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

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.