Effects of Counter-Ion Specificity in Salt-Aqueous Solutions of Dodecylsulfates of Alcali Metals

Abstract
New experimental data are presented for aqueous solutions of sodium, potassium and cesium dodecylsulfates containing salt additives; the latter are chlorides having the same cations as the surfactants, the salt concentration spans 0 – 30 mmol/l. The information obtained includes the critical micelle concentrations, the aggregation numbers, concentrations of sphere-to-rod transitions, degrees of counter-ion binding, thermodynamic functions of aggregation (at 40^oС and 45^oС); data on the Krafft boundary. Counter-ion effects are analyzed from the standpoint of ions specificity towards water structuring in their micro-environment. In the proposed version of the quasi-chemical model the specific contribution to the Gibbs aggregation energy is described through parameters of the counter-ions adsorption on the micelle surface. The results of model estimations are in a satisfactory agreement with the ex-periment and conform to the concept of ions chaotropy or cosmotropy.

Keywords: ionic surfactants, micellization, effects of salt additives, counter-ion specificity, molecular-thermodynamic modeling