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We calculate the electro-osmotic mobility and surfaceconductivityat a solid–liquid interface from a modified Poisson–Boltzmannequation, including spatial variations of the dielectric functionand the viscosity that where extracted previously from molecular dynamicssimulations of aqueous interfaces. The low-dielectric region directlyat the interface leads to a substantially reduced surface capacitance.At the same time, ions accumulate into a highly condensed interfaciallayer, leading to the well-known saturation of the electro-osmoticmobility at large surface charge density regardless of the hydrodynamicboundary conditions. The experimentally well-established apparentexcess surface conductivity follows from our model for all hydrodynamicboundary conditions without additional assumptions. Our theory fitsmultiple published sets of experimental data on hydrophilic and hydrophobicsurfaces with striking accuracy, using the nonelectrostatic ion–surfaceinteraction as the only fitting parameter. [ABSTRACT FROM AUTHOR]

We derive the theoretical framework to calculate thedielectricresponse tensor and determine its components for water adjacent tohydrophilic and hydrophobic surfaces using molecular dynamics simulations.For the nonpolarizable water model used, linear response theory isfound to be applicable up to an external perpendicular field strengthof ∼2 V/nm, which is well beyond the experimental dielectricbreakdown threshold. The dipole contribution dominates the dielectricresponse parallel to the interface, whereas for the perpendicularcomponent it is essential to keep the quadrupole and octupole terms.Including the space-dependent dielectric function in a mean-fielddescription of the ion distribution at a single charged interface,we reproduce experimental values of the interfacial capacitance. Atthe same time, the dielectric function decreases the electrostaticpart of the disjoining pressure between two charged surfaces, unlikepreviously thought. The difference in interfacial polarizability betweenhydrophilic and hydrophobic surfaces can be quantized in terms ofthe dielectric dividing surface. Using the dielectric dividing surfaceand the Gibbs dividing surface positions to estimate the free energyof a single ion close to an interface, ion-specific adsorption effectsare found to be more pronounced at hydrophobic surfaces than at hydrophilicsurfaces, in agreement with experimental trends. [ABSTRACT FROM AUTHOR]