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_I) of direct cylindrical micelles is observed to evolve to another hexagonal phase (H_II) of, again, direct cylindrical micelles with a lamellar phase ($L_{\alpha }$) 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 ($L_{\alpha }$). The molecular mechanism of appearance of such different phases has been discussed.

Graphical Abstract

KEYWORDS:

• Lyotropic liquid crystal
• surfactant
• ionic liquids
• rheology
• small angle x-ray diffraction

Acknowledgements

S. M acknowledges the financial assistance of the Department of Science and Technology (DST), India through INSPIRE fellowship and S. K. G acknowledges the financial support received from UGC-DAE CSR (Mumbai Centre). We also thank DST for financial support and Saha Institute of Nuclear Physics (SINP) for facilitating the experiments at the Indian Beamline, Photon Factory, KEK, Japan. S. M and S. K. G are thankful to Prof. Rupamanjari Ghosh and Prof. Sankar Dhar for their academic support and encouragement.

Disclosure statement

No potential conflict of interest was reported by the authors.