(104b) Insights into Surface Hydration through Parallel Measurements of Water Diffusivity and Surface Forces

The fundamental molecular origins of solvent-structural forces, such as hydration and hydrophobic interactions, remain a mystery. Surface forces measurements, while accurate, are complicated by the need to distinguish between simultaneous electrostatic, steric, and solvent-structural forces, a task that is particularly challenging for silica and lipid bilayer surfaces. Here, I will discuss our efforts to use two complementary nanoscale measurements of surface water diffusivity, using Overhauser Dynamic Nuclear Polarization (ODNP), and surface forces measurements, using the surface forces apparatus (SFA), to elucidate the mechanisms of surface hydration at silica and lipid bilayer surfaces. Bilayer surfaces were studied in the presence of varying bulk concentrations of dimethyl sulfoxide (DMSO), a commonly used cellular cryoprotectant. The surface water diffusivity increases with DMSO concentration, as does the adhesion force between bilayers. Both trends appear to be caused by a shrinking of the hydrated volume of lipid head groups by DMSO, which allows for extraction of quantitative surface hydration length scales. Similar insight was gained for silica surfaces by adjusting surface, rather than solution, properties. Silica (SiO₂) surfaces are composed of silanol (Si-OH) groups, which impart hydrophilic character, and siloxane (Si-O-Si) groups, which give rise to modest hydrophobicity. Over a range of silica surface compositions, water diffusivity near the surface increases with decreasing silanol density (increasing hydrophobicity), and the surfaces exhibit a corresponding decrease in the range of the hydration repulsion. None of the silica surfaces adhere in water due to a short-range (1-4 nm) repulsion, likely arising from the hydration of uncharged (deprotonated) silanols. Overall, our work suggests two contrasting paradigms for surface hydration: hydrated excluded volume of individual surface groups, and a collective hydrogen bond network.