Determination of the physico-chemical parameters and aggregation number of surfactant in micelles in binary alcohol-water mixtures

The effect of ethanol on the micellization of the anionic surfactant, sodium dodecyl sulfate (SDS), was investigated using potentiometric and pulsed field gradient-NMR spectroscopic techniques. Potentiometry studies showed that the critical micelle concentration (CMC) of surfactant decreases to a minimum value at around 10% ethanol (v/v) because of the co-surfactant effect. The mean values of the hydrodynamic radius, Rh, and aggregation number, NA, of micelles were determined by a combination of viscosity and self-diffusion coefficient measurements. The viscosity of the micellar solutions was approximately independent of ethanol concentration, indicating that the presence of the alcohol does not induce substantial changes in the micellar structure. The number of attachment of ethanol in the micellar core and the degree of dissociation of the counterion were evaluated using a potentiometric technique for alcohol concentrations between 5% and 40% v/v. In addition, an explicit expression is developed for the average size of micelles formed in aqueous solvents containing alcohol.