Counter ion binding in solutions of sodium dodecylsulfate in water–ethylene glycol mixtures and the Corrin–Harkins equation

Abstract

The aggregation behavior of sodium dodecylsulfate (SDS) in mixtures of water and ethylene glycol (EG) in the presence of varying amounts of sodium chloride has been studied by using surface tension, conductance and fluorescence methods. The critical micelle concentration (cmc) values obtained from the different methods are found to be comparable. The cmc value of SDS keeps on increasing with increase in concentration of EG due to the decrease in the solvophobicity of the surfactant, and it decreases with increase in the salt concentration due to reduction in the repulsive interaction between the head groups. The counter ion binding behavior of SDS in the mixed solvent is assessed by using the Corrin–Harkins (CH) equation. The applicability of the CH equation appears to be limited in the mixed solvent as deviation from this equation has been observed in the region of low concentration of NaCl when the EG amount becomes equal to or more than 50 w/w %. It is observed that this particular deviation from the CH plot occurs in the region where the total counter ion concentration at the cmc is controlled by the ionic surfactant.