(Invited) Double Salt Ionic Liquids Prepared by Mixing Partially Miscible Ionic Liquids: Tuning the Solubility of Lipophilic Molecules

Over the past twenty years, ionic liquids (ILs) have received much attention as interesting new solvents which can offer unique properties often unavailable in traditional molecular solvents.  Now, “IL/IL mixtures”, prepared by mixing two or more ILs, and their specific physicochemical properties are attracting increasing interest from the scientific community.  It is surprising that quite a few papers describing the properties of “IL/IL mixtures” tend to view these systems simply as mixtures rather than unique ionic compositions, even when unexpected properties are observed.  We have found that physical properties of “IL/IL mixtures”, such as viscosity, density, excess molar volume, etc., seem to be mainly controlled by the composition and the intrinsic ion interactions, and thus any ideal or non-ideal behavior depends on the particular property studied, the nature of the ions, the abundance of each ion, as well as competitive interactions (e.g., Coulombic interactions and hydrogen bonding) between the ions.  These “IL/IL mixtures” can be considered to be liquid analogs of double salts, salts containing more than one cation or anion, which are known to exhibit unexpected associations that yield non-ideal properties.  Based on this similarity, we propose that two or more ILs, when mixed homogeneously, should be viewed as “Double Salt Ionic Liquids” (DSILs) and should be defined by the specific ion combinations present, rather than considered as only a “mixture” of two ILs.  Solubility studies with molecular solvents and active pharmaceutical ingredients in DSILs demonstrate that solubility of selected solutes (protic, aprotic, acid, base, etc.) in the DSILs can be finely tuned via changes in the ion compositions.  These studies support the concept that each DSIL is a unique ionic fluid that is able to provide tunable solvent property sets derived from the ions present and that these properties can be controlled by choosing the nature and the abundance of the ions.