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Non-equilibrium electric double layers at the interface between two electrolytes

The charge distribution at the interface between two electrolytes is studied for the case of non-vanishing ion fluxes. The analysis is an extension of the established Verwey-Niessen theory to non-equilibrium situations. Applying matched asymptotic expansions for the region of the electric double layer and the bulk electrolytes, analytical expressions for the ion concentrations and the electrostatic potential are derived. It is found that the electric double layer is surprisingly stable. Even in cases where the applied electric field is so strong that it completely cancels the field of the charge clouds at the interface, two opposing space charge regions are formed. This phenomenon is qualitatively explained using a simple model incorporating ion partitioning and diffusive ion transport. Another notable phenomenon is that the interface acquires a net charge in the presence of an applied electric field, being of relevance for the hydrodynamic stability of interfacial flows.