Interactions between Zwitterionic Membranes in Complex Electrolytes

TL;DR

This study explores how zwitterionic membranes interact in mixed electrolytes, revealing salt-dependent forces and transitions influenced by multivalent ions and membrane surface properties, with implications for membrane engineering.

Contribution

The paper introduces a formalism that captures electrostatic interactions of zwitterionic membranes in complex electrolytes, including correlation effects and membrane binding transitions.

Findings

Monovalent salt is essential for finite electrostatic forces.

Salt concentration modulates membrane pressure non-uniformly.

Multivalent cations induce attractive interactions and binding transitions.

Abstract

We investigate the electrostatic interactions of zwitterionic membranes immersed in mixed electrolytes composed of mono- and multivalent ions. We show that the presence of monovalent salt is a necessary condition for the existence of a finite electrostatic force on the membrane. As a result, the mean-field membrane pressure originating from the surface dipoles exhibits a non-uniform salt dependence, characterized by an enhancement for dilute salt conditions and a decrease at intermediate salt concentrations. Upon addition of multivalent cations to the submolar salt solution, the separate interactions of these cations with the opposite charges of the surface dipoles makes the intermembrane pressure more repulsive at low membrane separation distances and strongly attractive at intermediate distances, resulting in a discontinuous like-charge binding transition followed by the membrane binding transition. By extending our formalism to account for correlation corrections associated with large salt concentrations, we show that membranes of high surface dipole density immersed in molar salt solutions may undergo a membrane binding transition even without the multivalent cations. Hence, the tuning of the surface polarization forces by membrane engineering can be an efficient way to adjust the equilibrium configuration of dipolar membranes in concentrated salt solutions.