Breakdown of hydration repulsion between charged surfaces in aqueous Cs⁺ solutions

Using a surface force balance, we have measured the normal and shear forces between mica surfaces across aqueous caesium salt solutions (CsNO₃ and CsCl) up to 100 mM concentrations. In contrast to all other alkali metal ions at these concentrations, we find no evidence of hydration repulsion between the mica surfaces on close approach: the surfaces appear to be largely neutralized by condensation of the Cs ions onto the charged lattice sites, and are attracted on approach into adhesive contact. The contact separation at adhesion indicates that the condensed Cs ions protrude by 0.3 ± 0.2 nm from each surface, an observation supported both by the relatively weak adhesion energies between the surfaces, and the relatively weak frictional yield stress when they are made to slide past each other. These observations show directly that the hydration shells about the Cs⁺ ions are removed as the ions condense into the charged surface lattice. This effect is attributed to the low energies-resulting from their large ionic radius-required for dehydration of these ions.