Effect of sodium chloride on micellization of sodium dodecylsulfate with sodium decylsulfate/ sodium tetradecylsulfate surfactants

Abstract Critical micelle concentration (CMC) values of binary mixtures of sodium dodecylsulfate (SDS) with sodium decylsulfate (SDeS) and sodium tetradecylsulfate (STS) in absence and presence of NaCl have been determined at 298 K employing conductance measurement method. In water the mixtures exhibited synergism. However, on increasing the NaCl concentration, the mixtures started exhibiting antagonism after a particular NaCl concentration dependent on bulk composition of the mixture. Rubingh’s model was utilized to calculate the mutual interaction parameter ( β M ) and composition ( x SDS ) in the mixed micelle. The attractive interaction between the surfactant components in the mixed micelle decreases with increase in the NaCl concentration and becomes repulsive at high NaCl concentration, an opposite trend to that reported for similar binary mixtures of SDS + Aerosol-OT system. The calculated micellar mole fraction values indicate enrichment of the mixed micelle with the component having lower CMC. Slope ratio method and Corrin-Harkins equation have been used to calculate the counterion binding constant of both pure and binary mixed systems. Micellization is enhanced thermodynamically by the addition of NaCl.