Shurui MIAO1, Ingo HOFFMANN2, Michael GRADZIELSKI3, Gregory WARR1
1School of Chemistry and Sydney Nano Institute, The University of Sydney, Sydney, Australia
2Institut Max von Laue-Paul Langevin, Grenoble, France
3Stranski-Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie, Technische Universität Berlin, Berlin, Germany
Ionic liquids (ILs) are a novel class of solvents with ultra-low vapour pressures and tunable liquid properties. Among them, protic ionic liquids (PILs) are particularly effective solvents for the self-assembly of surfactants and lipids into micelles, vesicles, liquid crystals, and microemulsions.[1-3] This is exemplified by PILs, which are cheap and easily prepared. Over the past decade, much has been learnt about the time-averaged structure of many PILs.[4] Some combinations of ions can yield bicontinuous networks of polar/charged and apolar moieties, rendering the solvent to be nanoscopically heterogeneous. Although such nanostructures have been extensively characterised, virtually nothing is known about their dynamics, or how the underlying nanostructure of the IL solvent affects the dynamics of amphiphile aggregates. In this work, we aim to reveal the effect of solvent nanostructure on the flexibility of lipid bilayers. We employed neutron spin echo spectroscopy (IN15, ILL) to probe the mesoscopic undulation of egg-PC vesicle bilayer membranes in ethylammonium formate (EAF), ethanolammonium formate (EtAF), and their water mixtures. For the first time, IN15 is configured to perform dynamic measurements of lipid bilayers up to near 1000 ns, which is essential due to the much slower motion we observed in PILs.[5] By comparing these ILs and water, we can elucidate the dominant factor that affects membrane stiffness and stability in ILs, and how IL nanostructure contributes to the dynamics of large amphiphile aggregates.
[1] M.U. Araos, G.G. Warr, Self-Assembly of Nonionic Surfactants into Lyotropic Liquid Crystals in Ethylammonium Nitrate, a Room-Temperature Ionic Liquid, The Journal of Physical Chemistry B, 109 (2005) 14275-14277.
[2] S.J. Bryant, K. Wood, R. Atkin, G.G. Warr, Effect of protic ionic liquid nanostructure on phospholipid vesicle formation, Soft Matter, 13 (2017) 1364-1370.
[3] S.J. Bryant, R. Atkin, M. Gradzielski, G.G. Warr, Catanionic Surfactant Self-Assembly in Protic Ionic Liquids, J. Phys. Chem. Lett., 11 (2020) 5926-5931.
[4] R. Hayes, G.G. Warr, R. Atkin, Structure and Nanostructure in Ionic Liquids, Chemical Reviews, 115 (2015) 6357-6426.
[5] S. Miao, I. Hoffmann, M. Gradzielski, G.G. Warr, Lipid Membrane Flexibility in Protic Ionic Liquids, The Journal of Physical Chemistry Letters, 13 (2022) 5240-5245.