Three-phase structure of hexadecane nanoemulsion formed with phospholipid–surfactant mixtures and its novel phase transition temperature TE

The data of DSC, X-ray diffraction (XRD) and particle size measurements have indicated formation of the three-phase emulsion composed of water, hexadecane oil particle, and DMPC bilayer phases. The DSC charts of such three-phase emulsion always show a characteristic endothermic peak named TE at the temperature slightly below the melting point of oil TH. The peak at the TE transition is attributable to a local melting of the nano-size oil domain adhering to the phospholipid bilayers that have fully solubilized oil molecules. case of the emulsifier of DMPC mixed with a cationic surfactant (C14TAB), it has been found that stable hexadecane nanoemulsions (10–100 nm) form when the mole fraction of hexadecane (XH) exceeds 0.33, the limit of solubilization of hexadecane in the mixed bilayer. However, submicron-emulsions have appeared when XH > 0.94. On the other hand, we have observed the TE transition in the nanoemulsion region of 0.33 < XH < 0.94, and observed both TE and TH at XH > 0.94. he emulsifier of DMPC mixed with a nonionic surfactant (C14(EO)8) is used, it has been also found that the stable hexadecane nanoemulsions form at XH > 0.5, the limit of solubilization of hexadecane in the mixed bilayer. When XH > 0.8, however, submicron-emulsions have appeared. Also we have observed the TE transition in the nanoemulsion region of 0.5 < XH < 0.8. rmation and stability of the three-phase nanoemulsion have been elucidated in terms of lamella–vesicle transformation of the mixed bilayer emulsifiers upon adhering to the hexadecane droplet phase. Thus, the emergence of the TE transition has been found to be a criterion of the formation of the three-phase nanoemulsion unless it is accompanied by the TH transition.