Finite-thickness-enhanced attractions for oppositely charged membranes and colloidal platelets

Within linearized Poisson-Boltzmann theory, we study the disjoining pressure of two oppositely charged parallel objects (membranes and colloidal platelets) in a 1:1 electrolyte, with a focus on the effects of their finite thickness. This extension of the standard Gouy-Chapman model from an interacting pair of double layers to a quartet (one on each side of the two interacting objects) is shown to enhance the regime of attractive interactions significantly, in particular, when the separation and the thickness are on the order of the Debye length of the solvent, provided the dielectric mismatch between objects and solvent is not too extreme. The enhancement of attractions occurs for objects with fixed charge as well as for those that exhibit charge regulation but not for those with a constant surface potential. The underlying mechanism for this enhancement for thin objects is the transfer of net ionic charge from the electrolyte in between to the other sides. For biological membranes in water, this effect is small; however, it is due to strong image charge effects.