Interfacial, Electroviscous, and Nonlinear Dielectric Effects on Electrokinetics at Highly Charged Surfaces

Abstract

The dielectric constant and the viscosity of water at the interface of hydrophilic surfaces differ from their bulk values, and it has been proposed that the deviation is caused by the strong electric field and the high ion concentration in the interfacial layer. We calculate the dependence of the dielectric constant and the viscosity of bulk electrolytes on the electric field and the salt concentration. Incorporating the concentration and field-dependent dielectric constant and viscosity in the extended Poisson–Boltzmann and Stokes equations, we calculate the electro-osmotic mobility. We compare the results to literature experimental data and explicit molecular dynamics simulations of OH-terminated surfaces and show that it is necessary to additionally include the presence of a subnanometer wide interfacial water layer, the properties of which are drastically transformed by the sheer presence of the interface. We conclude that the origin of the anomalous behavior of aqueous interfacial layers cannot be found in electrostriction or electroviscous effects caused by the interfacial electric field and ion concentration. Instead, it is primarily caused by the intrinsic ordering and orientation of the interfacial water layer.

Interplay of Interfacial Viscosity, Specific-Ion, and Impurity Adsorption Determines Zeta Potentials of Phospholipid Membranes

“…The inverse perpendicular dielectric response profile ε ⊥ −1 (z), which is obtained from equilibrium polarization fluctuations in neat water simulations as explained previously 49,50 and used for the computation of φ ε and φ η , is shown in Figure 2A. It has been shown recently that the presence of ions modifies the dielectric response only mildly, 51 which validates our usage of the neat water dielectric profile. As seen in Figure 2A, lipid molecules contribute significantly to the dielectric response only within a distance of 1 nm from the Gibbs dividing surface located at z = 0 nm (gray circles), where the charged headgroups are located.…”

Osmotic Transport at the Aqueous Graphene and hBN Interfaces: Scaling Laws from a Unified, First-Principles Description

“…Throughout this work we thus assume that continuum hydrodynamics is valid at the nanoscale and that the viscosity is homogeneous. While on hydrophilic and charged surfaces, osmotic flows are better described by assuming a larger viscosity in a subnanometric interfacial layer, , on hydrophobic, slipping surfaces such as the ones considered in this work, our assumptions have been shown to provide an accurate description of the velocity profiles even in the first molecular layers of the liquid …”

“…The ions’ density and the electrostatic potential profiles appearing in eqs – are determined from the solution of the Poisson–Boltzmann equation, , which is used to describe the EDL near electrified interfaces and is modified to include the free energy of ion adsorption, , here computed from first-principles simulations: where the dimensionless free energies of ion adsorption g ± ( z ) = Δ G ± ( z )/( k B T ) are the key terms that distinguish eq from the standard PB description of the EDL, and which importantly enable the possibility for nonzero solutions of eq even in the absence of charged surfaces, as is the case in our aqueous graphene and hBN interfaces. Although the form of eq assumes a constant value of the dielectric constant ε, we have also considered its possible spatial dependence at the interface , using a step model, , and we have computed the transport coefficients within this model in Figure S4. Whereas the electro-osmotic and diffusio-osmotic coefficients are not affected by the particular choice made for ε, the magnitude of the diffusio-osmotic conductivity is altered depending on the model used for the dielectric constant; still, the scaling as a function of concentration and the change of sign remain the same, thus not affecting the conclusion of our work (see the Supporting Information).…”

Specific Ion Effects in Different Media: Current Status and Future Challenges

“…Herewith, one can empirically introduce the missing polarizability of ions in MD simulations, such that the reduced charge of an ion can be written as with the dielectric constant ϵ r of the solution as consequence of polarization and charge transfer effects. , In addition to such static continuum approaches, also fully polarizable force fields in terms of Drude oscillators or bead–spring models for atomistic and coarse-grained MD simulations were introduced. − Thus, all of these approaches focus on fluctuating charge behavior which underlines the crucial role of ion–solvent interactions. In summary, one can conclude that the electronic polarizability in combination with charge transfer effects are important contributions for a more reliable description of specific ion effects in terms of the binding behavior, pairing effects, and ion distributions. − Moreover, it was also discussed that charges in solution introduce a locally varying dielectric permittivity which further affects the electrostatic interactions. , The corresponding higher order arrangement of the solvent molecules changes the local viscosity of the solution, such that also the dynamic behavior of the species is modified . Because of the strong influence of the aforementioned local interactions, it becomes clear that simple mean field electrostatic theories are not sufficient to explain such observations.…”