We employ second harmonic generation (SHG) experiments to probe the interactions of the negatively charged fused silica:aqueous interface with Al³⁺, Y³⁺, La³⁺, Mg²⁺, Li⁺, Cs⁺, NH₄^+,and Na⁺ cations at pH 4 to ensure clean ion speciation, and an ionic strength of 20 mM. In the absence of screening electrolyte, we observe a pronounced specific ion effect for the second-order nonlinear susceptibility χ⁽²⁾values, the total surface potential, and the adsorption-desorption kinetics as a function of flow rate. Notably, the adsorption of the monovalent cations varies considerably, with ammonium showing the most significant variation in the SHG intensity, I_SHG. The adsorption rate of trivalent cations also varies, with clear signs of a partially irreversible interaction. The time-dependence of the SHG intensity observed during the desorption of the monovalent cations indicates a decoupling between the Stern and diffuse layer dynamics. This effect is not observed at lower flow rates and for higher valence cations. From heterodyne-detected SHG amplitude and phase data we recorded, the total potential was computed to estimate the total number of absorbed monovalent cations, resulting in a maximum of 10¹³ cations cm^-2, or about 1 percent of a monolayer. A K_d adsorption model was fitted to the data yielding 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, respectively, corresponding to adsorption free energies of -13 to -18 and -24 kJ mol^-1, respectively.

Prof. Franz M. Geiger

Professor of Chemistry, Northwestern University

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