Specific ion effects in nonaqueous solvents: A copernican shift for specific ion effects

Specific ion effects describe the observation that ions influence physical, chemical, and interfacial properties of solutions in ways that cannot be explained solely by ionic charge or concentration. Historically, these effects were identified and extensively studied in aqueous systems, and described most notably in relation to the Hofmeister and lyotropic series. Over the past two decades, it has become increasingly clear that analogous ion-specific phenomena occur in non-aqueous solvents. These findings challenge traditional water-centric interpretations of the origins of specific ion effects and suggest that ion specificity arises fundamentally from the ions themselves and is perturbed by ion-solvent, ion-solute, ion-surface and ion-ion interactions. This article reviews experimental and theoretical studies of specific ion effects in non-aqueous solvents, examining how solvent properties modulate ion specific trends, how classical Hofmeister concepts and explanations extend beyond water, how ion-ion interactions influence ion specific trends and how emerging frameworks can rationalize ion specificity across diverse chemical systems – within limitations. As such we now have at hand useful semi-quantitative methods for understanding and predicting ion-specific effects, strategies for controlling ion specificity and conceptual frameworks upon which to build a more complete understanding of ion specificity in all solvents. We are still some distance from a widely applicable quantitative description of specific ion effects, but we have a better understanding of the boundaries of our present understanding. As such, studies in non-aqueous solvents have provided us with tools that are presently useful and a new perspective for future investigations and theories of ion-specificity.