Evaluation of the effective collision cross-section for ion-water thermal motion in a weakly non-equilibrium system

Water is essential for life and industry, often existing as aqueous solutions. Strong electrolytes fully dissociate in water, forming conductive solutions whose electrical conductivity depends on ion concentration. According to Onsager’s limiting law, ion mobility approaches a constant at infinite dilution. In dilute solutions of strong electrolytes, ion mobility is significantly influenced by the properties of the solvent water. This study investigates ion-solvent collisions in dilute KCl solutions under weakly non-equilibrium conditions. Conductivity measurements were performed at room temperature using a high-precision conductivity metre, and the limiting ion mobility was determined via data fitting. Based on molecular kinetics, the mean molecular collision time was calculated, and the equivalent hard-sphere model was applied to analyse ion-water collision cross-sections. Results indicate that, besides momentum exchange, Coulomb interactions significantly influence ion-water collisions, especially at low concentrations.