Destructed double-layer and ionic charge separation near the oil-water interface

TL;DR

This paper investigates how ionic charge separation and double-layer destruction occur near oil-water interfaces in colloidal suspensions, revealing effects on colloidal adsorption and droplet crystallization driven by ion partitioning and dielectric properties.

Contribution

It introduces a modified Poisson-Boltzmann theory accounting for image charges and ion partitioning, explaining recent experimental observations of charge behavior at oil-water interfaces.

Findings

Ion aversion to oil destroys colloidal double layers.

Salt concentration influences colloidal adsorption behavior.

Water-in-oil droplets can crystallize due to charge accumulation.

Abstract

We study suspensions of hydrophobic charged colloidal spheres dispersed in a demixed oil-water mixture by means of a modified Poisson-Boltzmann theory, taking into account image charge effects and partitioning of the monovalent ions. We find that the ion's aversion for oil can destroy the double layers of the oil-dispersed colloids. This affects the salt-concentration dependence of the colloidal adsorption to the oil-water interface qualitatively. The theory also predicts a narrow range of the oil-dielectric constant in which micron-sized water-in-oil droplets acquire enough charge to crystallize at volume fractions as small as $\sim 10^{-3}$ in the absence of colloids. These findings explain recent observations [M.E. Leunissen {\em et al.}, Proc. Nat. Ac. Sci {\bf 104}, 2585 (2007)].