Phase Behavior of the Anionic Surfactant [Bmim][AOT]-Stabilized Hydrophobic Ionic Liquid-Based Microemulsions and the Effect of <i>n</i>‑Alcohols

In this work, the fishlike phase diagram of the H2O/[Bmim]­[AOT]/[Bmim]­[PF6]/n-alcohol as a function of temperature (T) and the mass fraction of [Bmim]­[AOT] (with or without n-alcohol) in the total mixture (γ) has been observed for the first time at several mass ratios of [Bmim]­[PF6] to H2O (α) and with different n-alcohols. The larger area of the three-phase region occurs at α ≤ 0.500, and the resulting fish shapes are similar to each other. For a given α, a temperature scan (from lower to higher) at several γ values reveals that the present system forms an upper phase microemulsion first and then a lower phase microemulsion. The formation of hydrophobic ionic liquid-in-water (HIL/W) microemulsion at low temperature and water-in-hydrophobic ionic liquid (W/HIL) microemulsion at high temperature was confirmed by dynamic light scattering and small-angle X-ray scattering techniques. Here, the phase sequence that occurred during the temperature scan is opposite to that of a classic H2O/NaAOT/oil system. At the lower temperature, the H-bonding interaction is considered to be the main driving force for the aggregation; at the higher temperature, however, the main driving force may be the hydrophobic interaction. n-Alcohols with medium/long alkyl chains have a great influence on the fish-tail coordinates of the present systems. Compared with the ternary system without alcohol, the addition of n-alcohols (C4–C8) decreases the phase inversion temperature (T̃) and the surfactant efficiency. With the increase of the alkyl chain length of n-alcohols, however, the decrement in T̃ becomes smaller due to the increase of the interfacial rigidity. A comparison of these results with those obtained for the H2O/NaAOT/oil system indicates that there are some similarities and also some differences, depending on the relative density, polarity, or hydrophobicity among the HIL, oil, and n-alcohols. The above insight into the phase behavior of the present HIL-based system helps to formulate biocompatible HIL-based AOT-stabilized microemulsions which are important templates for the biosynthesis of conducting polymers