DLVO and hydration forces between mica surfaces in Li+, Na+, K+, and Cs+ electrolyte solutions: A correlation of double-layer and hydration forces with surface cation exchange properties

The forces between two molecularly smooth mica surfaces were measured over a range of concentrations in aqueous Li+, Na+, K+, and Cs+ chloride solutions. Deviations from DLVO forces in the form of additional short-range repulsive “hydration” forces were observed only above some critical bulk concentration, which was different for each electrolyte. These observations are interpreted in terms of the corresponding ion-exchange properties at the mica surface. “Hydration” forces apparently arise when hydrated cations adsorbed on mica are prevented from desorbing as two interacting surfaces approach. Dehydration of the adsorbed cations leads to a repulsive hydration force. A simple and remarkably successful method of analysis of the charging mechanism at the mica surface suggests a novel approach to the determination of the hydrated radius of adsorbed cations. Incorporation of this charge regulation in the exact DLVO force calculation gives much better agreement with experimental results, justifying the precise validity of the DLVO theory in cases where hydration forces are absent. The regulation-force theory allows for a more exact analysis of the net hydration force. Using this approach an exponential “hydration” force of decay length 1.0 ± 0.2 nm was inferred for the interaction of mica surfaces fully covered with hydrated K+ and Na+ ions. From these results the interaction energy between two hydrated ions can also be estimated.