Temperature Dependence of Proton Coverage and the Total Potential at Fused Silica:Water Interfaces from Phase-Resolved Nonlinear Optics

We study the temperature and ionic strength dependence of interfacial amphoterism via phase- and amplitude-resolved SHG measurements at the fused silica:water interface, finding that the total interfacial potential becomes increasingly negative when raising the temperature from 20 to 60 °C. The interfacial structure, reported by calibrated measurements of the second-order nonlinear susceptibility of the interface, remains largely temperature invariant. Our approach is presented in form of a blueprint for second harmonic generation (SHG) amplitude and phase measurements at buried aqueous interfaces using an affordable Y-crystal-based oscillator. A Galilean beam expander addresses the signal vs local oscillator mismatch, while a beam block eliminates otherwise interfering front reflections from the flat optical windows used here. The results show nearly linear voltage increases with temperature that rise faster at low vs high ionic strength and are consistent with temperature-dependent equilibrium constants governing the amphoteric silica:water interface. Proton surface coverages at pH 2.5, 6, and 11 increase with temperature as the surface becomes more negatively charged, reaching up to 10¹³ protons cm^-2 at 60 °C. These findings aid interfacial model development, benchmark atomistic simulations, explore temperature-dependent Hofmeister effects, and enhance understanding of interfacial electrocatalysis.