Mesoscopic theory for ionic liquids and concentrated electrolytes in a slit with charged walls

Abstract

Concentrated electrolytes and ionic liquids confined between parallel charged plates are studied within a theory combining density functional and field-theoretic methods, extended in this work to the slit geometry. We obtain charge- and density profiles, as well as the excess grand potential as a function of the slit width L in the self-consistent Gaussian approximation. For large density of ions, the obtained disjoining pressure exhibits oscillatory asymptotic decay for increasing L, with the decay length that agrees with the results of recent experiments. On a general level, we show that if the charge density decays with a distance from a charged surface in the oscillatory way, then the electrostatic energy of the slit with charged walls also decays with L in an oscillatory way with the same characteristic lengths. The oscillatory decay disagrees with the asymptotic repulsion obtained in the surface force balance experiments. We argue that in the experiment, the lack of the attraction between the confining surfaces for large L may follow from slow ordering of the isotropic fluid absorbed in the expanding slit into layers with alternating charge.