A Conductometric Study of Interaction between Sodium Dodecyl Sulfate and 1-Propanol, 1-Butanol, 1-Pentanol and 1-Hexanol at Different Temperatures

Size: px

Start display at page:

Download "A Conductometric Study of Interaction between Sodium Dodecyl Sulfate and 1-Propanol, 1-Butanol, 1-Pentanol and 1-Hexanol at Different Temperatures"

Ashlie Wilkinson
5 years ago
Views:

1 1 J. Surface Sci. Technol., Vol 24, No. 3-4, pp , Indian Society for Surface Science and Technology, India. A Conductometric Study of Interaction between Sodium Dodecyl Sulfate and 1-Propanol, 1-Butanol, 1-Pentanol and 1-Hexanol at Different Temperatures NEELIMA DUBEY Department of Chemistry, Kurukshetra University, Kurukshetra , India drneelimadubey@yahoo.com Abstract The critical micelle concentrations (cmc) of sodium dodecyl sulfate (SDS) in dilute aqueous solutions of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol at 298, 303, 308 and 313 K have been determined from conductance measurements in the range of ~0.002 to 0.90 alcohol mol%. A relatively weak effect of temperature was observed in increasing the cmc in the narrow composition range studied. Thermodynamic parameters of micellization, enthalpy ( Hº m ), entropy ( Sº m ) and free energy ( Gº m ) were calculated from temperature dependence of cmc. The dependence of these thermodynamic parameters on the concentration of alcohols is determined in terms of the effect of additives on micellization of SDS. In all the cases the negative value of Hº m decreases with increase in concentration, whereas the value of Gº m < 0 and remained practically constant over the entire alcohol concentration range studied. Hº m and Sº m values show increasing trend with increase in concentration of alcohols. Due to different structural consequences of intermolecular interactions caused by different chain-length of alcohols, both enthalpy and entropy are different in a mutually compensating manner so that Gº m is not significantly affected. The results obtained are analyzed in terms of the effect of chain length of alcohols on micellization of SDS molecules. Keywords : Critical micelle concentration, SDS micelles, alcohols, conductivity. INTRODUCTION The critical micelle concentration (cmc) is a narrow concentration range where the physical properties of the solution of an amphiphile show an abrupt change due to cooperative formation of micelles in the bulk solution [1]. The cmc in this narrow

2 140 Dubey concentration range depends on the physical property observed and the precision of the measurement. It is essential to employ physical methodologies which are highly sensitive to structural changes for determining the cmc. The existence of cmc indicates aggregation of amphiphilic molecules in solutions. The knowledge of the cmc is important for the calculation of the thermodynamic parameters, which confirms the scientific interest of a precise determination of the cmc [2]. The cmc in aqueous solution is influenced by the degree of binding of counter ion to the micelles. For aqueous systems, the increased binding of the counter ion to the surfactant causes a decrease in the cmc and an increase in the aggregation number [3]. The effect of alcohol on the self association processes of ionic surfactants and hence on the properties of the micelles formed has been investigated for several systems using a variety of techniques [4-10]. The behavior of SDS micelles in the presence of n-alcohols has been extensively investigated [11-18]. However limited work has been done on micellization of ionic surfactants in dilute aqueous solutions of alcohols of chain lengths, n = 3 6 due to poor solubility [2, 4-6]. Since wateralcohol-surfactant systems are frequently used as media in the studies of chemical equilibria and reaction rates, it is essential to investigate the effect of the nature of the alkyl groups in the alcohol on the cmc of the surfactants. Addition of alcohols to aqueous solutions of surfactants has allowed the investigation of the effect of hydrophobic interactions on the micellar structure [19]. Moreover, the measured changes in the micellar size or structure upon addition of medium chain length alcohols are not entirely consistent, suggesting that to a certain extent the conclusions depend upon the measuring technique used. The short chain alcohols are solubilized mainly in the aqueous phase and affect the micellization process by modifying the solvent properties. Medium chain alcohols typically decrease the cmc, as well as the size or aggregation number of ionic micelles. When the alcohol partitions between water and the micellar pseudophase, the fraction bound to the micelles replaces water molecules at the interface leading to increased electrostatic repulsion between surfactant head groups [7, 20]. According to Rubio et al [17] moderately hydrophobic alcohols in low concentration promote micellization probably by residing at the micellar surface and reducing unfavorable water hydrocarbon contacts. However at higher concentrations these alcohols destabilize micelles by displacing water from the surface, therefore decreasing its effective dielectric constant, increasing head group repulsions, and disrupting surfactant packing. In the present work, some preliminary investigations of the effect of n-alkanols on the micellization of an anionic surfactant, SDS in their dilute aqueous solutions at 298, 303, 308 and 313 K are reported in terms of critical micelle concentration (cmc) obtained by conductance measurements.

3 A Conductometric Study of Interaction between SDS and n-alkanols 141 EXPERIMENTAL Materials : The analytical grade 1-propanol, 1-butanol and 1-pentanol were obtained from Spectrochem Pvt. Ltd. (Mumbai). The claimed purity for the chemicals was more than 99 mass %. 1-Hexanol was an Acros (USA) product with stated purity of 98%. Alcohols were distilled before use. SDS was obtained from HiMedia Laboratories Pvt. Ltd. (Mumbai) and its minimum assay was 99%. It was used as received. Doubly distilled water of specific conductance, (1-2) x10-6 ohm -1 cm -1 at 298 K was used for all measurements. All solutions were prepared using water that was deionised, distilled and degassed. In all studies the SDS concentration was well beyond the cmc of SDS to minimize the effect of possible trace impurities, e.g., C 12 OH. Methods : Solutions were prepared by weight molal (~ mol %) SDS solution was prepared by weighing appropriate amount of the SDS in an electronic balance, (Afcoset-ER120A) with a precision of g. Bidistilled water and alcohols were separately degassed by vacuum pump shortly before sample preparation. Conductivities were measured using digital conductivity meter (306) of Systronics which is a microcontroller based instrument for measuring specific conductivity of solutions. The accuracy in conductance measurements is ± 1%. The conductivities were determined at 298, 303, 308 and 313 K. Measurements were carried out in a jacket containing conductivity cell of cell constant 1.0 cm 1. Water was circulated in the jacket from thermostat and the temperature was maintained within ± 0.01ºC. The cmc of SDS in an aqueous additive solution was taken as the SDS concentration at the break point in the plot of specific conductance vs. SDS concentration. The cmc of SDS in water was determined to be 8.2 mmol dm -3 at 298 K and was found to be in good agreement with the value reported in literature [21], whereas at 303, 308 and 313 K it was 8.45, 8.7 and 8.96 mmol dm 3 respectively and shows agreement with literature [22,23] when cmc values are taken in molar units. RESULTS AND DISCUSSION The experimentally determined cmc values for SDS in very dillute aqueous solutions of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol at 298, 303, 308 and 313 K are expressed in mole fraction (Table 1). From the temperature dependence of the cmc, the enthalpy of micellization ( Hº m ) is obtained through the van t Hoff relation :

4 142 Dubey TABLE 1. Critical micelle concentration (cmc) of sodium dodecyl sulfate (SDS) in very dilute aqueous solutions of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol at different temperatures and the corresponding thermodynamic parameters at 298 K. Additive cmc (molfraction) 10 4 Gº m Hº m Sº m (mol %) 298 K 303 K 308 K 313 K (kj mol 1 ) (kj mol 1 ) (J mol 1 K 1 ) 1-Propanol Butanol Pentanol Hexanol

5 A Conductometric Study of Interaction between SDS and n-alkanols 143 Hº m = RT 2 d[ln cmc] / dt (1) Free energy of micellization ( Gº m ) is obtained using relation : Gº m = RT ln cmc (2) Entropy of micellization ( Sº m ) was calculated using the equation: Gº m = Hº m T Sº m (3) where R is the gas constant and T is the absolute temperature. Thermodynamic results for micellization of SDS in very dilute aqueous solutions of n-alkanols are summarized in Table 1. It is evident that in case of 1-propanol and 1-butanol the decrease in cmc occurs steeply over a narrow composition range, mol%. In case of 1-pentanol and 1-hexanol also, the decrease in cmc is observed up to 0.27 mol% of alcohol concentrations. As solubility of 1-pentanol and 1-hexanol are small in water, (1-pentanol 2.19%w/w at 298 K and 1-hexanol 0.706% w/w at 293 K) [24], the solution shows turbidity on increasing concentration of these alcohols further. The cmc values obtained for system SDS + 1-propanol in molar concentrations in aqueous medium at 298 K have been compared to the work reported by Romani et al. [7] (at 0.4 M 1-propanol, cmc = 6.3 mm and at 0.6 M, cmc = 6.0 mm) to that reported in the present manuscript (where cmc = 6.1 mm at 0.5 M). For the system SDS + 1-butanol at 298 K the cmc reported by Rubio et al. [17] (cmc = 4.2 mm at M) has been compared to that reported here (cmc = 4.3 mm at 0.50 M). A good agreement is seen. However in case of systems containing 1-pentanol and 1-hexanol no literature could be found at the studied concentration range for comparison. A close look at Table 1 reveals that there is small difference in cmc due to the presence of the studied n-alkanols. The cmc lowering of the surfactants by the small addition of alcohols may be due to their direct action on water structure and the subsequent addition may cause secondary effects such as their solubilization in micelle and decrease of hydrophobic effect [5]. It can be noted from the Table 1 that the cmc values at the minimum alcohol concentration (0.002 mol%) decrease gradually with increase in the length of the hydrocarbon chain in the alcohol. This further supports the view [25] that the formation of the cavity of more ordered water molecules is favored by the long hydrocarbon chain of the alcohols. In presence of such a cavity, a decrease in cmc is expected. As a representative, the change of cmc of SDS as a function of mol % of 1- pentanol in aqueous solution at different temperatures is shown in Fig. 1. It is evident from Table 1 that in water alcohol mixtures cmc shows weaker temperature dependence as compared to pure water. The value of cmc increases with increase

6 144 Dubey (cmc / molfraction) pentanol / mol% Fig. 1. The plot of critical micelle concentration (cmc) of sodium dodecyl sulfate (SDS) as a function of mol% of 1-pentanol in aqueous solution at different temperatures in temperature from 298 to 313 K. Further the magnitude of cmc values in aqueous mixtures of alcohols are relatively smaller and suggest the solubilization of these additives in the hydrocarbon environment of the micelles. The change in cmc was small in all the studied systems. The thermodynamics of micellization in the studied systems shows that the addition of n-alkanols (C 3 -C 6 ) has an almost identical effect on Gº m, Hº m and Sº m values. From Table 1 it is evident that in all cases Gº m is negative and its magnitude remains practically constant over the entire alcohol concentration range. The Hº m

7 A Conductometric Study of Interaction between SDS and n-alkanols 145 Hº m / kj mol 1 alcohol / mol% Fig. 2. Dependence of Hº m (kj mol 1 ) on the composition of water- alcohol mixtures at 298 K. values are found to be negative and its negative magnitude decreases with increase in concentration in all the alcohol systems (Fig 2). Negative Hº m values indicate that the micellization process is exothermic. Sº m values are positive and shows increasing trend with increase in concentration. According to literature [2] the positive Sº m values are attributed to the disruption of water structure around the hydrocarbon part of these additive molecules as they transfer from the aqueous bulk phase to nonaqueous micellar interior. At the same time, water-water bonds are broken, resulting in an increase in Sº m. Thus, the decrease in cmc on addition of these additives should be seen in terms of additional hydrophobic interactions between the hydrophobic part of the surfactant and the additive molecules. In light of the above discussion, it can

8 146 Dubey be concluded that all the studied alcohols behave as penetrating additives and the penetration of these additive molecules into the micellar interior decreases in the order 1-propanol>1-butanol>1-pentanol>1-hexanol. This observation may be explained as due to reorganization of water molecules on micellar solubilization of alcohols. Such an effect would indeed cause smaller decrease in entropy and enthalpy, which is in agreement with the observed values of Hº m and Sº m. The relative change in the magnitude of Hº m on the addition of alcohols shows that Hº m values in water, kj mol -1 increases to -4.1, -4.2, -4.4 and -4.6 kj mol -1 in mixtures containing ~0.002 mol % 1-propanol, 1-butanol, 1-pentanol and 1-hexanol respectively. These results indicate Hº m changes in the increasing order 1-propanol>1-butanol>1- pentanol>1-hexanol. The order of Hº m values for alcohols (C 3 -C 6 ) thus, reveals not only the structural effects caused by the differences in their chain length, but also accounts for reasonable decrease in their cmc values [25]. CONCLUSION The result of this analysis thus shows the effect of chain length of alcohols on micellization of SDS molecules. The enthalpy and entropy changes are associated with the effects of alcohol additives on micellization of SDS molecules which consist of hydrophobic contributions. It may be concluded that the micellization process is governed primarily by the entropy increase and the driving force for micellization is mainly entropic, i.e., the tendency of the hydrophobic group of the surfactant to transfer from the solvent to the interior of the micelle. It is apparent from experimental results that the more hydrophobic alcohols are, the more marked decrease in cmc is observed. The cmc decreases sharply as the hydrocarbon chain length of alcohols becomes larger. ACKNOWLEDGEMENT Author is thankful to the Department of Science and Technology, New Delhi, India for the award of the project under Women Scientist Scheme A (WOS-A) No.SR/WOS- A/CS-87/2004.

9 A Conductometric Study of Interaction between SDS and n-alkanols 147 REFERENCES 1. M. Perez-Rodriguez, L. M. Varela, V. Mosquera and F. Sarmiento, J. Chem. Eng. Data, 44, 944 (1999). 2. M. S. Chauhan, G. Kumar, A. Kumar and S. Chauhan, Coll. Surfaces, 166, 51 (2000). 3. S. Pandey, R. P. Bagwe and D. O. Shah, J. Colloid Interface Sci., 267, 160 (2003). 4. A. Ali and A. K. Nain, J. Surface Sci. Technol., 13, 1 (1997). 5. H. N. Singh and S. Swarup, Bull.Chem. Soc. Jpn., 51, 1534 (1978). 6. S. Reekmans, H. Luo, M. Van der Auweraer and F. C. De Schryver, Langmuir, 6, 628 (1990). 7. A. P. Romani, M. H. Gehlen, G. A. R. Lima and F. H. Quina, J. Colloid Interface Sci., 240, 335 (2001). 8. E. Rodenas and M. L. Sierra, Langmuir, 12, 1600 (1996). 9. M. S. Akhter and S. M. Alawi, Coll. Surfaces, 196, 163 (2002). 10. S. K. Mehta, R. K. Dewan and Kiran Bala, Phys. Rev. E, 50, 4759 (1994). 11. M. Z. Zhou and R. D. Rhue, J. Colloid Interface Sci., 228, 18 (2000). 12. A. Patist, T. Axelberd and D.O. Shah, J. Colloid Interface Sci., 208, 259 (1998). 13. M. M. El-Banna and M. S. Ramadan, J. Chem. Eng. Data, 40, 367 (1995). 14. M. Manabe and M. Koda, Bull. Chem. Soc. Jpn., 51, 1599 (1978). 15. G. M. Forland, J. Samseth, M. I. Gjerde, H. Hoiland, A. O. Jensen and K. Mortensen, J. Colloid Interface Sci., 203, 328 (1998). 16. D. Attwood, V. Mosquera and V. Perez-Villar, J. Colloid Interface Sci., 127, 532 (1989). 17. D.A. R. Rubio, D. Zanette and F. Nome, Langmuir, 10, 1151 (1994). 18. C. Treiner, A. A. Khodja and M. Fromon, Langmuir, 3, 729 (1987). 19. S. E. Moya and P. C. Schulz, Colloid Polym. Sci., 277, 735 (1999). 20. M. I. Gjerde, W. Nerdel and H. Hoiland, Colloid Polym. Sci., 276, 503 (1998). 21. L. Magid In: K. L. Mittal, (Ed.) Solution Chemistry of Surfactants, Vol.1. Plenum Press, New York, P. C. Pradhan and B. K. Sinha, Indian J. Chem., 26, 691 (1987). 23. S. A. Markarian, L. R. Harutyunyan and R. S. Harutyunyan, J. Solution Chem., 34, 361 (2005).

10 148 Dubey 24. J. A. Riddick, W. B. Bunger and T. K. Sakano, Organic Solvents : Physical Properties and Methods of Purification, 4th ed., John Wiley & Sons, New York (1986). 25. C. Tanford, The Hydrophobic Effect, Formation of Micelles and Biological Membranes, John Wiley, New York (1973).

Pelagia Research Library

Pelagia Research Library Available online at www.pelagiaresearchlibrary.com Der Chemica Sinica, 2012, 3(3):8-635 ISSN: 0976-8505 CODEN (USA) CSHIA5 A study on solution behaviour of sodiumdodecyl sulphate and cetyltrimethylammonium