Netsu Sokutei, 42 (3), p. 90, (2015)



Hydrophobicity/Hydrophilicity of Constituent Ions of Ionic Liquids Based on Differential Thermodynamics

Some aqueous solutions of ionic liquids exhibit liquid-liquid phase transition with upper or lower critical solution temperatures. Aqueous solutions of [C4mim]BF4 and [P4444]CF3COO are examples for the solutions which show UCST-type and LCST-type phase transitions, respectively. Hydrophobicity and hydrophilicity of the constituent ions of the ionic liquids can be characterized in terms of their effects on molecular organization of H2O based on the 1-propanol probing methodology. The contributions of hydrophobicity/hydrophilicity are evaluated based on the third derivatives of the Gibbs energy. Amphiphilicity is defined as combination of the effects of hydrophobicity and hydrophilicity. Constituent ions composed of ionic liquids basically show amphiphilicity with strongly hydrophobic and equally hydrophilic contributions. The self-aggregation behavior of the cations in aqueous solutions is also discussed based on the third derivatives of the Gibbs energy. Furthermore, we introduce the fluctuation of mixing state in aqueous solutions of the ionic liquid, such as aqueous solution of [P4444]CF3COO. Aggregation aspect in the solutions is discussed in terms of inhomogeneity of molecular distribution. It is suggested that the evaluated hydrophobicity/hydrophilicity could be related to the mixing state even in the higher concentration range.