Distinguishing electrostatic from ion-specific effects at charged oxide/water interfaces by sum-frequency spectroscopy

Charged aqueous interfaces are ubiquitous, yet the ionic modulation of interfacial reactions is often empirically related to the bulk ion concentration without distinguishing pure electrostatic screening from ion-specific interactions. The Gouy-Chapman-Stern (GCS) model provides a mean-field description of the surface charge and ion distributions at such electrified interfaces, but direct experimental validation at the molecular level remains scarce. Here, we combine in situ sum-frequency spectroscopy of surface lattice vibrations with the modified Poisson-Boltzmann formalism to spectroscopically disentangle various ionic effects. Using the benchmark silica/water interface in NaCl solutions as a model system, we calibrated experimental deprotonation rates of surface silanol groups against the calculated surface proton concentrations. Below the Na+ concentration of ∼2 M, the apparent ion-induced acceleration of deprotonation is fully accounted for by electrostatic screening alone, with ion concentration dependence essentially reflecting the change in surface proton concentration. Above this threshold, the GCS model breaks down as ions specifically perturb the surface structure, causing the deprotonation rate to exceed the electrostatic prediction. This framework is further examined by calculations across different pKa values, ionic valences, and oxide species, providing a general basis for interpreting interfacial ionic effects beyond the mean-field limit.