Specific-ion effects in non-aqueous systems

It is widely acknowledged that specific-ion effects are ubiquitous in aqueous systems and undoubtedly are essential to the fundamental processes of life, although a deep fundamental understanding of specific-ion effects remains an important challenge. Specific-ion effects in non-aqueous solvents are known but have attracted far less attention, yet knowledge of specific-ion effects in non-aqueous systems is likely to provide important information for guiding, evaluating and testing our theories of specific-ion effects. Here, the literature on specific-ion effects in non-aqueous solvents is surveyed with a view to determining if the Hofmeister series or lyotropic series so universally observed in aqueous systems is widely evident in non-aqueous systems. Particular attention has been applied to experiments on non-aqueous systems that are known to exhibit Hofmeister series in aqueous systems with the aim of determining if a consistent ion ordering in the strength of specific-ion effects is observed in other solvents. We find that specific-ion effects are ubiquitous in non-aqueous solvents, that both Hofmeister and lyotropic series are widely observed, although not necessarily for the same class of experiment. Moreover, we find that Hofmeister and lyotropic series are observed in non-aqueous solvents even for experiments in which these series are not observed for water. Additionally, series reversal is seen for a given experiment when the solvent is changed. All this poses significant challenges for our understanding of specific-ion effects in aqueous and non-aqueous systems and also provides guideposts for future investigations.