Relative hydrophobicity of equilibrium phases in biphasic systems (ionic liquid plus water)

Abstract

Partition coefficients for a series of dinitrophenylated (DNP) amino acids in biphasic systems composed of hydrophobic ionic liquids and water were experimentally determined. The ionic liquids used were three1-alkyl-3-methylimidazolium tetrafluoroborates, [C(n)mim][BF4], with alkyl chain substituents hexyl, octyl, and decyl. The liquid-liquid phase diagram for the system ([C(10)mim][BF4] + water) was experimentally determined. DNP amino acids distribute preferentially to the IL-rich phase and ([C(10)mim][BF4] + water) was found to be the system with the lowest partition coefficients for the solutes studied. The experimental partition coefficients decrease as the size of the alkyl side chain in the ionic liquids increases. The free energy of transfer of a methylene group between phases was calculated through the partition coefficients, which provides a measure of the relative hydrophobicity of the equilibrium phases. It was found that the system ([C(10)mim][BF4] + water) presents a lower free energy (and thus a lower relative hydrophobicity) than the system ([C(8)mim][BF4] + water). In order to better understand this result, the micellar behavior of the three ionic liquids was studied. Electrical conductivities of several aqueous solutions of the ionic liquids were measured to determine the critical micelle concentration (CMC) and the degree of micelle ionization, alpha, of the three ionic liquids. From these two properties it was possible to obtain the free energy of micellization, Delta G(mic), for the ionic liquids.