Charging and Softening, Collapse, and Crystallization of Dipolar Phospholipid Membranes by Aqueous Ionic Liquid Solutions

Ionic liquids have a variety of unique controllable structures and properties. These properties may be used to tailor the self-assembly of charged and dipolar biomolecules. Using solution X-ray scattering, we measured the structure of Dilauryl(C-12:0)-sn-glycero-3-phospho-l-choline (DLPC), a dipolar (or zwitterionic) lipid, in the water-soluble room temperature ionic liquid Ethyl Methyl Imidazolium Ethyl Sulfate (EMIES) and mixtures of EMIES and water. We find that the interaction between the lipid bilayers is dominated by the balance between the charging of the polar headgroups by the ionic liquid, softening of the bilayer, and the osmotic pressure induced by the solvent. This balance leads to the following changes with increasing ionic liquid concentration: an incomplete unbinding transition from an attractive regime to a swollen regime of the lamellar phase formed by the bilayers. The swollen phase is followed by a collapse of the bilayers into a highly desolvated lamellar phase at some critical EMIES concentration, and eventually formation of lipid-crystalline phase, at very high EMIES concentrations. The latter phase is revealed by wide-angle X-ray scattering (WAXS) from the lipid solutions, showing multiple Bragg peaks, consistent with highly ordered structures. These structures were not observed in any other type of aqueous solutions containing monovalent or multivalent ions. The kinetics and temperature dependence of these transitions were also determined.