Mixing scheme of an aqueous solution of tetrabutylphosphonium trifluoroacetate in the water-rich region

Abstract

We have studied the mixing scheme of an aqueous solution of ionic liquid tetrabutylphosphonium trifluoroacetate, [P_4,4,4,4]CF₃COO, in the water-rich region. The mixture shows phase separation with a lower critical solution temperature. To learn how the solute [P_4,4,4,4]CF₃COO species interact with each other in the dilute region, the third derivative quantities of Gibbs energy in terms of enthalpy, H^E_IL–IL, and volume, V^E_IL–IL, are evaluated. In the present study using up to the third derivative quantities, we conclude that [P_4,4,4,4]CF₃COO turns out to be an extremely strong hydrophobe. This finding may hint that the present IL is not dissociated in this water-rich region, despite the large dielectric permittivity of solvent water and as such it works as an extremely strong hydrophobe. An earlier 1-propanol (1P) probing study showed that [P_4,4,4,4]⁺ is a significant amphiphile with a strong hydrophobicity and an equally strong hydrophilicity, and CF₃COO⁻ is a modest amphiphile. However, the 1-propanol (1P) probing methodology effectively uses the fourth derivative of G, and is capable of providing deeper information on the effect of the test solute on H₂O. To reconcile the present conclusion with the earlier observations described above, we propose to conduct the 1P-probing of the ionic liquid salt, [P_4,4,4,4]CF₃COO.