal mixture in the bulk and
in the interface, on the basis of regular solution theory (RST). The Rubingh model
treats the mixed micelles as a regular solution. Though these theories are
satisfactory but are questioned on thermodynamic grounds [206-208]. Motomura
[209] considered the mixed micelles as a macroscopic bulk phase and proposed his
thermodynamic model to describe the mixed micellar properties as a function of
excess thermodynamic quantities, defined with reference to the spherical dividing
surface. This model is independent of nature of surfactants and their counterions
and is suitable for prediction of micellar composition. Maeda [210] explained that
a mixed ionic-nonionic surfactant system often has a lower cmc much lower than
the cmc of the pure components. This can be attributed to the decrease in the ionic
head group repulsion caused by the presence of non-ionic surfactant molecules
between the head groups. Maeda suggested that besides regular solution
interaction parameter, there could be another parameter that actually contributes to
the stability of mixed micelles and put forward an equation to calculate the
thermodynamic stability of ionic-nonionic mixed micelle through free energy of
micellization function of micellar mole fraction of ionic components in the mixed
micelles. By the introduction of values of interaction parameter and micellar mole
fraction from different models, thermodynamic stability of mixed micelles can be
evaluated.
Georgiev‟s model is based on Markov‟s chain model [211] for
polymerization process of mixed micelles, and has introduced two molecular
parameters instead of one as in RST. Blankschtein [212, 213] has
thermodynamically formulated models for mixed-surfactant systems (nonideal
mixtures) to evaluate various physico-chemical parameters. This is based on the
cmc, chemical structure of hydrophobic and hydrophilic moieties of individual
components, surfactant concentration, temperature, salt effect, etc. This theory
helps to find out the cmc of binary surfactant mixtures, size and shape of the
micelles and phase behavior of solutions.
Molecular thermodynamic theory has a quantitative basis than RST, and
can be extended to multicomponent systems, expected to work better to know
exact information on a mixed surfactant system [213].
Clouding Phenomenon in Aqueous Solutions
The cloud point (CP) of a fluid is the temperature at which dissolved solids
are no longer completely soluble, precipitating as a second phase giving the fluid a
cloudy appearance [214]. This term is relevant to several applications with
different consequences. Phase separation results from the competition between
entropy, which favors miscibility of micelles in water, and enthalpy, which favors
separation of micelles from water [215, 216]. Depending on the variation of these
two contributions with temperature, either a lower or an upper consolute point can
result [215, 216].
Binary liquid mixtures that display partial miscibility exhibit critical
solution temperatures (CST) or consolute temperatures. CST‟s are of two kinds:
upper critical solution temperature (UCST), above which the liquid pair is
completely miscible and below which phase separation occurs, and lower critical
solution temperature (LCST), below which the two components are completely
miscible while above it the two components become partially miscible and form
two separate phases. The appearance of LCST is a rather rare phenomenon for
solutions of small molecules but is more frequent when at least one of the
molecules is large. The importance of the entropy of mixing, which favors mixing,
becomes relatively less important when the molecules get larger and the mixing
process becomes more dominated by the enthalpy term, which may favor or
disfavor mixing.
If the solution is cold, a temperature below which the amphiphile is not
really very soluble (Fig.1.6) at all is reached, known as the Krafft temperature
[217-220]. If, on the other hand, the temperature is raised, especially for non-ionic
amphiphiles or those with some non-ionic polar groups, a two-phase region is
encountered, above what is known as the CP [221-223] where two liquid
(micellar) phases are in equilibrium. Finally, if we increase concentrations at
ambient temperature, one starts to encounter, usually at amphiphile concentrations
above 40 % by weight, a series of mesomorphic phases sometimes called liquid
crystalline phases.
Cloudy micellaremulsion (2-phase)
Micelles Liquid crytals (mesophases)
Crystals
T
0 100Concentration (%)
Fig.1.6: Schematic temperature (T)–concentration phase diagram illustrating the
types of amphiphilic aggregates encountered by moving away from the micellar
region.
One may find from the literature that there are two classes of surfactants
and the temperature effect on the solution behaviors of each class are in sharp
contrast. The first class is the non-ionic surfactants, the micellar size of which
increases (or at least does not decrease) with increase in temperature. Such
systems always have cloud point properties [19, 224-232]. This behavior has been
attributed to dehydration of the hydrophilic group of the surfactant with increasing
temperature [227]. The absence of long range electrostatic interactions between
aggregates and the decreasing hydration of non-ionic head groups with increasing
temperature result in spontaneous phase separation.
The ionic surfactants belong to the second class in which the cloud point
phenomenon is rarely observed [228]. Until recently it was thought that this type
of behavior was not possible in binary ionic surfactant-water solutions due to large
electrostatic repulsions between the aggregates.
The clouding behavior of amphiphiles is strongly affected by the presence
of various additives in the solution either by changing the structural properties of
the micelles by solubilization in the micellar aggregates or by dissolving in water
phase and thus changing the environment of the micelles [229-234].
The mechanism of CP has not been exactly known and has been discussed
from two view points. One is that the aggregation number of the micelles increases
and intermicellar repulsions decrease with the increase in temperature [14, 235].
As the temperature increases, micellar growth and increased intermicellar
attraction cause the formation of particles, e.g., rod like micelles that are so large
that the solution becomes visibly turbid [236]. Turbidity measurements [224, 237-
239] have been performed in order to determine the micellar size. However, this
interpretation has been criticized [240, 241] because the turbidity must show a big
increase owing to the presence of CP, a phenomenon that has nothing to do with
an increase in the molecular weight of the micelles.
Other methods used to determine the micellar size [239, 242, 243], e.g.
diffusion, viscosity and sedimentation measurements and osmometry, have, in
general, confirmed that the micelles are small at low temperatures but larger at
higher ones. However, these methods are also subject to difficulties in
distinguishing between molecular weight and intermicellar interactions.
Corti et al. [222] explained the behavior as critical fluctuation. The
interpretation is that as the CP is approached the micelles come together, and
above the CP they separate out as second phase. They were able to evaluate the
critical exponents from the light scattering data. The existence of attractive
interactions between poly(oxyethylene) alkyl ethers at elevated temperatures has
been demonstrated convincingly by Claesson et al. [244]. Lindman [245] has
advanced a model based on conformational changes in the poly(oxyethylene)
chain with changing temperature, from which a change in the dipole moment of
the hydrophilic chain arises. This, in turn, leads to a decrease in the polarity or
hydrophilicity of the surfactant and, hence, to phase separation. Kjellender [246,
247] has argued persuasively that attractions between spherical micelles cannot
give rise to the low observed critical concentrations but are a consequence of
attraction between anisotropic micelles.
High concentration of salts can cause ionic surfactant solutions to separate
into immiscible surfactant-rich and surfactant-poor phases [19]. This phenomenon
has been investigated since the 1940s and was first observed for mixtures of the
cationic surfactant Hyamine 1622 with salts such as potassium thiocynate (KSCN)
and potassium chloride (KCl) [248, 249]. The phase separation is typically of the
upper consolute type, i.e., it occurs on cooling below a characteristic temperature,
which, in turn, increases with salt content. Later on, few more studies on ionic
surfactant-salt combinations were performed to show CP phenomenon (lower
consolute type) [250, 251]. Appell and Porte [228] found cloud points at high
concentrations of sodium chlorate (NaClO3) in aqueous solutions of either
cetyltrimetylammonium bromide (CTAB) or cetylpyridinium bromide (CPB). In a
later study Warr et al. [250] showed that quaternary ammonium surfactants with
tributyl head groups exhibit cloud points in binary aqueous solution. Adding salt
to these surfactant solutions lowers the cloud point temperature. They [250]
advanced a mechanism involving hydration shells to account for cloud points,
whereas Appell and Porte [228] interpreted the clouding of CPB/NaClO3 mixtures
as being analogous to the phase separation of polymers in a poor solvent.
Recently, Raghavan et al. [252] has reported the clouding behavior in ionic
amphiphiles in presence of salts with hydrophobic counterions. The plausible
hypothesis given is that the binding of hydrophobic counterions promotes micellar
branching. As free micellar ends are incorporated into a branched network, the
viscosity of the solution drops, and the entropic attraction between junction
eventually causes phase separation. Interest has been focussed on the possibility
that upper or lower critical points could occur within ordered liquid crystal phases.
An upper consolute loop within a lamellar phase has been reported for binary
anionic [253] or cationic [254] surfactants in water. However, lower consolute
loops are much rarer. Some ionic surfactants are reported [250] to undergo phase
separation without the addition of electrolyte, and their micelles remain spherical
or near spherical throughout their region of stability. The parameters, which
govern the phase equilibria and particularly the lower consolute behavior of ionic
surfactant solutions, have been detailed [251].
According to Yu and Xu [255], there may be four factors responsible for
the CP phenomenon in ionic amphiphiles: van der Waals attraction, electrical
repulsion, solvation layer and hydrophobic interactions. They proposed a
mechanism for clouding in tetrabutyltetradecyl sulfate. They postulated that butyl
chains belonging to TBA+ associated with one micelle could cross-link to another
micelle helping overcome the effects of electrostatic repulsion and an energetic
barrier due to oriented water near the surfaces of the two micelles. To be operative
geometrically, it appears that the two micelles would have to approach one another
intimately due to the limited extent of short butyl chains.
Zana et al. [256-259] opposed the mechanism for clouding in TBADS [or
SDS + TBA+] which explains that reducing the water hydrating the micelles is
responsible for CP. The hypothesis is supported by the fact that alkylammonium
counterions are quite bulkyl and ought to expel water from the palisade layer in
the same way that bulky head groups do [260]. Measurements of the water content
of the palisade layer will be needed to confirm the expectation [261]. If indeed
clouding is induced somehow by dehydrating the micelle surface, then the
interpretation of the data will be further complicated by presence of Na+ because
increasing the aggregation number of globular micelle leads to less water per
surfactant [259, 261]. This is a consequence of fact that the surface area of a
micelle grows more slowly than its volume, leading to a smaller volume per
surfactant in which to fit the water [259, 261]. Adding Na+ increases the
aggregation number of SDS. Therefore, there would be two competing
mechanisms to dehydrate: one displacement of water by the counterions and
another by the geometric constrictions due to micelle growth. Therefore, one
would need to know the details of the micelle aggregation numbers in the presence
of both TBA+ and Na
+ to gain a clear understanding.
To the previously suggested mechanisms for clouding in ionic micelles,
they suggested [256] that a second layer of TBA+ is loosely attached outside the
polar shell of the TBADS micelle because steric restrictions did not appear to
allow enough available volume to house a sufficient number of counterions. If this
second layer is in fact available, the cross-linking between micelles could take
place between butyl groups of the TBA+ ions in the second layer This possibility is
supported by the tendency of TBA+ ions to self-associate [262-265].
Recently, Kim and Shah [266-268] have observed the CP phenomenon in
amphiphilic drug amitriptyline hydrochloride solutions and have explored the
effect of additives. Also, Kabir-ud-Din and his group [269-271] have studied CP
phenomenon in some amphiphilic drug solutions and examined the effects of
different additives.
Relevance of the Research Problem
Although the pharmacological activity of the amphiphilic drugs appears at
low concentration where aggregation is negligible [201], the accumulation of drug
can occur at certain site of organism after long period of administration, giving
rise to formation of aggregates that are unable to pass through membranes;
decreasing transport rate and consequently leading to adverse effect on health.
Thus, the study of physico-chemical properties of amphiphilic drugs is important
from physical, chemical, biological and pharmaceutical point of view for their
implication. As most of the drugs that we have so far considered form micelles at
concentrations which they do not attain in vivo, it is most likely that it is their
surface-active characteristics which are most important biologically, although the
propensity of the molecules to form associations by hydrophobic bonding will
manifest itself. The study of the properties of surface active drugs in solution
provides an opportunity to investigate the influence of the structure of the
hydrophobe on the mode of association of amphiphiles.
A most common challenge faced by pharmaceutical scientists as well as
industry is to design and develop drugs with good aqueous solubility while
simultaneously retaining potency and selectivity [272]. The absorption properties
(which cause also bioavailability) of these compounds are influenced by their
physico-chemical and biological properties. A number of methodologies have
been adopted to improve the aqueous solubility (and hence bioavailability) of
drugs. This problem can be overpowered by the use of mixed micelles of drug–
surfactant or surfactant–surfactant. Drug delivery systems which have attracted
much attention for their potential to improve the pharmacological properties of the
drug include nanocapsules [273], cell ghost [274], liposomes [275] and micelles
[272, 276].
Micellar solubilization is the most convenient method to increase the
solubility of drugs. A mixed amphiphile system can exhibit surface and colloidal
properties different from those of the pure individual components. Nonideal
mixing of amphiphilic components often causes synergism in the properties of the
mixtures that may be exploited in their applications. When a mixed amphiphile
system shows lower critical micelle concentration (cmc) values than that of pure
components, the system is said to be synergistic. As a result, mixed micelles are
commonly used in pharmaceutical formulations, in industries, and in enhanced oil
recovery processes [72, 277, 278].
Nowadays, gemini surfactants [26] are gaining wide attention due to their
superior properties. These surfactants contain two hydrophobic tails and two
hydrophilic heads joined with a spacer (of variable nature) at the level of head
groups. These surfactants have cmc values 10–100 times lower than conventional
surfactants.
Keeping the above in view and the fact that surfactant micelles, like many
other amphiphilic substances, are potentially important encapsulating/solubilizing
agents, we have performed conductometric measurements on some amphiphilic
drug-surfactant mixed systems.
The solution properties of amphiphiles are sensitive to the presence of
additives. The values of cmc are found to depend on the type and nature of
additives. To optimize the applications of amphiphile mixtures, it is, therefore,
important to understand the interplay of forces that govern the micellization
behavior in the presence of additives. It was shown by Mouritsen and Jorgensen
[279] and Tieleman et al. [280] that amphiphilic drugs insert into membranes as
interstitial components and they affect the organization of lipids. As the electrolyte
concentration in the membranes may vary, their presence and concentration may
affect the micellization tendency of the drug. Therefore, it is important to have
knowledge of drug‟s association behavior with temperature and also when present
with electrolytes.
In order to understand the effect of additives on the micellar properties of
amphiphilic drugs, we have, therefore, determined the cmc in presence of
inorganic salts and urea/thiourea as well as with the change in temperature by
conductometric method.
The amphiphilic drugs, because of their surfactant-like nature, exhibit
concentration, temperature and pH-dependent phase separation [254-256]. It was
observed that cloud point (CP) can vary with additives. When using these drugs it
should be kept in mind that normal body temperature is typically 12 degrees above
ambient. Even if the CP of pure drug in buffer is above this temperature, it may
decrease in presence of additives, especially surfactants which are used as drug-
carrier. As clouding concentrates the drug in a small volume, it may affect the
activity of drugs and, therefore, it is important to have knowledge of clouding
behavior of the drugs in designing more effective drug-carrier combinations. With
this idea in mind, effects of various additives, viz., electrolytes, non-electrolytes,
alcohols, surfactants, sugars, and amino acids have been examined on CP behavior
of some amphiphilic drugs.
Layout of the Thesis
This thesis consists of five chapters including this one which is concerned
mainly with the general introduction of amphiphiles. Experimental details are
provided in Chapter II.
Chapter III presents a detailed and systematic study of micellar properties
of two antidepressants (AMT and IMP) and a phenothiazine (PMT) drugs in
presence of surfactants (conventional and gemini) in aqueous media. Surfactants
are generally used as drug carriers in pharmaceuticals but presence of surfactants
may alter the micellization tendency of a drug as surfactants form mixed micelles
with the drug: this may affect the activity of the drug. Therefore, it is important to
have knowledge of the effect of surfactants on micelle formation of drugs and the
related energetics.
Chapter IV presents the effect of additives (salts and ureas) on the
association behavior of the above mentioned drugs. Electrolytes and urea are
found in the body and study of their effect on micellization will allow the better
designing of effective therapeutic agents.
Studies on the effect of various additives on the CP values of the above
mentioned drugs are described in Chapter V.
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