Dielectric properties of water at rutile and graphite surfaces: Effect of molecular structure

The dielectric properties of interfacial water adjacent to the surfaces of hydrophobic graphite and the 110 surface of hydrophilic rutile (α-TiO ₂) are investigated by means of nonequilibrium molecular dynamics simulations. The dielectric behavior of water is found to arise from its local density and molecular polarizability in response to an external field, and can be rationalized in terms of the number and strength of water-surface and water-water H-bonds. The interplay of local density and polarizability leads to a particularly strong dielectric response, exceeding the external field, of the water layer directly contacting the surfaces, while the second layer exhibits a reduced response. Consequently, dielectric profiles near surfaces cannot be correctly described by implicit solvent models valid for bulk water. The overscreening response of the contact water layer has been observed in previous simulation studies and implies the local permittivity (dielectric constant) of that layer is negative. However, the negative permittivity of the contact water layer is counterbalanced by the positive permittivities of the surface depletion layer and the second water layer such that the calculated Stern layer capacitance is positive and compatible with experimental data. Moreover, the electrostatic potential profile matches well the profile calculated for an aqueous salt solution at the charged rutile (110) surface, thus supporting the "water centric" view of aqueous electrical double layers.