NaCl, MgCl 2 , and AlCl 3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 from Nonlinear Optics.

We employ amplitude- and phase-resolved second harmonic generation 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 quantify the second-order nonlinear susceptibility and the total interfacial potential in the presence and absence of a 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions are observed in the absence of the screening electrolyte. The total potential is then employed to estimate the total number of absorbed cations cm ^-2 . The contributions to the total potential from the bound and mobile charges were separated using Gouy-Chapman-Stern model estimates. All three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the K _d adsorption model fits the data with binding constants of 3-30 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 are observed and found to be cation-specific.