Surface charge reversal and hydration forces explained by ionic dispersion forces and surface hydration

Abstract

The first direct measurements of forces between mica surfaces in electrolytes showed an apparent short range repulsion additional to the predictions of DLVO theory. The origins of such “secondary hydration forces” have remained a mystery. We show they can be explained as an repulsive entropic force appearing due to a hydration surface layer with longer range secondary hydration forces appearing as a consequence of ionic dispersion forces. Ionic dispersion forces are calculated from dynamic polarisabilities and finite ion sizes. Both are determined by ab initio quantum chemical methods. An hydration model is applied to describe the hydration layer of cosmotropic ions as well as hydration of the mica surface. Strongly hydrated cosmotropic ions are allowed to penetrate the surface hydration layer. Weakly hydrated chaotropic ions are excluded from the surface hydration layer. The combination of ab initio ionic polarisabilities and the hydration model allows the extended theory to account for secondary hydration forces. Ionic dispersion forces are also shown to enhance surface adsorption of ions leading to charge reversal. Repulsive ionic dispersion forces for cations at a hydrocarbon surface reverse the entropic force, making it attractive rather than repulsive.