ionic Surfactants by Conductometry -...

Abstract

A conductivity method for CMC determination of non

copolymer using C

alkyl chain length or anion on IL are optimized in such a way that

should be negligible effect on CMC values of non ionic surfactants

composed of different hydrophobic part and common polyethylene oxide

block. The obtained CMC values are compared with the CMC values

evaluated using classical conventional method such as s

Ionic Liquid as a Probe for the

Determination of C

ionic

Chapter 6

A conductivity method for CMC determination of non ionic surfactant/block

copolymer using C4PyCl is proposed in this Chapter. The concentration and

alkyl chain length or anion on IL are optimized in such a way that




evaluated using classical conventional method such as surface tension.


termination of CMC of Non

ionic Surfactants by

Conductometry

ionic surfactant/block

in this Chapter. The concentration and

alkyl chain length or anion on IL are optimized in such a way that there




urface tension.


of Non-

Surfactants by

Chapter 6

197

6.1 Introduction

Surfactants are amphiphilic molecules that contain a non polar segment, commonly called

“the tail” and polar segment called “the head” also called surface active agents. Because

of this amphiphilic character, they undergo aggregation. At low concentrations the

molecules exist as on individual entities, and at high concentration the molecules tends to

associate to from aggregates. Due to their unique properties and versatile availability,

surfactants have applications in many areas, including chemistry, biology, pharmacy,

material science etc.1,2

In polar solvent like water, this dual character of the amphiphilic leads to self-association

or micellization, micelles where the hydrophobic tails of the surfactant associate, leaving

the head group (hydrophilic) exposed to the solvent (Figure 6.1).

Figure 6.1 Schematic representations of block copolymer monomer and micelle

The hydrophobic tails of the aggregates from the ‘cone’ of the micelle, while the polar

head groups are located at the micelle-water interface in contact with and hydrated by a

number of water molecules. The transition form a monomeric solution to an aggregated

form can be seen as a change in the slope of plots against surfactants concentration of

many physical properties such as viscosity, conductance, surface tension etc.3-8 The

concentration above which micelles form (the change in physical property take place) is

called the critical micelle concentration (CMC). Once the micelles are formed, further

increase of the surfactant concentration does not significantly change the concentration of

the free monomer, above CMC, monomers and micelles exist in dynamic equilibrium.

Depending upon the chemical structure of the surfactant, its micelle can be cationic,

anionic, zwitterionic or . There are various methods for CMC determination

Chapter 6

198

based on the change in physical properties at micelle formation.3-8 The methods listed in

Table includes classical as well as some newly developed methods based on specific

instruments along with the properties or parameters obtained during these measurements.

Table 6.1 shows a list of methods used for CMC determination for all kind of

surfactants/amphiphilic block copolymer. These techniques are mainly classified into two

classes; (i) direct method where change in physical properties as a function of

concentration are measured (surface tension, density, conductivity, ultrasonic etc.), and

(ii) indirect method where probes or tracers are used and changes in their properties are

measured for determination of CMC (dye solubilization, fluorescence etc.).

Table 6.1 Various methods for the determination of CMC along with equipments and information deduced from each methods

Sr. No.

Methods Instrument Required

Physical Parameters

Information deduced

Applicable for surfactant

Ref.

1. Surface tension Tensiometer Surface tensing

CMC, γcmc, πcmc, G°mic, τmax, a

sm, pC20

Non-ionic, Ionic

9

2 Light Scattering SLS/DLS Light intensity

CMC, effective charge,

aggregation number

Non-ionic, Ionic

10

3 Conductivity Conductometer Conductivity CMC Ionic 11,12

4 Capillary electrophoresis

P/ACE 5000 HPCE electrophoresis

electrical current

CMC Ionic 13,14

5 Chromatography (LE-MEKC)

CE Instrumental setup, CE-971 UV-

Detector

Elution time CMC Ionic (Anionic)

15

6 Refractometry Ray light-Haber-Lowe type

Interferometer

Refractive indices

CMC Ionic, Non-ionic

16,42

7 Micro calorimetry

Micro calorimetry Heat capacity

CMC, ∆H, ∆S, ∆G

Ionic,

Non-ionic

17,18,

19,37

8 Density Densimeter Density CMC Ionic,

Non-ionic

20,21, 22

Chapter 6

199

Sr. No.

Methods Instrument Required

Physical Parameters

Information deduced

Applicable for surfactant

Ref.

9 Sound Velocity (Speed of sound)

Pulsecho-overlap technique

Speed of sound

CMC Ionic,

Non-ionic

23

10 Viscosity Viscometer Viscosity CMC Ionic, Non-ionic

24

11 Solubilization UV-visible spectrophotometer

Absorbance CMC, λmax Ionic, Non-ionic

25

12 NMR (Magnetic resonance method)

NMR δppm ∆G°m, ∆H°m, ∆S°m, CMC

Ionic, Non-ionic

26,42

13 Potentiometry Potentiometer with electrode

Emf CMC Ionic (Cationic)

27,36

14 Cyclic Volammetry

Cyclic Volameter (Three

compartment electrolytic cell,

SCE, Pt)

Potential, Current

CMC Non-ionic, Ionic

28,35

15 Polarography Sargent model XV Polarograph

Emf, id (diffusion

current)

CMC Ionic, Non-ionic

29,30

16 Fluorescence Shimadzy flurometer, Spectro

flurometer, Luminescence spectrometer

fluorescence intensity

(I/I3)

CMC Ionic, Non-ionic,

gemini surfactant

31,32,

42

17 Spectral method UV-spectrometer Absorbance CMC Non-ionic, Ionic

33,34

18 Equilibrium dialysis

Spectrophotometer Optical density

CMC Ionic 38

19 Kinetics Baunch & Lomb monochrometer

Time CMC 39

20 Impedance spectroscopy

technique

Schlumberger

(SI-1286)

Z', Z" Conductivity

CMC Ionic, Non-ionic,

Mixed (Ionic & Non-ionic)

40

21 SANS Neutron Scattering instrument

(SANS)

Scattering Intensity

Micellar shape and size,

Aggregation number, volume

fraction, poly dispersity

Ionic, Non-ionic

41

Chapter 6

200

In direct methods, surface tension (ST) and ionic conductivity are the most popular

methods. For CMC determination of surfactant solutions, the surface tension is the most

widely used technique since the surface tensiometer is easy to operate and the evaluation

of CMC from surface tension vs concentration curve is clear. Beside this, ST is suitable

for all kind of surfactant solution like ionic, nonionic, zwitterionic, gemini and mixed

surfactants. Along with ST, ionic conductivity is also extensively employed method for

determining the CMC, but it is strictly limited for ionic surfactant solution only. A clear

slope change which corresponds to CMC is observed in the specific conductivity (κ) vs

concentration curve. Conductance measurement has been employed for the evaluation of

CMC of gemini surfactants,43 and mixed surfactant systems,44 in aqueous and non

aqueous media.45 It is well known that conductance method is not appropriate for the

CMC determination of nonionic surfactants, but Svitova and Pletnev46 used this method

for the nonionic surfactant solutions and observed that a change in slope of straight line in

the specific conductance vs concentration diagram was not clear. The inflection point

becomes clear when an electrolyte is added to the solution of nonionic surfactants which

normally affects the CMC value of the same. Except the conductance method all the

major techniques used for CMC determination requires sophisticated professional

equipments.

On the other side, in indirect methods, dye solubilization and fluorescence

spectrophotometry are the most widely used methods. In case of dye solubilization

method, dyes, used as probe are added to the surfactant solutions and the change in λmax

of probe is measured as a function of surfactant concentration. But it is well documented

that, the presence of dye in the spectral change method causes a little lowering in CMC of

surfactants which may be explained in terms of induced micelle formation.47 Similarly, in

case of fluorescence spectrophotometry method, fluorescence probes (e.g. rhodamine B,

pyrene etc) have been used for CMC determination, which causes similar problem like in

dye solubilization method. Therefore, it is a necessity to develop an environment friendly

method in which use of a probe or tracer should have very negligible effect on the

micellization of nonionic surfactants.

Ionic liquids (ILs) which are considered as salts with a melting point below 100 °C, have

attracted considerable interest during the past decade due to their unique properties, such

as low vapor pressure, wide liquid range, good conductivity and large electrochemical

Chapter 6

201

window.48 Due to their unique features, they have become very much attractive in fields

such as catalysis, in basic electrochemical studies of organic compounds and inorganic

compounds, formation of metal nanostructures, analytical chemistry, including sensor,

bioanalytical chemistry and for electrochemical biosensors.49 This variety of applications

shows that ILs offers a high solvating, yet non-coordinating medium in which a number

of organic and inorganic solutes may be dissolved which make it an appropriate media to

work with. In the current understanding, a typical IL is composed of a bulky organic

cation and an inorganic or organic anion. The ability to tune the physicochemical

properties of ILs by changing the structure of the ions has led to their being called

“designer” solvents.50 The properties of both cation and anion are useful tools for fine-

tuning the properties of the resulting IL for desired properties. The choice of anion along

with alkyl chain length on cation decides the nature of ILs in terms of hydrophilic and

hydrophobic character which in turn predicts the physical properties of ILs which are

very important for various studies.

In view of this, we present a simple and easy approach for determining CMC values of

nonionic surfactants using IL as probe in conductivity method. The method is validated

for surfactants with different molecular weights having variety of hydrophobic part and

also different hydrophobic to hydrophilic ratio. The CMC values obtained by this method

were also compared with that obtained by other methods. SANS, DLS and 1H NMR are

used to understand the interaction between IL and surfactant as well as its effect on

micellar size and shape.

6.2 Experimental

The detailed specifications of all nonionic surfactants/block copolymers are given in

Table 6.2. The CMC measurements were examined at 30 oC ± 0.1 oC by testing the

nonionic surfactants, Triton X-100 (>99% purity, Fluka), Brij-98 (>99% purity, Sigma-

Aldrich), nonionic silicon surfactants (Degussa, Germany), Amphiphilic block

copolymers Pluronics® F127, P123, P105 (Sigma-Aldrich) and (EO-BO-EO) B – 1 and

B – 2 (Dow Corning Chemicals, USA). The ionic liquid C4PyCl (99.99+, Merck) was

used as a probe for the determination of CMC of all above mentioned surfactants. All the

chemicals were used as received. All surfactant solutions were prepared in de-ionized

water having conductivity ~ 5 × 10-6 S.cm-1 to which constant volume of C4PyCl was

Chapter 6

202

added in all solutions in such a way that the net concentration of C4PyCl becomes

15 mM. The conductivity of the aqueous surfactant solutions in presence of IL were

measured using digital conductometer (Equiptronics, India). The conductivity cell (cell

constant, 0.1 cm-1) was immersed in surfactant solutions which were kept in constant

temperature (~ 30 oC) water bath. The measurements were repeated three times for each

concentration of surfactants. The surface tensions of all solutions were also measured by

surface tension on tensiometer (Data Physics, Germany) at the same temperature. The

specific and relative viscosities for surfactants/polymers with and without IL were

measured using Ubbelhold Viscometer in constant temperature bath at 30 oC.

Table 6.2 Molecular formula, molecular weight and % of EO for various surfactants/block copolymers

Sr. No.

Surfactant/

Block copolymer

Formula Mol. wt.

gm.mol-1

% EO

1 P123 (EO)20(PO)70(EO)20 5750 30

2 P105 (EO)99(PO)69(EO)99 6500 50

3 F127 (EO)37(PO)56(EO)37 12600 70

4 B – 1 C4H9O-(BO)n-(EO)m-OH

m = 16, n = 9

1500 50

5 B – 2 HO-(EO)m-(BO)n-(EO)m-OH

m = 43, n = 14

5000 80

6 Triton X - 100 C8H17−C6H4−O(CH2CH2O)9.5H 625 80

7 Brij - 98 C18H35−(OCH2CH2)nOH, n~20 1149 80

8 SS – 1

For SS – 1 m = 5, n = 13, x = 12, y = 0

For SS – 2 m = 5, n = 20, x = 10, y = 4

4360 100

9 SS - 2 5600 75

EO = Ethylene Oxide, PO = Propylene Oxide, BO = Butylene Oxide

Chapter 6

203

SANS measurements were carried out on the surfactant solutions prepared in D2O at the

SANS facility at DHRUVA reactor, Trombay. The mean incident wavelength was 5.2 Å

with ∆λ/λ = 15%. The scattering was measured in the scattering vector (q) range of

0.017–0.3 Å-1. The measured SANS data were corrected for the background, empty cell

contributions, and transmission and were placed on an absolute scale using standard

protocols. The detailed data analysis were made using core-shell hard sphere model.51

DLS measurements were carried out in the H2O medium using a Malvern 4800 Autosizer

with 7132 digital correlator. The light source was an argon ion laser operated at 514.5 nm

with a maximum output power of 2 W. The average decay rate was obtained by analyzing

the electric field autocorrelation function, g1(τ) vs time data using a modified cumulants

method as has recently been proposed.52a The apparent equivalent hydrodynamic radii of

the micelles were calculated using the Stokes-Einstein relationship. All 1H NMR

experiments were conducted on a Bruker Avance 400 spectrometer at a frequency of

400.13 MHz in D2O and TMS as internal standard.

6.3 Result and Discussion

6.3.1 Concentration Optimization for Ionic Liquid

In the determination of CMC by conductometry method, selection of IL is crucial and we

have selected pyridinium based IL with alkyl chain having four carbon atoms. In recent

study carried out by us,52b where the effect of alkyl chain length on cationic head group in

CnPyCl (n = 4, 6, 8), effect of various cationic head groups and anions of various ILs on

the micellization of P123 in aqueous solutions was studied. Among all the variable

parameters, alkyl chain having four carbon atoms and pyridinium head groups have

almost negligible effect on the micelles of P123. In addition to this, C4PyCl IL was

unable to form stable aggregates while IL with C6 and C8 alkyl chain form stable

aggregates at room temperature.53 The concentration of IL is selected in such a way that it

falls very well below to the CMC value of ILs and it should have negligible effect on

morphology of micelles and less interaction with hydrophobic/hydrophilic blocks i.e. the

concentration of IL can only charge the micelles so that it has an impact of conductivity

in pre-micellar and micellar phase. In order to optimize the concentration of IL (C4PyCl),

we started with P123 (EO20-PO70-EO20) system which is well known for formation of

well organized spherical micelles.54

Chapter 6

204

The SANS and DLS methods were used to measure the effect of IL on morphology of

P123 micelles in aqueous solutions while NMR study gives an idea about the location as

well as interaction of IL with surfactants. Figure 6.2 represent the SANS curves for 15%

(w/w) of P123 solutions (well above from CMC value i.e. 0.015% (w/w)) in presence of

different concentrations of IL at 30 oC.

0.01 0.110-1

100

101

102

P123 P123 + 100 mM C4PyCl

dΣ/Σ/ Σ/Σ/

dΩ

, Ω

, Ω

, Ω

, cm

-1

q, Å-10.3

Figure 6.2 SANS spectra of 15% (w/w) P123 in D2O and 100 mM C4PyCl

Figure 6.2 reveals that upon addition of 100 mM IL, there was no appreciable change in

SANS profile which can be easily observed by no change in peak position as well as

intensity. The SANS curves were fitted to hard sphere core-shell model52 and the

parameters obtained from the fits are depicted in Table 6.3.

Table 6.3 Values of Core radius, RC, Shell Thickness, RS, Hard Sphere Radius, RHS,

Polydispersity, σ, Volume fraction, φ, Hydrodynamic Radius, Rh, for 15% (w/v) aqueous solutions of P123 in presence and absence of C4PyCl

System

Bkg

(cm-1)

RC

(Å)

RS

(Å)

RHS

(Å)

σσσσ

Volume fraction,

φφφφ

Rh

(Å)a

P123 + D2O 0.14 36.3 ± 0.2 6.8 ± 0.1 79.7 ± 0.3 0.49 0.152 94.5 ± 0.4

P123 + 50 mM C4PyCl

0.15 36.2 ± 0.2 6.7 ± 0.1 79.5 ± 0.3 0.49 0.153 94.1 ± 0.4

P123 + 100 mM C4PyCl

0.18 36.8 ± 0.2 6.7 ± 0.1 79.2 ± 0.3 0.48 0.157 93.1 ± 0.3

a Obtained from DLS measurements

Chapter 6

205

Table indicates that up to 100 mM concentration of IL, there is no change in size and

shape of P123 micelles (only little increase in background was found which is expected

due to the incoherent scattering from IL), which indicates that the IL has no prominent

interaction with PEO/PPO parts of P123. The same was also confirmed with the help of

DLS measurements. The plots of the intensity correlation function for 15% (w/w) P123 in

absence and presence of IL (50 mM) at a scattering angle of 90o is shown in Figure 6.3.

Figure reveals that the addition of 50mM IL was unable to make any shift in the

correlation function vs time plot which manifests pristine micellar morphology of P123.

Analysis of the same is carried out using the regularized non-negatively constrained

method, CONTIN55 which shows a unimodal distribution of relaxation rate. The

hydrodynamic radius (Rh) parameters obtained by this method are depicted in column 8 of

Table 6.3 and it clearly shows negligible effect of 50 mM IL concentration on the P123

solution. From the above measurements and reasonable discussion, we decided to take 15

mM of IL as an appropriate concentration for CMC determination using conductometry

method.

10 100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

g1 , ττ ττ

Time, µµµµs

P123 P123 + 25 mM C4PyCl

600

Figure 6.3 Representative plot of the intensity correlation function for 50 mM C4PyCl with 15% (w/w) P123 at a scattering angle of 90°. The solid line is a fit to the data using the method of CONTIN

In order to know the interaction and position of ILs in micelles, the concentration

dependent 1H NMR spectra for P123 block copolymer in presence of 15 mM C4PyCl are

measured. (Figure 6.4). The concentration of block copolymers were selected in such a

way that they were below and above CMC values. From the 1H NMR spectra, the δppm

Chapter 6

values at 4.80, 3.61, 3.49, 1.24 corresponds to HDO, EO

respectively which are in good agreement with previous assignments

signals are attributed to IL.

Figure 6.4 1H NMR spectra of Pluronic P123 in presence of 15 mM CCMC value

The presence of different multiplets at lower concentration (below CMC) of P123 is

because of efficient motion of polymer

values at 4.80, 3.61, 3.49, 1.24 corresponds to HDO, EO –CH2–, PO

respectively which are in good agreement with previous assignments

signals are attributed to IL.53b

NMR spectra of Pluronic P123 in presence of 15 mM C


because of efficient motion of polymeric chain which indicates that even in presence of

Below CMC

Above CMC

206

, PO –CH2–, PO –CH3

respectively which are in good agreement with previous assignments56 and the remaining

NMR spectra of Pluronic P123 in presence of 15 mM C4PyCl below and above


ic chain which indicates that even in presence of

CMC

Above CMC

Chapter 6

207

IL, all segments of the solvated polymer can move freely. This has marked effect on the

conductivity of IL. When the concentration is increased above a certain value (CMC), the

chemical shifts of PO –CH3 and PO –CH2– were slightly shifted towards downfield while,

PO –CH– was slightly shifted towards upfield, which indicates that PPO domain of P123

micelle is not completely interacting with the ILs.56 In order to find out the position of IL

in micelles we have monitored the change in chemical shifts of terminal methyl (–CH3)

group of butyl chain as well as aromatic ring hydrogen of head group of IL (Figure 6.5).

Figure 6.5 Aggregation of Pluronic in presence of 15mM C4PyCl by 1H NMR Spectroscopy. δppm is the observed chemical shift (a) for aromatic proton (b) for the proton of the terminal methyl unit

δ (ppm)

Pure 15mM C4PyCl

Above CMC

(a) (b)

Chapter 6

208

Figure reveals, increase in the chemical shifts of both types of hydrogen. The observed

downfield shift of the terminal –CH3 of butyl chain protons is a manifestation of increase

in hydration i.e. it might not enter the apolar PPO core but reside at the interface of

PPO/PEO where it is in the influence of hydration. Moreover, the broadening of the peaks

after the CMC values indicates reduction in the mobility of –CH3 of butyl chain57 which

ultimately affects the conductivity of IL upon micellization.

6.3.2 CMC determination by Conductometry

As discussed in experimental section, all surfactant solutions were prepared in aqueous

solution of 15 mM C4PyCl and have specific conductivity 948 µScm-1 at 30 oC. The

conductivity of the C4PyCl solutions changes upon addition of nonionic surfactants. The

conductivity curves of the C4PyCl in the presence of the increasing amount of nonionic

surfactants were studied and found to be changing markedly like surface tension. The

specific conductivity vs concentration plots for the aqueous solution of surfactants

(having different hydrophobic groups) in presence of 15 mM C4PyCl are shown in

Figure 6.6. For the comparison purpose we have put surface tension data of the same

systems in the Figure 6.6. A perusal of figure reveals that a marked change in

conductivity of C4PyCl was noticed until the stable formation of micelles (i.e. complete

aggregation of surfactant molecules). The conductivity remains almost constant upon

further addition of surfactant above the CMC values. The concentration at which

conductivity attained the constant values is equal to CMC. The exact CMC values are

obtained by plotting the conductivity of the C4PyCl-surfactant mixtures vs concentration

of the surfactants. The obtained CMC values for different surfactants from conductivity

method along with surface tension and other methods are depicted in Table 6.4.

It can be clearly seen that the CMC values obtained by variation in conductivity of

C4PyCl is in good agreement with the literature data as well as obtained by surface

tension method. Unlike other additive methods where dyes or organic molecules were

used as probe, this method does not make any effect on the micellization processes. In

order to check versatility of this method, we have selected block copolymers/surfactants

in such a way that they have different hydrophobic moieties and along with this some

industrially important surfactants like Triton X-100, Brij-98 are also used. The change in

conductivity of the C4PyCl may be due to the restriction in the motion of IL.

Chapter 6

209

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007

905

910

915

920

925

Conductivity Surfacetension

Conc. of Surfactant, g/dl

Sp

.Co

nd

uct

ance

, κ, κ,

κ,

κ, µµ µµ

S/c

m

P123

CMC

(a)

35

40

45

50

Su

rface tensio

n, γγ γγ, m

Nm

-1

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07910

915

920

925

930

935

940

945

Conductivity Surface tension


Sp

.Co

nd

uct

ance

, κ, κ,

κ,

κ, µµ µµ

S/c

m

B - 2

CMC

(b)

30

35

40

45

50

55

Su

rface tensio

n, γγ γγ, m

Nm

-1

0.00 0.01 0.02 0.03 0.04 0.05 0.06

910

915

920

925

930

Conductivity Surfacetension


Sp

.Co

nd

uct

ance

, κ,κ, κ,κ, µµ µµ

S/c

m

CMC

30

35

40

45

50

55

Su

rface tensio

n, γγ γγ, m

Nm

-1TX-100 (c)

0.00 0.01 0.02 0.03 0.04 0.05

915

920

925

930

935

940



Sp

.Co

nd

uct

ance

, κ, κ,

κ,

κ, µµ µµ

S/c

m

CMC

(d)

45

50

55

60

65

70

Su

rface tensio

n, γ,γ, γ,γ, m

Nm

-1

Brij-98

0.00 0.01 0.02 0.03 0.04 0.05 0.06910

915

920

925

930

935

940



Sp

.co

nd

uct

ance

, κ, κ,

κ,

κ, µµ µµ

S/c

m

CMC

(e)

30

35

40

45

50

Su

rface tensio

n, γγ γγ, m

Nm

-1

SS - 1

Figure 6.6 Specific conductance vs. concentration of surfactants at 30 oC, (a) P123, (b) B – 2, (c) Triton X-100, (d) Brij – 98 and (e) SS – 1 (conductivity of 15 mM C4PyCl is 948 µS/cm2 in water)

As discussed in previous section, through 1H NMR study we found that the IL molecules

are located at hydrated PEO/PPO interface, which restrict the motion of IL molecules

which in turn decreases the conductivity of the IL.

Chapter 6

210

Table 6.4 Critical Micelle Concentration of surfactants and block copolymers obtained from Conductivity method and surface tension method at 30oC

Surfactants Critical Micelle Concentration (g.dl-1)

Surface tension Conductivity Lit. Ref.

P123 5.7±0.3 × 10-3 5.9±0.3 × 10-3 5.0 × 10-3 [a]

1.9 × 10-3[b]

60

61

P105 3.7±0.2 × 10-2 3.5±0.2 × 10-2 2.5 × 10-2 [a] 60

F127 4.0±0.2 × 10-2 3.9±0.2 × 10-2 3.9 × 10-2 [b] 61

B - 1 1.9±0.1 × 10-3 1.5 ±0.1× 10-3 2.2 × 10-3 [b] 62

B - 2 2.5±0.1 × 10-2 2.3±0.1 × 10-2 2.9 × 10-2 [b] 62

Triton X-100 2.6±0.1 × 10-2 2.5±0.1 × 10-2 1.4 × 10-2 [c]

1.8 × 10-2 [d]

63

64

Brij-98 2.5±0.1 × 10-2 2.4±0.1 × 10-2 3.0 × 10-2 [b] 65

SS - 1 4.2±0.2 × 10-2 3.8±0.2 × 10-2 4.2 × 10-2 [b] 66

SS - 2 1.1±0.1 × 10-2 1.1±0.1 × 10-2 2.5 × 10-2 [b]

@25 oC

67

a Dye solubilization, bSurface tension, c Kinetic approach, dCyclic voltammetry

It is very well documented that, Pluronic block copolymers in surface tension curves

shows two distinct break points58 and generally second break point is considered as the

CMC value. Alexandridis et. al.59 reasoned that Pluronic EPE copolymer molecules at

very low copolymer bulk concentration adsorbs at the air/water interface in an extended

conformation. And when the bulk concentration increases, the interface is covered fully

with the copolymer molecules, thus changing the extended conformation to an inverted

‘∩’ with two ends blocks contact with water and the middle hydrophobic block

protruding into air. This change in conformation would manifest as first break point in

surface tension-concentration plots. Other indirect methods like dye solubilization, cyclic

voltammetry, fluorescence spectroscopy etc. were insensitive to measure double break

points. Figure 6.6(a) and Figure 6.7(a, b) shows the formation of double break point in

Pluronic block copolymers.60,61 Figures reveal that the first break point observed by

conductivity method is also in good agreement with the one obtained by ST, which

indicates that the method is sensitive towards this kind of critical phenomenon. The

double break point in conductivity measurements can be explain as; the quaternary

ammonium ion of C4PyCl is known for making coordination bonds with etheric oxygen

of PEO and PPO.

Chapter 6

211

As mentioned above, at very low concentration of polymer, hydrophobic part will reside

at air/water interface thus it is less available in to bulk solution, which results into rare

chances of interaction of quaternary ions with lone pair of oxygen from PPO i.e. it is

paving the free mobility of ionic species of IL which leads to less change in conductivity.

At high concentration of polymer, due to the saturation of PPO molecules at air/water

interface, they are more available in bulk in addition to PEO and ions of IL can interact

with PPO which ultimately reduces the mobility and resulting into suppression of

conductivity. Therefore, we can say that our method is simple as well as sensitive towards

such kind of changes along with the determination of CMC values of nonionic surfactants

where sophisticated instrumentation is not required.

0.00 0.01 0.02 0.03 0.04 0.05 0.06

965

970

975

980

985

990



Sp

.Co

nd

uct

ance

, κ,

κ,

κ,

κ, µµ µµ

S/c

m

P105

CMC

(a)

35

40

45

50

55

60

65

70

Su

rface tensio

n, γγ γγ, m

Nm

-1

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07910

915

920

925

930

935



Sp

.Co

nd

uct

ance

, κ,

κ,

κ,

κ, µµ µµ

S/c

m

F127

CMC

(b)

40

45

50

55

60

Su

rface tensio

n, γγ γγ, m

Nm

-1

0.00 0.01 0.02 0.03 0.04

915

920

925

930



Sp

.Co

nd

uct

ance

, κ,κ, κ,κ, µµ µµ

S/c

m

CMC

(c)

20

30

40

50

60

70

Su

rface tensio

n, γγ γγ, m

Nm

-1

SS-2

0.000 0.001 0.002 0.003 0.004 0.005

910

915

920

925

930

935



Sp

.Co

nd

uct

ance

, κ κ κ κ, µµ µµ

S/c

m

B-1 (d)

35

40

45

50

55

Su

rface tensio

n, γγ γγ, m

Nm

-1

CMC

Figure 6.7 CMC measurements from Conductivity and Surface tension Curve of (a) P105, (b) F127, (c) SS-2, (d) B-1 (conductivity of 15 mM C4PyCl is 948 µS/cm2 in water)

It is worth to mentioned that the viscosity effect on conductivity of ILs has not been

consider in the change of conductivity as a function of concentration of block

Chapter 6

copolymers/surfactants because the variation in viscosity of polymer with concentration is

not prominent.(Figure 6.8)

0 10 20 30

0.005

0.010

0.015

0.020

0.025

ηη ηη sp

/c

Conc. of C

Figure 6.8 Reduced viscosities of (a) 5% (w/w) P123 in various concentration of C(b) various concentration of P123 in water at 30

Figure 6.9 1H NMR spectra of pure P123 in D


(Figure 6.8)

40 50 60 70 80 90 100 110

Conc. of C4PyCl, mM

5% w/w P123 + C4PyCl

0 1 20.0

0.5

1.0

1.5

2.0

2.5

3.0

ηη ηηsp

/c

Conc. of P123, gm/dl

Reduced viscosities of (a) 5% (w/w) P123 in various concentration of C(b) various concentration of P123 in water at 30 oC

H NMR spectra of pure P123 in D2O at above CMC

212


3 4 5 6

Conc. of P123, gm/dl

P123 in water

Reduced viscosities of (a) 5% (w/w) P123 in various concentration of C4PyCl,

Chapter 6

213

Figure 6.10 1H NMR spectra of 150 mM pure IL (C4PyCl) in D2O

6.4 Conclusions

A simple and novel method has been devised for the determination of Critical Micelle

Concentration values for nonionic surfactants using conductometry where IL, C4PyCl was

used as a probe. Selection of IL and optimization of its concentration was carried out

using SANS and DLS measurements. 1H NMR measurements gave an insight into the

location as well as interaction of IL inside the micelles of surfactants. From these

experiments we have found that IL does not affect the morphology of micelles by making

strong interaction with hydrophobic or hydrophilic part of the surfactant. This method

was checked for various nonionic surfactants having different hydrophobic parts and

hydrophobic/hydrophilic ratio. The CMC values obtained by this method are in good

agreement with the one obtained by other versatile methods and its sensitivity was proved

by observing double break points in the Pluronic block copolymer system. Thus, we can

say that the conductivity method can be employed for the determination of CMC of

nonionic surfactants using IL as conducting probe.

Chapter 6

214

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