Here, R is the gas constant and T is the temperature (298K). Fig. shows that for all cations we surveyed the surface coverages are quite similar, all the way up to 10 12 -10 13 per square centimeter at the highest concentrations. This saturation level range roughly corresponds to roughly 1% to 10% of the number of silanol groups on fused silica (4.6 x 10 14 cm -2 ), or the number of deprotonated silanol groups on a silica surface held at pH 4. The similarity in the overall surface coverages at a given ion concentration seems to indicate that local field effects, such as lateral cation-cation interactions, are not detectable within the uncertainty and intra-sample variation of the data.