Three-phase Emulsification of Hexadecane with Dimyristoylphosphatidylglycerol Sodium Salt in Water: An Interpretation by New Phase Transition in Bilayer Assembly

摘要

Properties of the three-phase emulsion of hexadecane have been investigated at 40℃ as a function of mole fraction of hexadecane (X) in the mixture of hexadecane and dimyristoylphosphatidylglycerol sodium salt (NaDMPG). NaDMPG used as an emulsifier was the aqueous dispersion of the IM or the Gel-2 state. Here the IM state was a nematic-like metastable state that appeared above the critical temperature T~* (31.7℃), while the Gel-2 state was a stable gel state that was established after approximately 15 day aging of the IM state. DSC data showed that hexadecane was solubilized into the NaDMPG bilayers up to approximately X = 0.5, and that at X > 0.5 stable hexadecane emulsions were formed regardless of the states of the NaDMPG dispersions. XRD data showed that hexadecane emulsions were stabilized with the NaDMPG multibilayers solubilizing hexadecane in their palisade layers. In the region of X < 0.5, hexadecane solubilized into the IM phase promoted formation of a gel structure of the IM phase, while hexadecane solubilized into the Gel-2 phase promoted hydrolytic cleavage of the ester bonds between glycerol and myristic acid in NaDMPG molecules. The emulsion prepared with the NaDMPG dispersions in the Gel-2 state were more stable than these prepared with the dispersions in the IM state, reflecting the different physical states of the NaDMPG dispersions.