A mixture theory of electrolytes containing solvation effects

•New continuum mixture theory including solvation effects.•Pressure and potential dependence of the ionic charge density in equilibrium.•Prediction of the space charge layer structure at the metal/electrolyte interface.•New definition of the Stern layer•Comparison to Poisson-Boltzmann- and ideal incompressible mixture-models.

In this work we present a new mixture theory of a liquid solvent containing completely dissociated ions to study the space charge layer of electrolytes in contact with some inert metals. We incorporate solvation shell effects (i) in our derivation of the mixing entropy and (ii) in the pressure model. Chemical potentials of ions and solvent molecules in the incompressible limit are then derived from a free energy function. For the thermodynamic equilibrium the coupled equation system of mass and momentum balance, the incompressibility constraint and the Poisson equation are summarized. With that we study the space charge layer of the electrolytic solution for an applied half cell potential and compare our results to historic and recent interpretations of the double layer in liquid electrolytes. The novelties of the model are: (i) the potential and pressure dependence of the free charge density in equilibrium (ii) the calculation of entropic contributions due to solvated ions, and (iii) the natural prediction of a solvated anion saturation layer which we finally reinterpret as Stern layer.

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