Electric contribution to the surface tension of a dielectric liquid containing ions

We evaluate the ionic contribution to the surface tension of a liquid. In our analysis we assume that the sample is in the shape of a slab and that the limiting surfaces adsorb, selectively, ions of a given sign. It is shown that the selective ion adsorption is responsible for a non-homogeneous distribution of ions across the sample, giving rise to a spatially-dependent electric field. The electrostatic energy relevant to the electric field is localized in two surface layers, whose thickness is comparable with Debye's screening length. It can be considered a surface energy of electrostatic origin, that renormalizes the true surface tension. We show that the surface energy due to the ions separation depends on the thickness of the sample and that the effective surface tension of a liquid containing ions is not only a surface property. The predictions of our model are compared with the experimental data relevant to a nematic liquid crystal. The agreement between theory and experiment is reasonably good.