Measurements of long-range attractive forces between hydrophobic surfaces and atomic force microscopy tips

We have measured the force acting on neutral tips as function of distance to hydrophobic surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer). The exchange of a volume of this region with a dielectric permittivity ϵint by the tip with a dielectric constant ϵtip is responsible for the tip attraction when it is immersed in the hydration layer. Hydrophobic hydration layers, characterized by a variable dielectric permittivity profile, have measured widths of ∼4 and ∼8 nm for hydrophobic silicon and CTAB monolayer covering mica surfaces, respectively.