Influence of Short-Chain Alcohols on the Micellization Parameters of Sodium Dodecyl Sulfate (SDS) M. A. Safarpour*, A. A. Rafati, H. Gharibi and M. Rezaie Sameti Department of Chemistry, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran The influence of short-chain alcohol on the aggregation behavior of Sodium Dodecyl Sulfate (SDS) is investigated within a wide range of alcohol concentration by potentiometric technique. It is observed that aggregation behavior of SDS is a function of alcohol concentration. Variation of micellization parameters are studied in a wide range of alcohol concentration and the trend of changing of micellization parameters have been investigated. The variations of micellization parameters show several regularities. Finally, the relationships between these regularities and alcohol-surfactant interactions have been explained. INTRODUCTION Surfactants form micelles after a critical concentra- tion, called critical micelle concentration (cmc). 1 Various environmental factors (solvent, pH, additive, temperature, etc.) affect the micellization process. Several parameters of micellization such as aggregation number (n), degree of dis- sociation of counterion ( ), cmc, etc., vary by changing the environmental conditions. 2-9 In order to investigate micellization phenomena and characteristics of surfactants, the parameters of micellization (cmc, n, , etc.) have been studied under various conditions. Many studies on the properties of SDS solutions have been published recently. 10-12 In the present work, a number of properties of SDS such as n, , cmc, and Gibss free en- ergy, in the presence of ethanol (EtOH) and 1-propanol (PrOH) were studied. In this research, interaction between SDS and small al- cohols are investigated in the micellar region. In order to in- vestigate this interaction, micellization parameters are con- sidered. The trend of variation of micellization parameters lead to understanding of the alcohol-surfactant interactions. Experimental data have been obtained by potentiometric technique. Among the several methods used to study the micellization phenomena, the use of potentiometric method has several advantages. This method has been used to determination of ions concentra- tion (surfactant ions and counterions). EXPERIMENTAL SECTION The SDS was obtained from Merck and was used as re- ceived. Ethanol (EtOH) and 1-propanol (PrOH) was also obtained from Merck. They were 98% pure. All solutions were prepared in doubly distillated water. In all ex per i- ments the temperature was controlled to within 0.1 °C by circulating thermostated water through the jacketed glass cell, and the sample solution was continuously stirred by magnetic stirrer. The surfactant membrane electrode selective to SDS used in this work was constructed by a method described previously. 13 In order to achieve a suitable membrane to make the surfactant selective reversible to anionic surfactant ions, PVC with cationic end groups (synthesized in Tarbiat Modarres university) according to Daividson’s method, was made. 14 All SDS Solutions were doped with 10 -4 mole dm -3 sodium bromide. It is assumed that the free concentration of the bromide ions is constant. In these ex- periments, the emf of surfactant electrode was measured relative to a commercial bromide ion electrode (Corning solid state ISE 30-35-00). This electrode acts as a reference electrode. Measuring the emf of the surfactant electrode relative to a commercial sodium electrode (Corning 476210), yields information about counterion binding. The emf measurements on the former cell measures the ratio of “surfactant activity / bromide ion activity”, un- der experimental conditions. It is expected that the activity coefficients of the ions are approximately equal, in which case the “surfactant monomer concentration / bromide ion concentration” is measured. Since the concentration of bro- mide is constant, the emf of this cell is given by 2.303RT E 1 = E o 1 – log(m 1 ) (1) F where m 1 is the concentration of free surfactant, and E o 1 is a constant. Fig. 1 shows typical experimental data taken from this cell. Micellization of Sodium Dodecyl Sulfate J. Chin. Chem. Soc., Vol. 46, No. 6, 1999 983