Attraction between Like Charged Surfaces Mediated by Uniformly Charged Spherical Colloids in a Salt Solution

Abstract

Like-charged macromolecules repel in electrolyte solutions that contain small (i.e. point-like) monovalent co- and counterions. Yet, if the mobile ions of one species are spatially extended instead of being point-like, the interaction may turn attractive. This effect can be captured within the mean-field Poisson-Boltzmann framework if the charge distribution within the spatially extended ions is accounted for. This has been demonstrated recently for rod-like ions. In the present work, we consider an electrolyte solution that is composed of monovalent point-like salt ions and uniformly charged spherical colloids, sandwiched between two planar like-charged surfaces. Minimization of the mean-field free energy yields an integral-differential equation for the electrostatic potential that we solve numerically within the linear Debye-Huckel limit. The free energy, which we calculate from the potential, indeed predicts attractive in- teractions for sufficiently large spherical colloids. We derive an approximate analytical expression for the critical colloid size, above which attraction between like-charged surfaces starts to emerge. (doi: 10.5562/cca1824)