Counting the number density of ions and water molecules at aqueous: solid interfaces remains a fundamental challenge, especially under conditions of varying ionic strength. Here, we employ an all-optical non-contact approach based on phase- and amplitude-resolved second harmonic generation spectroscopy to estimate the cation and anion coverages and the number density of net-aligned water molecules at fused silica surfaces in contact with aqueous solutions held at pH 5. 8 and a wide range of ionic strengths of NaCl, NaClO4, Na2SO4, and Na2HPO4 up to 8 M. At 8 M ionic strength of NaClO4, the data are consistent with an interface that is comprised of half of a monolayer of ions (θ (Na+) +θ (ClO4-) ≈ 5 x 10¹4 cm^-2) and half of a monolayer of water molecules (θ_ (H2O) ≈ 4 x 10¹4 cm^-2), the latter having flipped their dipolar orientation from "protons to the surface" to "oxygens to the surface". The interfacial water: total ion ratio is close to ten times smaller than in the bulk solution at these high ionic strengths. The Helmholtz free energy associated with water disordering increases abruptly from near 0 kJ mol^-1 below 2 M ionic strength to 10 kJ mol^-1 at 8 M ionic strength, indicating perhaps a Kirkwood-like transition from a hydrated to a dehydrated interface.
Gonzalez et al. (Wed,) studied this question.