A. SHG Amplitudes and Phases. Figures and show the SHG amplitude and the SHG phase for the range of NaCl, MgCl2, CaCl2, SrCl2, and BaCl2 concentrations indicated. Our earlier homodyne-detected SHG measurements indicate that the cation surface coverage increases with increasing cation concentration up to an estimated saturation level of approximately 10 12 ions per square cm. This coverage corresponds to the number of negatively charged adsorption sites (SiO -groups) on silica at circumneutral pH reported from XPS measurements. Our current HD-SHG measurements reveal a nonmonotonic trend in the recorded SHG amplitude for the aqueous NaCl solution, as recently reported. In contrast, the divalent chloride salts show a less pronounced, weakly non-monotonic trend with increasing ionic strength in the SHG amplitude.