Extensive measurements of the force acting between two approaching silica surfaces in solution as a function of pH, salt type, and salt concentration provide new insight into the way surface hydration governs specific cation adsorption to the surface and consequently affect surface charging and surface-surface interaction. The results disclose that cation adsorption to the surface is determined by the relative strength of bulk water hydrogen bonds compared with surface-water hydrogen bonds. At low pH, when the former are stronger than the latter, excess adsorption of chaotropic cations (normal Hofmeister series) takes place, leading to surface neutralization at intermediate salt concentrations and surface charge reversal at higher salt concentrations. At high pH, surface deprotonation leads to the formation of a tight hydration layer that repels chaotropic ions from the surface and leads to the reversal of the Hofmeister series. The normal Hofmeister effect is hence traced by our experiments to hydrophobic expulsion of chaotropes from solution to weakly hydrated surfaces while the reverse Hofmeister order is associated with strongly hydrated surfaces. These findings account for all the experimental data known to us, including overcharging by excess adsorption of chatropic cations (previously unexplained), the opposite Hofmeister order in high and low pzc oxides, and the reorientation of interfacial water molecules as a function of pH.
phsivan@tx.technion.ac.il
