Ions and water molecules in an electrolyte solution in contact with charged and dipolar surfaces

• Orientational ordering of finite sized water dipoles in electrolyte in saturation regime.
• Analytical models of spatial variation of permittivity in electrolyte solution.
• Differential capacitance of electric double layer and water ordering.
• Osmotic pressure of the electrolyte solution between charged surfaces.
• Attractive interactions between like-charged surfaces mediated by macroions.

The electrolyte-charged surface interface is described within the Langevin–Poisson–Boltzmann (LPB) and Langevin–Bikerman models. It is shown that in the saturation regime close to the charged surface, water dipole ordering and depletion of water molecules may result in a strong local decrease of permittivity. Analytical expressions for the space dependence of relative permittivity are derived for both models. The differential capacitance as a function of the surface potential is calculated within the modified Langevin–Bikerman model and compared to the prediction of the classical Gouy–Chapman theory. As an example of the application of the models described, a zwitterionic lipid surface with non-zero dipole moments in contact with an electrolyte solution of monovalent salt ions and water dipoles is studied within the LPB model. An analytical expression for the osmotic pressure of the electrolyte solution between the zwitterionic lipid surface and a charged particle (macroion) is derived. Some of the predictions of the described electric double layer mean-field theoretical considerations are evaluated using the results of a molecular dynamics simulation. At the end a theoretical description of the possible origin of the attractive interactions between like-charged surfaces mediated by charged macroions with distinctive internal charge distribution is given.

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