We found that all three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the Kd adsorption model fit the data with binding constants of 3 to 30 mol -1 and ~300 mol -1 for the low (<0.1 mM) and high (0.1 -3 mM) concentration regimes, corresponding to adsorption free energies of -13 to -18 and -24 kJ mol -1 at room temperature, respectively. The maximum surface coverages are around 10 13 cations cm -2 , matching the number of deprotonated silanol groups on silica at pH 4. Clear signs of decoupled Stern and diffuse layer nonlinear optical responses were observed and found to be cation specific.