Hydrophobic surfaces probed by atomic force microscopy

We have measured the force acting on neutral tips as a function of distance to hydrophobic surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to the electrostatic response of the aqueous fluid structure (hydration layer) at the interface. The local electric field in an interfacial region is a manifestation of the distribution of surface polar residues, and we have assumed that the polarization (hydration) of the hydrophobic surface immersed in water is predominantly driven by the direct water binding. The simplest electrostatic description of the coupling between the interfacial polarization charges and the corresponding polarization charges of the solvent molecules is expressed here as the spatially variable dielectric permittivity epsilon(int). The exchange of a volume of the interfacial region with epsilon(int) by a tip with a dielectric constant epsilon(tip) is responsible for the tip attraction. The variable dielectric permittivity profiles for the following interfaces were measured in order to clarify the origin of the long-range attractive forces: water/air, water/CTAB covered mica, and water/hydrophobic silicon.