Chloride ions in aqueous solutions

Abstract

In spite of much experimental and theoretical work during the past 80 yr, there is at present no generally agreed structural model for concentrated aqueous solutions, even for a relatively simple system like NaCl dissolved in H₂O (ref. 1). Aqueous solutions contain four chemical species and so require 10 pair correlation functions to describe the structure, thus complicating attempts to isolate, for example, ion–water correlations unambiguously from diffraction experiments. The combination of long-range Coulomb potentials, quantum-mechanical hydrogen bonding effects and the intractability of the liquid state has proved an obstacle to theoretical understanding. We have investigated the coordination of water molecules around Cl^– ions in aqueous solution by the isotopic substitution method. A range of counter-ions and concentrations was used to answer questions about the sensitivity of Cl^– hydration to the nature of the counter-ion and the ionic strength. We show here that Cl^– hydration is essentially independent of the cation and of ionic strength with the possible exception of a transition metal counter ion (Ni²⁺). The insensitivity of Cl^– hydration to cation type should lead to a major simplification in the theory of solutions.