Surface properties of fluids of charged platelike colloids

TL;DR

This study uses density functional theory to analyze how charged platelike colloids interact with charged surfaces, revealing complex wetting and drying behaviors influenced by electrostatic and hard-core interactions.

Contribution

It extends a recent density functional to model charged platelike colloids, providing new insights into surface phase diagrams and electrostatic potential profiles.

Findings

First-order wetting for small macroion charges and isotropic bulk

First-order drying for large macroion charges and nematic bulk

Crossover from monotonic to non-monotonic electrostatic potentials

Abstract

Surface properties of mixtures of charged platelike colloids and salt in contact with a charged planar wall are studied within density functional theory. The particles are modeled by hard cuboids with their edges constrained to be parallel to the Cartesian axes corresponding to the Zwanzig model and the charges of the particles are concentrated in their centers. The density functional applied is an extension of a recently introduced functional for charged platelike colloids. Analytically and numerically calculated bulk and surface phase diagrams exhibit first-order wetting for sufficiently small macroion charges and isotropic bulk order as well as first-order drying for sufficiently large macroion charges and nematic bulk order. The asymptotic wetting and drying behavior is investigated by means of effective interface potentials which turn out to be asymptotically the same as for a suitable neutral system governed by isotropic nonretarded dispersion forces. Wetting and drying points as well as predrying lines and the corresponding critical points have been located numerically. A crossover from monotonic to non-monotonic electrostatic potential profiles upon varying the surface charge density has been observed. Due to the presence of both the Coulomb interactions and the hard-core repulsions, the surface potential and the surface charge do not vanish simultaneously, i.e., the point of zero charge and the isoelectric point of the surface do not coincide.