Re-entrant direct hexagonal phases in a lyotropic system of surfactant induced by an ionic liquid

Abstract

In the present study, an ionic liquid (IL), known as green solvent, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF₄) with four carbons in the hydrophobic chain (C-4) has been used as an additive to modify the self-assembled aggregates of an anionic surfactant sodium dodecyl sulfate (SDS) and then the evolution of the liquid crystalline phases has been investigated. Small angle synchrotron x-ray diffraction study has revealed an unprecedented phase sequence where a hexagonal phase (H_ll) of direct cylindrical micelles is observed to evolve to another hexagonal phase (H_l) of, again, direct cylindrical micelles with a lamellar phase (L_a) as the intermediate. The rheological data and the theoretical calculations have confirmed the phases. To understand such a phase behaviour of the system, the work is extended by considering the ionic liquids with the same head group but of shorter (C-2) and longer (C-10) hydrocarbon chains compared to BMIM-BF4. While the shorter chain IL causes the hexagonal phase to form rectangular (R) phase, the longer chain is observed to induce the lamellar phase (L_a). The molecular mechanism of appearance of such different phases has been discussed.