Effect of alcohols on counteriono association in aqueous solutions of sodium dodecyl sulfate

Effect of Alcohols on Counterion Association in Aqueous Solutions of Sodium Dodecyl Sulfate INTRODUCTION The study of counterion association is of importance as it throws light on the parameters affecting micellar behavior. A number of methods (1) have been used to determine the extent of counterion association and they often tend to give widely different values. Ionselective membrane electrodes of adequate mechanical and chemical stability which are not poisoned by surfactants are expected to give a direct measure of counterion activity. A number of membrane electrodes (2-6) have been investigated to determine counterion association. In the present communication, we report on the use of a new membrane electrode system of adequate mechanical and chemical stability which is not poisoned by surfactants and alcohols. We investigate systematically the effect of alcohols on counterion association of sodium dodecyl sulfate (SDS).

EXPERIMENTAL

Reagents and materials. The SDS was a K o c h Light product, which was purified by recrystallization with an alcohol-water mixture several times. The purity was checked by the absence of a minimum in

the plot of surface tension vs concentration. Alcohols were of analytic reagent grade and further purified by standard methods. The chromium ferricyanide exchanger was prepared as reported in Ref. (7). Stock solutions of SDS and various alcohols in conductivity water were prepared, mixed, and diluted to obtain desired concentrations. Membrane potential measurements. Srivastava and co-workers (7) have recently observed that the chromium ferricyanide membrane responds to alkali metal ions. Therefore, membrane of this exchanger was used to determine sodium ion concentration in SDS solution. The chromium ferricyanide exchanger was equilibrated with 0.02 M SDS for about 4 days. The equilibrated exchanger was filtered, washed with water, and dried. It was then used to prepare a heterogeneous membrane containing 40% Araldite content by the method described earlier (7). Equilibration of the exchanger was necessary to avoid the poisoning of the membrane electrode by surfactant anions. The membrane was cemented to one end of a Pyrex glass tube with Araldite. Ten milliliters of an internal reference solution of 0.1 M NaC1 was poured into the tube which was then immersed in a 50-cm3 Pyrex beaker containing one of various test solutions. Saturated calomel electrodes were used as reference electrodes and the potential of the cell so set up

External I NH4NOa t Test Membrane 0.1 M NaC11 Internal reference salt solution internal reference electrode bridge solution electrode

was measured on a Toshniwal potentiometer in conjunction with a sensitive suspended coil galvanometer at 25 --+-0.2°C. The response time of the membrane was found to be less than a minute and the potential observed was reproducible with a standard deviation of -+0.5 mV. The potential of the cell was determined for different concentrations of SDS taken as test solution. In order to find the concentration of Na + from the potential data, the membrane was calibrated using different concentrations of NaC1 as a test solution. Further, as the membrane potential is a function of sodium ion activity, it was therefore necessary to

prepare different calibration plots under the same experimental conditions operative while measuring potentials of SDS solutions. These different calibration plots could take care of the solvent effect on sodium ion activity. However, as the effect of anionic micelles on counterion activity could not be corrected for, the so-determined sodium ion concentrations remain apparent ones. Conductance measurements. The conductance of SDS solution in the presence of alcohols was measured on a Toshniwal conductivity bridge type CL01/01A.

536 0021-9797/81/060536-04502.00/0 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of CoUoid and Interface Science, Vol. 81, No. 2, June 1981

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Typical plots for the effect of butanol on free sodium ion concentration are shown in Fig. 2. Similar plots were also obtained for the effect of other alcohols on SDS. Effect o f alcohols on CMC. The CMC calculated from the breaks in potential vs log (concn) plots are given in Table I together with values obtained by the conductance method. The CMC values obtained from the potential data are in excellent agreement with those obtained from conductance data. It is seen from Table I that the CMC decrease in the presence of alcohols apparently due to the incorporation of alcohols. These results are in agreement with those of the earlier workers (8-11). Effect o f alcohols on the micellar dissociation. The degree of dissociation (c0 of SDS micelles has been calculated using the equation of Botre et al. (12):

110

10C

9C

8C

~ 7c

g- 6c s(

4(

3c

CNa+ = CMC + ~(Ct - CMC), I

-&O

I

-2.5 log SDS Concn

I

-2£)

-1.5

FIG. 1. Plot of potential vs tog concentration (M). (0) SDS; (©) NaC1.

RESULTS AND DISCUSSION In Fig. 1, the variation of the cell potential vs SDS/NaC1 concentration in aqueous solution is given. Similar plots were also obtained in the presence of various alcohols. It is seen from Fig. 1 that both the plots of SDS and the plots of NaCl coincide up to a certain concentration, indicating the same concentration of free sodium ions in solution of SDS and NaC1. Above this concentration, the SDS undergoes micellization causing immobilization of a certain amount of sodium ions on the micellar surface and thus reducing the concentration of free sodium ions. Evidently the break in the plot is the CMC of the surfactant. The free sodium ion concentration above the CMC has been calculated with the help of calibration plots and plotted against total concentration of SDS.

where CNa+ refers to free sodium ion concentration and Ct to total SDS concentration. We have omitted the activity coefficient term from the original equation of Botre et al., as calibration plots under identical conditions have been drawn. The calculation of ~ is subject to the approximation that monomer concentration remains constant above the CMC. The degree of dissociation of SDS micelles so calculated is given in Tables II and III. It is seen from Table II that the micellar dissociation of SDS in water increases with increasing concentration. Varying values of c~for SDS have been obtained by different methods and our results are in agreement with those reported (5, 11, 12). Further it is seen that hexanol is most effective in increasing c~, and propanol least. This behavior can be explained on the basis of alcohol penetration in the micelles. The organic part of the alcohol molecule is expected to be anchored in the micelle while the - O H group is sandwiched between the negative end groups. Such an orientation would result in the decrease in micellar charge density and consequently liberation of counterions. Due to the larger hydrophobic content, hexanol would have maximum tendency to pass into micellar phase and, therefore, would be

TABLE I Effect of Alcohols on the CMC (mM) of SDS (25 --- 0.2°C) Propanol

Potential measurements Conductance

Butanol

Pentanol

Hexanol

Water

0.1M

0.5M

1.0M

0.1M

0.2M

0.3M

0.02M

0.05M

0.1M

0.01M

0.02M

0.03M

8.0 8.2

6.4 5.9

5.8 5.6

4.7 4.7

6.0 6.1

5.0 5.0

2.8 3.0

5.1 5.6

3.9 4.0

3.0 3.1

4.3 4.3

2.9 3.0

1.1 1.4

Journal of Colloid and Interface Science. Vol. 81, No. 2, June 1981

1°3'~18[

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-

SOS Conch (M)

FIG. 2. Plots o f free s o d i u m ion v s total SDS concentration in ([3) water; ( × ) 0.1 M butanol; (A) 0.2 M butanol; (©) 0.3 M butanol. T A B L E II Effect of n-Propanol and n-Butanol on the Degree of Dissociation (c0 o f SDS Micelles at Various Concentrations (25 _+ 0.2°C) Otprooanol S D S co n cn (raM)

asps ~. a,o

4 6 8 10 12 14 16 18 20

. --0.25 0.27 0.30 0.31 0.32 0.32

.

0.1 M

0.5 M

. -0.40 0.37 0.36 0.36 0.35 0.35 0.35

. 0.52 0.47 0.44 0.41 0.39 0.37 0.36 0.36

~butanol 1.0 M

.

0.1 M

0.2 M

0.3 M

-0.63 0.58 0.54 0.52 0.50 0.48 0.47

0.78 0.70 0.66 0.63 0.60 0.57 0.55 0.54

0.87 0.82 0.78 0.72 0.69 0.67 0.66 0.65 0.64

. 0.61 0.55 0.51 0.47 0.43 0.39 0.36 0.37

T A B L E III Effect of n-Pentanol and n - H e x a n o l on the Degree of Dissociation (a) of SDS Micelles at Various Concentrations (25 -+ 0.2°C) O~oenta~l S D S eo n cn (mM)

2 4 6 8 10 12 14 16 18 20

0.02 M

. -0.53 0.48 0.45 0.44 0.43 0.43 0.43 0.42

O~hex~ol

0.05 M

.

0.1 M

. 0.65 0.60 0.56 0.53 0.51 0.50 0.50 0.49 0.49

. 0.77 0.70 0.64 0.60 0.57 0.55 0.54 0.54 0.53

538 Journal of Colloid and Interface Science, Vol. 81, No. 2, J u n e 1981

0.01 M

0.02 M

0.03 M

-0.60 0.53 0.48 0.44 0.44 0.43 0.43 0.43

0.70 0.64 0.60 0.57 0.55 0.55 0.55 0.54 0.54

0.80 0.74 0.68 0.64 0.62 0.60 0.58 0.57 0.57 0.56

.

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of the plot after the break becomes more steep, thereby showing increase in release of counterions. Thus our results show that alcohols are incorporated in the micelles and the extent of incorporation is a function of concentration, as well as the nature of the alcohol.

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REFERENCES

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2

4

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6 8 SDS Conch (M)

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10

12

14

16x10-4

FIG. 3. Plots of specific conductance vs concentration of SDS in (×) water; (O) 0.1 M butanol; (V1) 0.2 M butanol; (A) 0.3 M butanol.

more effective in increasing micellar dissociation. For a given alcohol, the value of a increases with increasing alcohol concentration due to greater partitioning of alcohol into the micellar phase. Similar results h a v e also been obtained by Miyagishi (13) for cationic surfactants. Further it is seen from Tables II and III that the value of c~ decreases with increasing SDS concentration at a constant alcohol concentration. With increasing SDS concentration, redistribution of alcohols in the micelle occurs. As a result, the micelles become progressively poorer in alcohol content and, thereby, micellar dissociation is decreased. The release of counterions on the addition of alcohol has also been confirmed by specific conductance data. In Fig. 3, plots showing the effect of butanol on SDS conductance are given. Similar plots were also obtained for other systems. Figure 3 shows that as the amount of butanol is increased, the slope

1. Fisher, L. R., and Oakenfull, D. G., Quart. Rev. Chem. Soc. London, 16 (1977). 2. Botre, C., Hall, D. G., and Scowen, R. W.,Kolloid Z. Z. Polym. 250, 900 (1972). 3. Saski, T., Hattori, M., Sasaki, J., and Nakma, K., Bull. Chem. Soc. Japan 48, 1397 (1975). 4. Vikingstad, E., J. Colloid Interface Sci. 72, 68 (1979). 5. Lawrence, A. S. C., and Pearson, J. T., Trans. Faraday Soc. 63, 495 (1967). 6. Brun, T. S., Hoiland, H., and Vikingstad, E., J. Colloid Interface Sci. 63, 590 (1978). 7, Srivastava, S. K., Jain, A. K., Agrawal, S., and Singh, R. P., J. Chem. Tech. Biotechnol. 29, 379 (1979). 8. Shirahama, K., and Kashiwabara, T., J. Colloid Interface Sci. 36, 65 (1971). 9. Emerson, M. F., and Holtzer, A., J. Phys. Chem. 71, 3320 (1967). 10. Shirahama, K., Hayashi, M., and Matuura, R., Bull. Chem. Soc. Japan 42, 1206 (1969). 11. Larsen, J. W., and Tepley, L. B., J. Colloid Interface Sci. 49, 113 (1974). 12. Botre, C., Crescenz, V. L., and Mele, A., J. Phys. Chem. 63, 650 (1959). 13. Miyagishi, S., Bull. Chem. Soc. Japan 47, 2972 (1976). AJAY K. JAIN R. P. B. SINGH

Department of Chemistry University of Roorkee Roorkee-247672, India Received August 15, 1980; accepted October 10, 1980

Journal of Colloid and Interface Science, Vol. 81, No. 2, June 1981