Image-charge forces in thin interlayers due to surface charges in electrolyte

Abstract

The surface forces arising in wetting films of nonpolar liquids or in thin air interlayers between an electrolyte and a nonpolar medium in the case of discrete charging of the dielectric-electrolyte interface are considered. The contributions of polarization effects to the distribution of the electrostatic potential in the three contacting media were calculated. Within the Debye-Hückel approximation, the analytical solutions were derived for the disjoining pressure in thin films, for the case of either dilute or relatively concentrated electrolyte solutions in the aforementioned systems. Analysis of the analytical and numerical results demonstrated that for dilute solutions the contribution of image forces to the disjoining pressure may significantly exceed the van der Waals forces for films from a few to tens of nanometers thick.

Image Charge Effects in the Wetting Behavior of Alkanes on Water with Accounting for Water Solubility

“…The distribution of potentials in contacting media, induced by each type of electric source, was obtained on the basis of simultaneous solution of the Poisson equations in three contacting media, as presented in [2,3,18]. …”

“…For water or diluted 1-1 electrolyte aqueous solutions, when inverse Debye length κ << 1/ z 0 , the potential at arbitrary point $z,\rho$ in the film, induced by a single point charge q and its image charges can be written within the Debye-Hückel approximation as [3]: $\phi false(z,\rho false)=\frac{2q}{{\epsilon }_2}true\sum _{k=0}^{\infty }{false(-{\beta }_{13}false)}^k\left[\frac{1}{{\left({\rho }^2+{false(z+2khfalse)}^2\right)}^{+1\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}+\frac{{\beta }_{13}}{{\left({\rho }^2+{false(z-2z_0-2false(k+1false)hfalse)}^2\right)}^{+1\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}\right]$ where ${\beta }_{13}=false({\epsilon }_1-{\epsilon }_{3}false)/false({\epsilon }_1+{\epsilon }_{3}false)$. Note, that the Equation (1) is accurate for lateral distances $\rho <<1/\kappa$.…”

“…Considerable attention in recent studies was paid to experimental and theoretical studies on wetting of the surface of water and aqueous solutions by hydrocarbons and to the analysis of stability of alkane wetting films [1,2,3,4,5,6,7,8,9]. …”

“…Earlier it was suggested in the literature [10] that, in some cases, the coexistence between the droplet and thin liquid film (the pseudo partial wetting) can be explained by invoking the short-range forces of a different nature, in addition to van der Waals forces. In our previous theoretical studies [2,3] we have shown that some of the peculiarities of wetting of water and brine solutions by alkanes from pentane to octane can be described by taking into account the image charge forces. The solubility of water in alkanes, although small, gives rise to the formation of a water-enriched adsorption layer at the interfaces of alkane films.…”

“…The interaction of dipoles of water molecules in the adsorbed layer with their own images in confining phases and/or with discrete surface charges and their images contributes to the abovementioned image charge forces. The great importance of accounting the discreteness of surface charge was widely discussed in the literature when considering the electric potential distribution near charged interfaces, behavior of particles in electric fields, colloid aggregation, particle deposition, and adsorption phenomena [2,3,4,11,12,13,14,15,16,17,18,19,20,21]. Particularly, the polarization of alkane film boundaries by the electric field of dipoles of water dissolved in the film results in an appearance of image charge interaction between water molecules and film surfaces.…”