New Information on the Ion-Identity-Dependent Structure of Stern Layer Revealed by Sum Frequency Generation Vibrational Spectroscopy

We observed that the structure of water in the Stern layer depends on the identity of the cation, which cannot be predicted with classical double layer models. The ability of a cation to displace the hydration water on a silica surface is in the order of Mg 2+ > Ca 2+ > Li + > Na +, which is consistent with the trend of the acid dissociation constant ( K a ) of the salt. Our data suggest that ions with a high p K a, such as Mg 2+ and Ca 2+, have a local electrostatic field strong enough to polarize water molecules in the hydration shells of the ions which form linkages with the negative charges on the silica, Si-O – ···H +δ ···OH –δ ···M 2+, and displace the hydration water on the silica surface. Ions with a low p K a, such as Na + and Li +, are unable to displace the hydration water on the silica surface.