Miscibility of lactylated monoacylesters of propylene glycol with charged bilayer-forming colipids in aqueous solution

Lactylated monoacylesters of propylene glycol (LMP) are nonionic synthetic surfactants that form crystalline phases, when dispersed in aqueous solution. In this study the possibilities of controlled generation of lipid mesophases from LMP and two synthetic ionic colipids, dihexadecyldimethylammonium bromide (DHDAB) and sodium dihexadecylphosphate (DHP), were investigated applying differential scanning calorimetry (DSC) and small-angle X-ray diffraction (SAXD). We found that the bilayer forming, positively charged DHDAB is capable of forming mixed lipid phases with LMP. However, DSC data also revealed the formation of DHDAB-rich and LMP-rich domains. In mixtures of LMP with the negatively charged colipid, DHP, the DSC scans suggested the immiscibility of lipids at all mixing ratios. Since the hydrocarbon chain length is the same for both colipids, the observed differences in the thermotropic phase behaviour and the mixing behaviour were ascribed to the different steric features of the headgroups. The formation of unilamellar vesicles in the LMP/DHDAB mixture (60 : 40 w/w) was evidenced by the SAXD data. The ability to form mixed lipid phases and to self-assemble into vesicles upon hydration with an appropriate amount of colipid makes this non-phospholipid (LMP) a potential candidate for a phospholipid substitute in practical applications.