The many origins of charge inversion in electrolyte solutions: effects of discrete interfacial charges

TL;DR

This paper explores the universal phenomenon of charge inversion in electrolyte solutions, identifying three electrostatic mechanisms and providing detailed molecular dynamics analysis of complexation effects.

Contribution

It introduces a comprehensive framework for understanding charge inversion origins, including detailed molecular dynamics simulations of complexation regimes.

Findings

Charge inversion occurs due to three main electrostatic mechanisms.

Molecular dynamics simulations reveal detailed complexation behavior.

Different regimes apply depending on ionic and interfacial conditions.

Abstract

We show that charge inversion, i.e. interfacial charges attracting counterions in excess of their own nominal charge, is a general effect that takes place in most charged systems next to aqueous solutions with multivalent ions and identify three different electrostatic origins for this effect 1) counterion-counterion correlations, 2) correlations between counterions and interfacial charges and 3) complexation. We briefly describe the first two regimes and provide a detailed characterization of the complexation regime from united atom molecular dynamics simulation of a phospholipid domain in contact with an aqueous solution. We examine the expected conditions where each regime should apply and describe a representative experimental example to illustrate each case. We point out that our results provide a characterization of ionic distributions irrespectively of whether charge inversion takes place and show that processes such as proton release and transfer are also linked to ionic correlations. We conclude with a discussion of further experimental and theoretical implications.