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 suffice is in the order of Mg2+ > Ca2+ > 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 pK(a), such as Mg2+ and Ca2+, 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-center dot center dot center dot H+delta center dot center dot center dot OH-delta center dot center dot center dot M2+, and displace the hydration water on the silica surface. Ions with a low pK(a), such, as Na+ and Li+, are unable to displace the hydration water on the silica surface.