Overcharging and charge inversion: Finding the correct explanation(s)

Both overcharging and charge inversion denote a general observation that the sign of a surface charge can flip in the presence of interacting species such as surfactants, polyelectrolytes, proteins and multivalent ions. Moreover, charge inversion of proteins through charge regulation, is one explanation for protein adsorption to similarly charged surfaces. While overcharging and charge inversion have been long studied, the explanations for these phenomena are often still debated. Broadly these explanations can be categorized as "chemical" where specific attractive interactions are seen as the cause of charge inversion, and "physical" where purely electrostatic interactions and constraints of geometry are used as explanation. In this review, charge inversion is discussed from a very broad viewpoint, where we draw connections between the various explanations proposed for very different systems. Especially, we highlight the work of Johannes Lyklema, who always carefully balanced between the competing chemical and physical explanations, and demonstrated that only few experimental systems allow just a single explanation.