Using the solvation parameter model to characterize functionalized ionic liquids containing the tris(pentafluoroethyl)trifluorophosphate (FAP) anion

Ionic liquids (ILs) containing the tris(pentafluoroethyl)trifluorophosphate anion [FAP](-) have attracted increased attention due to their unique properties including ultrahigh hydrophobicity, hydrolytic stability, and wide electrochemical window. In this study, the solvation parameter model is used via gas chromatography to characterize the solvation interactions of seven ILs containing amino, ester, and hydroxyl functional groups appended to the cation and paired with [FAP](-), as well as three ILs containing the bis[(trifluoromethyl)sulfonyl]imide anion [NTf2](-). The role of the functional groups, nature of the counter anion, and cation type on the system constants were evaluated. ILs containing [FAP](-) possessed lower hydrogen bond basicity than NTf2-based ILs having the same cationic component; in the case of hydroxyl-functionalized cations, the presence of [FAP](-) led to an enhancement of the hydrogen bond acidity, relative to the NTf2-analogs. The system constants support the argument that [FAP](-) weakly coordinates the cation and any appended functional groups, promoting properties of the cation which might be masked by stronger interactions with other anion systems. The chromatographic performance of the IL stationary phases was evaluated by examining the retention behavior and separation selectivity for chosen analytes. The results from this work can be used as a guide for choosing FAP-based ILs capable of exhibiting desired solvation properties while retaining important physical properties including high thermal stability and high hydrophobicity.