The impact of surface charge on the ionic dissociation of common salt (NaCl)

Abstract

The ionic dissociation of NaCl has long been thought to be fully understood in terms of the simultaneous removal and hydration of the constituent ions from the surface. However, there are two observations that trouble the current model of ionic dissociation. The first of these observations is that the surface is charged. This has been established by several of studies using laser doppler electrophoresis and membrane electrochemistry. This has not been accounted for in descriptions of ionic dissociation, dissolution or crystallization of salt. The second observation is that the current model, which envisages sodium and chloride ions hydrating and departing the surface, does not yield a rate law consistent with experimental data. These two observations are resolved in this work. It is proposed that the removal of ions form ionic vacancies while deposition forms excess ions on the surface. Vacancies and excess ions are the source of surface charge. The effect of this surface charge affects the rate of removal of ions from the surface, resulting in a feedback mechanism: ionic vacancies are formed by the removal of ions, which affects the rate at which ions are removed. From this model, expressions are derived for the establishment of the charge on the surface and for the kinetics of ionic dissociation, dissolution and crystallization of NaCl.