Three-Dimensional Characterization of Layers of Condensed Gas Molecules Forming Universally on Hydrophobic Surfaces

Understanding the solvation layer of hydrophobic surfaces is essential for elucidating the interaction between hydrophobic surfaces in aqueous solutions. Despite their importance, little is known on these layers due to the lack of lateral resolution in spectroscopic or scattering experiments and probe instability in the static scanning probe methods used in most experiments. Using a high-resolution FM-AFM with stiff cantilevers and hydrophilic tips, we overcome this instability to provide the first detailed 3d maps of the solvation/hydration layer of two archetypal hydrophobic surfaces: graphite (HOPG) and self-assembled fluoro-alkane monolayer (FDTS). In degassed solutions we find different tip-surface interactions for the two surfaces; hydration oscillations superimposed on van der Waals attraction with HOPG and electrostatic repulsion with FDTS. Both are similar to interactions observed with hydrophilic surfaces. In solutions equilibrated with atmospheric air or high-pressure nitrogen, the tip-surface interaction changes dramatically, disclosing the formation of a 2-5 nm thick layer of condensed gas molecules adsorbed to the hydrophobic surfaces. This layer leads to strikingly similar tip-surface interactions for HOPG and FDTS with only weak dependence upon the concentration of dissolved gas molecules, indicating universality in the way hydrophobic surfaces present themselves to nondegassed aqueous solutions. Measurements at low cantilever oscillation amplitudes reveal the inner structure of the layer of condensed gas molecules with an average distance between its constituents, 0.5-0.8 nm, agreeing with recent molecular dynamics calculations. In addition to the uniform condensed layers, we probe sparse nanobubbles found on the surface. Those show distinct interaction with the tip, different from that with the flat layer.