Interactions between molecularly smooth gold and mica surfaces across aqueous solutions

Using a surface force balance, we measured the forces between an ultrasmooth (0.2 nm rms roughness) templatestripped gold surface and a molecularly smooth mica surface. Comparison of these forces in both low salt (conductivity water, equivalentto10^-6-10^-5 M 1:1 salt) and high salt (10mMKClO₄) regimes enabled us to examine the properties of water layers confined between a metal and a dielectric to films of a few nanometers or less in thickness. We find that the long-range forces between gold and mica are similar to those between two mica surfaces, indicating a net effective negative charge density on the gold similar to that on the mica. Differences were more pronounced at small separations, manifested by the larger jump-in distance in pure water and the weaker hydration repulsion in high salt between a gold and a mica surface compared with two mica surfaces. However, despite these short-ranged differences, replacing one mica surface with gold does not measurably alter the viscosity of nanoconfined water layers, either as free molecules or as bound hydration layers, relative to their confinement by two mica sheets.