Direct and indirect interactions in aqueous solutions of alkali metal chlorides

Abstract

Direct, pairwise forces do not control the ion-probe (p-nitroaniline is the probe) and cation-anion association process in all systems examined at low electrolyte concentrations (LiCl, NaCl, KCl, and CsCl) as might have been expected on the basis of accepted concepts in chemical and statistical thermodynamics. Indirect, multiple correlations of the solute species through water molecules are responsible for the control of this process. It is illogical to represent such cooperative interactions as chemical equilibria and they cannot be described strictly by the law of mass action. Indirect interactions also control the distribution of charge and probe molecules in the aqueous systems. The ions and p-nitroaniline molecules are nonuniformly distributed over the structural water and solution microzones. Direct, pairwise coulombic forces are involved in the control of association and charge distribution processes only for concentrations of lithium and sodium chloride above 0.5-1.0 mole/liter. Indirect cation-anion interactions apparently remain stronger than direct interactions in concentrated KCl and CsCl solutions. The chemical processes are described using two-particle, three-particle, and multiple correlation functions.