Study of micelles and microemulsions formed in a hydrophobic ionic liquid by a dielectric spectroscopy method. I. Interaction and percolation

Abstract

Dielectric measurements were carried out for binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and Triton X-100 (TX-100, a nonionic surfactant with a polyoxyethylene chain), and [bmim][PF₆]/TX-100/water ternary systems in a wide frequency range, to study the molecular interactions and percolation in these systems. Striking dielectric relaxations were observed. The dc conductivity data (obtained from the total dielectric loss spectra) have interesting dependencies on the variation of sample composition. In TX-100/[bmim][PF₆] solutions, the dependence of dc conductivity on volume fraction of TX-100 was analyzed in light of the Bruggeman's effective-medium approximation, which indicates that the number of imidazolium cations associated with every TX-100 molecule is ten. The water-in-IL, bicontinuous, and IL-in-water micro-regions of the microemulsions were identified by the dependence of dc conductivity on water mass fraction. The dc conductivity data were partly explained by the percolation theory, which suggests that a static percolation occurs in this hydrophobic IL microemulsion. When the mass concentration of water is more than 80 wt%, dc conductivity linearly decreased with the increase of water concentration, which implies that [bmim][PF₆] may dissolve in water rather than forming an ionic liquid micro-pool. The dependencies of dc conductivity as a function of IL-to-TX-100 molar ratios in three different sub-regions were explained by the microscopic interaction mechanism, which infers that the hydrophilicity of poly(oxyethylene) chain in TX-100 is stronger than its IL-loving nature, and the microemulsions are “softer” in the W/IL micro-region.