Effect of sodium chloride on the aggregation, adsorption and counterion binding behavior of mixtures of sodium dioctylsulfosuccinate and sodium dodecylsulfate in water

Sodium dioctylsulfosuccinate (AOT) is a special surfactant and has a special counterion binding (SCB) behavior as one of its special properties. The SCB refers to a sudden two-fold increase in the value of counterion binding constant when the concentration of added NaCl reaches about 0.015 mol kg−1. In order to examine the SCB of AOT when it is mixed with another surfactant which does not possess the SCB, aggregation and adsorption characteristics of mixtures of AOT and sodium dodecylsulfate (SDS) were studied in aqueous medium as a function of concentration of added NaCl by using surface tension method. The mixed systems with bulk mole fraction of AOT ≥ 0.245 have the SCB similar to that of neat AOT. Thus AOT controls the counterion binding behavior of AOT + SDS mixed surfactant system. It has been shown that the Corrin–Harkins equation used for determining the counterion binding constant of single ionic surfactants takes a modified form in the case of mixed surfactants. The compositions of the mixed micelle and adsorption layer were estimated. The added NaCl has a tendency to increase the attractive interaction between the two surfactants in both the micellar phase and the adsorption layer. Sodium ion has been found to have different specificities for sulfonate and sulfate headgroups.