Dielectric constant measurements of interfacial aqueous solutions using atomic force microscopy

Abstract

Dielectric constant variation at aqueous solution/mica interfaces is shown to be responsible for the force acting on tips immersed in the double layer. The exchange of the volume of a region of the electric double layer of a mica surface immersed in aqueous solutions, with a dielectric constant, by the silicon nitrite tip, with a dielectric constant ε Tip, is responsible for the repulsion at large distances from the surface (starting at ∼l00 nm, diffuse layer) and followed by an attraction when the tip is immersed in the inner layer (≤10 nm). The force versus separation measured curves were fitted to the expression of the dielectric exchange force derived by using a continuum theory for a sharpened conical tip immersed in a spatially variable dielectric constant double layer electric field. The dielectric exchange effect gives a consistent description of the force acting on the tip by assuming a double layer region of water with ε DL≈80 at distances far away from surface (∼100 nm), followed by a region of lower dielectric constant at the inner layer. Support for the proposed model (dielectric exchange force) is given by the observation of an attractive force when metal (platinum) coated tips ( ε Tip≈∞) are immersed in the mica double layer and the measurements of only repulsive force components when silicon nitride 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.