41

CHAPTER - 3

Micellization of Ionic Surfactants in Mixed Aqueous Organic Solvent

Media

3.1 Introduction

The role of co-solvents in the process of formation of micelles in surfactant

solution is of considerable interest both from fundamental and applied view

points since the interfacial phenomena and application of surfactants in many

industrial processes largely depend on it.1-5 The formation of micelles is

generally understood in terms of hydrophobic effect, which is the main

driving force behind the formation of micelles in solution.5,6 Besides the

hydrophobicity of the surfactant molecule, hydrophobicity of the solvent

media is also of importance in understanding the process of micellization.1,7,8

Addition of small amount of organic solvent has been known to produce

marked changes in the critical micelle concentration (CMC) of ionic

surfactants due to the tendency of the added organic solvent either to break

or make the water structure through solvation of the hydrophobic tail of the

surfactant by the hydrocarbon (hydrophobic) part of the organic solvent.5,9 A

perusal of the literature reveals that formation of micelles has been observed

in solvents having high degree of hydrogen bonding such as hydrazine,

glycol, formamide, etc.10-16 Contrary to the claim of Evans et al17,18 that the

ability of the solvent to form hydrogen bonds is a prerequisite for

micellization to occur, formation of micelles has been reported in solvents

such as N,N-dimethyl formamide, dimethylsulphoxide, which has little or no

42

hydrogen bonding ability.19,20 In addition to the criterion of the solvent’s

ability to form hydrogen bond, changes in the polarity or hydrophobicity of

the solvent media are also expected to play a critical role in determining the

micellar behavior of ionic surfactants.10 Studies on the micellar behavior of

ionic surfactants in aqueous and aqueous organic-solvent media, therefore,

assume significance in understanding the micellization process. In the

chapter, we report the result of the conductometric as well as surface tension

studies on the micellar behavior of cationic surfactants, namely, Cetyl

Trimethyl Ammonium Bromide (CTAB) and Tetradecyl Trimethyl

Ammonium Bromide (TTAB), and anionic surfactant, Sodium Dodecyl

Sulfate (SDS) in mixed aqueous organic solvent media at 303.15K. The

solvent chosen for the study are ethylene glycol (EG), 2-methoxy ethanol

(ME), 2-ethoxy ethanol (EE), dioxane (DIO), tetrahydrofuran (THF),

monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine

(TEA).

3.2 Experimental

Materials

The cationic surfactants, CTAB and TTAB and the anionic surfactant, SDS

procured from Sigma-Aldrich were used as received. The organic solvents

chosen for the study include EG, ME, EE, MEA, DEA, TEA, THF and DIO.

All the solvents were purified following standard procedures as described

earlier in Chapter 2 of the thesis.21 The solvents were fractionally distilled

43

prior to their use. Double distilled water was used all through the study. All

the solutions were prepared by weight.

Method

Surface tension measurements were performed using a Du-Nouy type

tensiometer, STT-78 (SC Dey & Co., Kolkata, India) equipped with a

platinum ring. The surface tension of water was measured periodically to

check the accuracy of the tensiometer. The measurements were reproducible

to within ± 0.5 dyne cm -1. Conductance measurements were carried out

using a Conductivity Meter (Eutech Con-510, Oakton, USA). The

conductivity cell was calibrated with standard KCl solution. The precision of

the measurements was within ±0.003 mS cm–1. All the measurements were

carried out at 303.15 ±0.1 K. Details of the measurements have been

described in Chapter 2.

3.3 Results and Discussion

The CMC of the cationic surfactants, CTAB and TTAB were evaluated from

the inflection point in the plots of conductance or surface tension against

concentration or logarithm concentration of the surfactant. Typical

representative plots of the conductance or surface tension against

concentration or logarithm concentration of the surfactant in aqueous

solution are shown in Figure 3.1. The observed CMC values of 0.98 and 3.67

mM for the CTAB and TTAB respectively are in closed agreement with the

literature values.5,22 We have measured the conductance and surface tension

44

of the CTAB and TTAB in different mixed aqueous organic solvent media in

order to bring out the influence of the added organic solvent in the

micellization process.

Figure 3.1: Plot of (a) Conductance vs. concentration of surfactant and (b) Surface Tension vs. log [surfactant] in aqueous media at 303.15K

A typical plot of the conductance or surface tension against concentration of

the CTAB in mixed aqueous-ME media is presented in Figure 3.2. Shown in

the Figure 3.3 is the conductance or the surface tension plots of TTAB in

mixed media containing ME. Such plots for the surfactants in other mixed

media under the present study are shown as supplementary data in the end of

the Chapter. The CMC of the surfactant solutions in different mixed media

was determined from the break points in the conductance or the surface

tension plots. It was observed that the break point in the conductance plots

became less well defined with increase in the solvent concentration and

micelle formation of the surfactant could not be detected beyond a certain

level of incorporation of the organic solvents.

a b

45

Figure 3.2: Plots of (a) Conductance Vs. Concentration and (b) Surface Tension Vs. Log[Conc] of CTAB in mixed media containing ME.

Figure 3.3: Plots of (a) Conductance Vs. Concentration and (b) Surface Tension Vs. Log[Conc] of TTAB in mixed media containing ME.

Addition of the organic solvent lowers the surface tension of water and the

maximum lowering was observed in solutions containing EE or THF. The

CMC of the surfactants in mixed solvents containing EE or THF, therefore,

could not be determined precisely from surface tension measurements.

a b

b a

46

The degree of counter ion binding, β was computed from the ratio of slop of

the lines above and below the CMC in the conductance vs. concentration

plots. From the surface tension plots, the maximum surface excess

concentration, Гmax, and from the maximum adsorption, the minimum area

per molecule, Amim, were computed using the following relations:

Γmax = - 0.5 / RT [∂γ/∂lnC] (3.1)

Amin = 1018 /N. Γmax (3.2)

where, R is the Gas constant, N is Avogadro’s number and C is the

concentration of the surfactant in solution.

The estimated values of the CMC of CTAB and TTAB solutions from both

the conductance and tensiometric methods for the systems under study are

recorded in Table 3.1 and Table 3.2 respectively along with their respective

β, Γmax and Amin values. There is, by and large, good agreement between the

conductometrically and tensiometrically measured CMC values.

47

Table 3.1: Values of the CMC, counter ion binding, β, maximum surface excess concentration, Гmax (mol/m2) and minimum area per molecule, Amin(nm2) for CTAB in different mixed aqueous media. %

CMC(mM) β Гmax x106

Amin % CMC (mM) β Гmax x106

Amin a b a b

EG THF* 0 0.98 0.97 0.266 2.53 0.656 5 0.99 - 0.421 - - 5 1.05 1.04 0.286 2.13 0.779 10 1.29 - 0.459 - - 10 1.22 1.17 0.317 1.82 0.912 15 1.48 - 0.663 - - 15 1.39 1.28 0.321 1.79 0.927 20 1.61 1.45 0.331 1.73 0.957 ME MEA 5 1.12 1.04 0.318 1.96 0.85 0.2 0.46# 0.42 - 1.55 1.07 10 1.31 1.21 0.421 1.59 1.04 0.5 0.39# 0.37 - 1.40 1.19 15 1.58 1.42 0.436 1.41 1.18 1.0 0.33# 0.32 - 1.38 1.20 20 1.99 1.50 0.489 1.42 1.17 2.0 - 0.31 - 1.37 1.21 EE* DEA 2 0.83 - 0.356 - - 0.2 0.61 0.49 0.643 1.67 0.99 5 1.03 - 0.610 - - 0.5 0.51 0.46 0.735 1.25 1.33 7 1.15 - - - - 1.0 0.44 0.41 0.844 1.22 1.36 2.0 0.43 0.37 0.901 1.27 1.31 DIO TEA 5 1.19 1.18 0.332 1.91 0.869 0.2 0.78 0.73 0.436 1.97 0.84 10 1.52 1.36 0.442 1.69 0.982 0.5 0.70 0.57 0.445 1.50 1.11 15 2.16 1.94 0.589 1.49 1.114 1.0 0.61 0.50 0.566 1.32 1.26 20 2.92 - 0.632 - - 2.0 0.53 0.42 0.567 0.84 1.98 3.0 0.52 0.40 0.608 0.89 1.87 a - conductance measurement, b - surface tension measurement * - cmc not measurable due to low surface tension of the solvent # - values less precise because of less abrupt changes in the conductance

48

Table 3.2: Values of the CMC, counter ion binding, β, maximum surface excess concentration, Гmax (mol/m2) and minimum area per molecule, Amin (nm2) for TTAB in different mixed aqueous media. %

CMC(mM) β Гmax x106

Amin % CMC(mM) β Гmax x106

Amin a b a b

EG

DIO

0 3.67 3.34 0.272 2.73 0.61 5 4.46 3.79 0.360 1.92 0.86 5 4.24 3.98 0.289 2.44 0.68 10 5.27 4.11 0.448 1.06 1.57 10 4.50 - 0.301 - - 15 6.74 4.36 0.521 1.67 0.99 15 4.97 4.46 0.315 2.49 0.67 20 8.07 5.61 0.752 1.56 1.06 25 6.15 - 0.441 - - 30 6.67 6.20 0.675 2.29 0.72 ME THF* 5 4.25 3.72 0.319 2.13 0.78 5 3.81 - 0.373 - - 10 4.70 4.01 0.331 1.85 0.89 10 4.02 - 0.477 - - 15 5.69 4.17 0.425 1.52 1.09 15 4.87 - 0.510 - - 20 6.26 - 0.528 - - 20 6.39 - 0.555 - - EE* 2 3.04 - 0.352 - - 5 3.14 - 0.513 - - 7 3.53 - 0.588 - - 10 4.34 - 0.734 - - a - conductance measurement, b - surface tension measurement * - cmc not measurable due to low surface tension of the solvent

49

The experimental CMC values of the surfactants, CTAB and TTAB, in

aqueous-organic ether mixed media as a function of the percentage

composition of the organic solvent are presented graphically in Figure 3.4

while the variation of CMC of CTAB in water-alcoholamine mixed media is

shown in Figure 3.5 respectively.

Figure 3.4: Variation of the observed cmc of aqueous solution of the surfactant as a function of the amount of the added organic ether in the mixed solvent

(a) CTAB and (b) TTAB

Figure 3.5: Variation of the observed cmc of aqueous solution of CTAB as a function of the amount of the added alcohol amine in the mixed solvent.

a b

50

For convenience sake, we refer to the conductometric CMC values all

through the discussion hereafter unless specified otherwise. It is observed

from Figure 3.4 that CMC of the surfactant increases with increased

percentage of the added organic ethers except for the solutions containing

EE, which showed an initial decrease.

A decrease in CMC of the surfactant was, however, observed in the solutions

containing an alkyl ethanol amine as is evident from Figure 3.5. The results

clearly indicate that change in the hydrophobicity of the solvent media play

an important role in the micellization process. It is well known that addition

of solvents, which act as water structure breakers decrease the hydrophobic

effect resulting into an increase in the CMC of ionic surfactants.9 The present

result would suggest that EG, the mono methyl substituted glycol, ME and

the cyclic ethers like DIO or THF, apparently act both as co-solvent and

structure breaking solute.

Breaking of the water structure by the organic solvent would facilitate

interactions between the hydrophobic tail of the surfactant molecules and the

hydrophobic (hydrocarbon) part of the organic solvent and consequently, the

local concentration of the organic solvent molecules around the surfactant

monomers becomes larger than the average of the bulk. Solvation of the

surfactant molecules by the hydrophobic part of the organic solvent would,

therefore, lead to delaying the aggregation of the surfactant monomers to

51

form micelles and hence the increase in the CMC of the surfactant.

Introduction of one methyl group in the ethylene glycol was found to result

into further increase in the CMC of the surfactants.

This may be attributed to increased solvation effect due to presence of

methyl group in the EG. However, in case of ethyl substituted glycol, EE, the

CMC of the surfactant showed an initial decrease and then increases through

a minimum with increased amount of the solvent. In view of the fact that EE

has relatively much lower surface tension compared to EG or ME, the initial

decrease in the CMC of the surfactant at lower concentration may be due to

dissolution of the ethyl group at the interstitial sites in water, which causes

the surfactant molecules to micellize at lower concentration.4 With increased

concentration of EE and after all the interstitial sites have been occupied, the

solvation effect would once again dominate resulting into an increase in the

CMC of the surfactants. It is also evident from Figure 3.4 that the increase in

CMC is more prominent in solvent containing DIO as compared to those

containing THF. Increase in CMC in DIO mixed media as compared to those

containing THF is attributed to the larger hydrophobic surface area coupled

with the fluxional nature of the dioxane molecule, which increases the

solubility of the hydrophobic tail of the surfactant monomers delaying the

formation of micelles in the solution.23

52

A comparison between the behaviors of the two cationic surfactants revealed

that the extent of increase in CMC in case of TTAB with increased

percentage composition of the solvent was more prominent than those of

CTAB. This is in accordance with the dependence of CMC on the length of

hydrophobic tail of the surfactants.1

Addition of ethanolamine at very low concentration was found to result into

a decrease in the CMC of the surfactants. At the same level of incorporation,

the CMC values were found to be in the order, TEA > DEA > MEA, the

minimum decrease being observed in solvent containing TEA.

Because of the presence of –CH2CH2OH group attached to the N-atom,

ethanolamines have greater affinity for water or hydrophilicity. Addition of

ethanolamine would, therefore, lead to reinforcement of the water structure

through increased network of intermolecular hydrogen bond interaction.

Strengthening of the water structure would lead to increasing the

hydrophobic effects forcing the surfactant molecules to micellize at lower

concentration and hence the lowering of CMC of the surfactants. The

decrease in CMC of the surfactants in alcoholamine mixed media would also

imply that alcoholamines preferentially interact with the cationic micelles

that may even lead to formation of mixed micelles. But with more alkyl

groups in the alcoholamine, the H-bond interaction of the alkyl ethanolamine

with water gets weakened due to increased hydrocarbon content in the

53

ethanolamine and also steric effect resulting into an increase in the CMC of

the surfactant. The increase in the CMC of the surfactants in TEA with three

–CH2CH2OH groups as compared to those in DEA with two such groups,

which in turn is more than the ethanolamine with only one group, clearly

indicates that the presence of more alkyl groups in the ethanolamine imparts

more hydrophobicity to the solvent media promoting more solvation of the

surfactant monomers besides increasing steric effects. This leads to an

overall decrease in the hydrophobic effect and hence an increase in the CMC

of the cationic surfactant.9,10

Figure 3.6: Variation of β for surfactant solutions with the amount of the organic solvent added (a) CTAB and (b) TTAB

The computed values of β for the CTAB and TTAB in aqueous–organic

ether mixed media have been plotted against percentage composition of the

added organic solvent in Figure 3.6. For all the system under study, the

counter ion binding, β was found to increase with percentage composition of

the added organic solvent as evident from Figure 3.6. For solvent containing

a b

54

Γ

EE, the increase in β is rather steep as compared to the other solvents. The

presence of organic solvents generally reduces the dielectric of the medium

and thus decreases the dissociation of the surfactant monomers and micelles

as well. The steep increase in case of EE supports our earlier observation that

at lower concentration, the ethyl groups of EE remained buried in the

interstitial sites in water.

Figure 3.7: Variation of Γmax for CTAB solutions with the amount of the organic solvent added.

Figure 3.7 shows the plot of the computed values of Γmax for the CTAB

solutions in aqueous–organic ether mixed media against percentage

composition of the added organic solvent. It is evident from the Figure 3.7

that there is decrease in Γmax with increased amount of the organic solvent.

The decrease in Γmax indicates that the hydrophobic tail of the surfactant

interacts with the hydrophobic (hydrocarbon) part of the organic solvent

causing a shift of the surfactant molecules from the interface to the bulk of

the solution.

55

The effect of the added organic solvent in the micellisation process of an

anionic surfactant, SDS has also been studied from conductometric

measurements only. The CMC value of SDS in pure aqueous solution was

found to be 8.13 mM at 303.15K which is in closed agreement with the

literature value.11 Shown in the Figure 3.8 is the representative plot of the

conductance verses concentration of SDS in mixed media containing ME .

The estimated values of CMC of SDS from the conductance measurement in

different mixed media along with the respective β values are recorded in

Table 3.3.

Figure 3.8: Dependence of specific conductance on the concentration of SDS in mixed aqueous ME media

56

Table 3.3: Values of the CMC, counter ion binding, β, for SDS in different mixed aqueous organic media

% CMC(mM) β % CMC(mM) β

EG MEA

0 8.13 0.36 0.2 6.58 0.64 5 8.35 0.39 0.5 5.77 0.81 10 8.68 0.42 0.8 5.41 0.85 15 9.27 0.43 20 10.01 0.45

ME DEA

5 8.08 0.55 0.2 6.23 0.59 10 8.79 0.56 0.5 5.38 0.60 15 9.59 0.61 0.8 5.19 0.61 20 10.95 0.67

EE TEA

5 5.37 0.75 0.2 5.81 0.50 10 7.63 0.69 0.5 5.13 0.51 15 10.03 0.77 0.8 4.93 0.55 20 11.39 0.85

57

The observed CMC values of SDS in mixed media containing organic ether

as a function of percentage composition of the organic solvent is graphically

shown in Figure 3.9 while Figure 3.10 is the corresponding plot of CMC in

mixed media containing alcohol amine.

Figure 3.9: Variation of the observed CMC of aqueous solution of SDS as a function of the amount of the added organic ether in the mixed solvent.

Figure 3.10: Variation of the observed CMC of aqueous solution of SDS as a function of the amount of the added alcohol amine in the mixed solvent.

58

Presence of homologues of ethylene glycol leads to an increase in the CMC

of SDS with increased percentage of the organic ether. However in solvent

containing ME and EE, there is an initial decrease in the CMC at low

percentage of the organic solvent which is more prominent in solvent

containing EE. A similar behavior was observed in case of the cationic

surfactants under study. It may, therefore, be concluded that micellisation of

the ionic surfactants is delayed in presence of ethylene glycol or its

homologues causing an increase in the CMC of the ionic surfactants. Further,

it is also abundantly clear that the increase in CMC is more with increasing

hydrophobic character of the mixed solvent.

It is observed from the Figure 3.10 that the CMC of SDS in solutions

containing alcohol amine was found to decrease with increased percentage of

alcohol amine as was the case with cationic surfactants. However, unlike the

cationic surfactants, the CMC values of SDS were found to be in the order

MEA > DEA > TEA at the same level of incorporation. The decrease in

CMC of SDS in alcoholamine mixed media with more -CH2CH2OH groups

in the alcoholamine would imply that there is significant decrease in the

hydrophobic solvation of the surfactant due to increased stearic effects,

which counteracts the effects due to changes in the hydrophobic character of

the solvent media.

59

Reference

1. L. L. Schramm, E. N. Stasink, D. G. Marangoni, Surfactants and their Applications, Annu. Rep. Prog. Chem., Sect C, 99, 3, 2003

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3. D. Myers, Surfactant Science and Technology, VCH, New York, 1998 4. B. C. Paul, S. S. Islam, K. Ismail, J. Phy. Chem. B, 102, 7807, 1998 5. R. Palepu, H. Gharibi, D. M. Bloor, E. Wyn-Jones, Langmuir, 9, 110,

1993 6. R. Nagarajan, C. C. Wang, Langmuir, 16, 5242, 2000 7. R. Atkin, V. S. J. Craig, E. J. Wanless, S. Biggs, J. Colloid Inter. Sci. 266,

236, 2003 8. F. J. Mojtaba, S. N. Alizadeh, J. Chem. Thermodynamics, 32, 755, 2000 9. K. Gracie, D. Turne, R. Palepu, Can. J. Chem.,74, 1616, 1996 10. A. Callaghan, R. Doyle, E. Alexander, R. Palepu, Langmuir, 9, 3422,

1993 11. M. Ramada, D. F. Evans, R. Lumry, S. Philson, J. Phy. Chem., 89, 15,

1985 12. M. Ramada, D. F. Evans, R. Lumry, J. Phy. Chem., 87, 4538, 1983 13. R. Jha, J. C. Ahluwalia, J. Phy. Chem., 95, 7782, 1991 14. A. Ray, Nature, 231, 313, 1971 15. M. Almgren, S. Swarup, J. E. Lofroth, J. Phy. Chem., 89, 21, 1985 16. R. Gopal, J. R. Singh, J. Phy. Chem.,71, 554, 1973 17. D. F. Evans, A. Yamauchi, R. Roma, E. Z. Casassa, J. Colloid Inter. Sci.,

88, 89, 1982 18. A. Beesley, D. F. Evans, R. G. Langhlin, J. Phy. Chem., 92, 791, 1988 19. H. N. Singh, S. M. Saleem, R. P. Singh, K. S. Birdi, J. Phy. Chem., 84,

219, 1980 20. R. Gopal, J. R. Singh, Kolloid ZZ Polym., 239, 699, 1970 21. J. A. Riddick, W. B. Bunger, Organic Solvents: Physical Properties and

Methods of Purifications, Vol. II, 4th Ed., John Wiley and Sons, Inc., 1986 22. K. Holmberg, B. Jonsson, B. Kronberg, B. Lindman, Surfactants and

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23. K.B. Singh, Thermodynamic Investigation on Ternary Systems of Some Selected Liquids in Tetrahydrofuran and Dioxane Mixed Solvents, Ph. D. Thesis, Submitted to Manipur University, Canchipur, 2005

60

Supplementary Data A. Dependence of Conductance and surface tension on the concentration of CTAB in different mixed media

THF

EG

DIO

DIO

EE

EG

61

DEA

TEA

TEA

MEA

DEA

62

B. Dependence of Conductance and surface tension on the concentration of TTAB in different mixed media

EG

DIO

DIO

EE

THF

63

C. Dependence of Conductance on the concentration of SDS in different mixed media

EG EE

MEA DEA

TEA