The Electrosolvation Effect: The Medium Is the Message

It is a well-known principle that like charges repel, whether positive or negative. However, experiments on electrically charged matter suspended in liquids have frequently reported the opposite: attraction between like-charged particles, rather than repulsion. Experiments have also shown that the sign of the force at large separations can depend on the sign of the particle charge, e.g., in water negative particles attract, while positives repel as expected, and this trend generally reverses in polar organic solvents such as alcohols. These observations cannot be reconciled with standing theories of the electrostatic interaction that regard the intervening solvent medium as a structureless, featureless dielectric continuum. Recent work has called for the traditional continuum view of the medium to be relinquished, attributing the observed attraction to an "electrosolvation force" - a term that describes interactions bearing electrical signatures that depend on the properties and structuring of the molecular solvent close to an interface. Here we discuss a range of recent experiments that shed light on the mechanistic underpinnings of this previously unanticipated interaction. We also discuss developments on the theoretical front that not only capture the observed attraction between like-charged particles but also recover the canonical monotonic repulsion - a hallmark of standing continuum-dielectric theories. Thus, the traditionally anticipated repulsion represents a distinct and important regime within a general theoretical framework that accounts for the solvent in substantially greater detail. Given the fundamental importance of the forces that drive structure and organization from molecular and particulate building blocks, this emerging discovery is likely to have broad implications across a wide range of fields. Interface-governed properties that generate long-range signatures in fluids may also prove relevant to electrokinetic and electrochemical processes.