Surface adsorption and vesicle formation of dilauroylphosphatidylcholine in room temperature ionic liquids

Surface chemical properties of a phospholipid, dilauroylphosphatidylcholine (DLPC), in two ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) and hexafluorophosphate (bmimPF(6)), were investigated by means of surface tension, dynamic light-scattering, and freeze-fracture transmission electron microscopy. It was found that DLPC shows finite solubility in the ILs and spontaneously forms vesicles with size distribution around 400 nm in diameter above the critical vesicular concentration (CVC) of 0.040 wt.% (in bmimBF(4)) and approx. 0.08 wt.% (in bmimPF(6)). Other than the CVC value, anion specificity of the ILs was also seen in the temperature effect on the vesicular aggregation; that is, a temperature-induced reversible aggregation was observed in bmimPF(6), but not in bmimBF(4). The differences in the vesicular stability against the temperature-induced aggregation could be attributed to differences in the interaction between anion species of the ILs and zwitterionic phosphatidylcholine head groups. The apparent molecular area occupied by DLPC at the air/solution interface was estimated to be 0.37 nm(2) in bmimBF(4) and 0.20 nm(2) in bmimPF(6) by applying the Gibbs adsorption equation. These values are much smaller than the molecular area of 0.69 nm(2) reported for the hydrated DLPC bilayer of lamellar liquid-crystalline phase. This result is not consistent with the traditional Gibbs adsorption model, but can be interpreted in terms of a picture for the surface adsorption of soluble amphiphiles proposed by Moroi et al. Differential scanning calorimetric study is also reported regarding the phase transition behavior of DLPC bilayer solvated by the ILs.