Relative hydrophobicity and hydrophilicity of some "Ionic liquid" anions determined by the 1-propanol probing methodology: A differential thermodynamic approach

The excess partial molar enthalpy of 1-propanol (1P), H-1P(E), was experimentally measured in ternary 1P-[NaPF6, NaCF3SO3 (OTF) or NaN(SO2CF3)(2) (TFSI)]+H2O system. From the H-1P(E) data, the enthalpic 1P-1P interaction function, H-1P-1P(E), which is the compositional derivative of H-1P(E), was evaluated graphically. On H-1P-1P(E) showed a characteristic change. This induced addition of the Na salt, the x(1P)-dependence pattern of H-1P-1P(E) change is used as a probe to elucidate the effect of the sample Na-salt on H2O. Because we know the effect of Na+ from our previous work, we show that each anion works as an amphiphile with hydrophobic and hydrophilic effects. Furthermore, the present method can quantify its relative hydrophobicity and hydrophilicity separately. The results indicate that the relative hydrophobicity ranking was in the order of TFS1(-) > PF6- approximate to OTF-, and the hydrophilicity TFSI- > PF6- > OTF-. Namely, TFSI- is the strongest amphiphile with the strongest hydrophobicity and the strongest hydrophilicity among the ionic liquid (IL) anions studied here. Using our earlier similar studies for normal ions, we map their relative hydrophobicity/hydrophilicity scales on a two-dimensional map together with those of the IL ions. The resulting map shows that the typical constituent ions for "ionic liquids" are strong amphiphiles; with more strongly hydrophobic and more strongly hydrophilic propensities than normal ions. Although the number of data points is limited, the melting points of ionic liquids consisting of TFSI- with the strongest hydrophobicity and the strongest hydrophilicity within the anions studied here are the lowest.