Electrostatic response of hydrophobic surface measured by atomic force microscopy

The arrangement of water molecules at aqueous interfaces is an important question in material and biological sciences. We have measured the force acting on neutral tips as a function of the distance to hydrophobic silicon surfaces and cetyltrimethylammonium bromide monolayers covering mica surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer) which is generated by the reorientation of water molecular dipoles. The exchange of a volume of this region with a dielectric permittivity (εint) by the tip with a dielectric permittivity (εtip) is responsible for the tip attraction when it is immersed in the polarization (hydration) layer. Variable permittivity profiles starting at ε≈11 at the interface and increasing to ε=80 about 10 nm from hydrophobic silicon surfaces and about 50 nm from cetyltrimethylammonium bromide monolayer covering mica surfaces were measured.