The governing role of surface hydration in ion specific adsorption to silica: an AFM-based account of the Hofmeister universality and its reversal.

AFM measurements of the force acting between silica surfaces in the presence of varied alkali chloride salts and pH's elucidate the origin of the Hofmeister adsorption series and its reversal. At low pH, electrostatics is shown to be insignificant. The preferential adsorption of Cs(+) to the silica surface is traced to the weak hydration of neutral silanols and the resulting hydrophobic expulsion of weakly hydrated ions from bulk solution to the interface. The same interactions keep the strongly hydrated Na(+) and Li(+) in solution. As pH is increased, a tightly bound hydration layer forms on deprotonating silanols. Cs(+) is correspondingly expelled from the surface, and adsorption of small ions is encouraged. The deduced role of surface hydration is corroborated by hydration repulsion observed at high pH, surface overcharging at low pH, and data in other oxides.