Dielectric exchange force: a convenient technique for measuring the interfacial water relative permittivity profile

Water relative permittivity profiles perpendicular to mica surfaces have been measured by atomic force microscopy using the force acting on uncharged tips when immersed in the mica double-layer. This force is modelled by the gradient of the electrostatic energy variation (dielectric exchange force) involved in the immersion of the tip with a relative permittivity εTip in the double layer region with εDL. The measured variable permittivity profile starting at ε≈4 at the interface and increasing to ε≈80 about 10 nm from the surface suggests a reorientation of water molecule dipoles in the presence of mica interfacial charges. Changes in water polarization are therefore responsible for the hydration or structural forces acting on the tips immersed in the inner double layer. Corroboration for the proposed model (dielectric exchange force) is given by the observation of an attractive force when metal-coated tips (εTip≈∞) are immersed in the mica double layer. Support for the change in the water relative permittivity at the interface is given by measurements of only a repulsive force component when silicon nitride and silicon tips are immersed in solvent where there is no interaction between the mica surface and the solvent and, consequently, no solvent structuring at the interface.