In conclusion, we employed amplitude-and phase-resolved SHG experiments to probe interactions of fused silica:aqueous interfaces with Al 3+ , Mg 2+ , and Na + cations at pH 4 and as a function of metal cation concentration. We quantified the second-order nonlinear susceptibility and the total potential in the presence and absence of 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions were observed in the absence of screen electrolyte. The total potential was then employed to estimate the total number of absorbed cations. The contributions to the total potential from the bound and mobile charge were separated using Gouy-Chapman-Stern model estimates.