Interaction between Heterogeneously Charged Surfaces: Surface Patches and Charge Modulation

When solid surfaces are immersed in aqueous solutions, some of their charges can dissociate and leave behind charge patches on the surface. Although the charges are distributed heterogeneously on the surface, most of the theoretical models treat them as homogeneous. For overall non-neutral surfaces, the assumption of surface charge homogeneity is rather reasonable, since the leading terms of two such interacting surfaces depend on the non-zero average charge. However, for overall neutral surfaces, the nature of the surface charge distribution is crucial in determining the inter-surface interaction. In the present work we study the interaction between two charged surfaces across an aqueous solution for several charge distributions. The analysis is preformed within the framework of the linearized Poisson-Boltzmann theory. For periodic charge distributions the interaction is found to be repulsive at small separations, unless the two surface distributions are completely out-of-phase with respect to each other. For quenched random charge distributions we find that due to the presence of the ionic solution in between the surfaces, the inter-surface repulsion dominates over the attraction in the linear regime of the Poisson-Boltzmann theory. The effect of quenched charge heterogeneity is found to be particularly substantial in the case of large charge domains.