Study on Higher Order Phase Transition of Phospholipid Bilayrs and Stability of Emulsion Formed with These Disperisons

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This work was carried out to make clear the thermal properties of lamellar liquid crystals of dimyristoylphosphatidylcholine (DMPC) and dimyristoryphosphatidylglycerol (DMPG) dispersed in water, and emulsions of hexadecane formed with these lipids. Main measurements for the lipid dispersions were made on the temperature dependence of heat capacity at constant pressure, equilibrium apread pressure on water, X-ray diffraction analysis, ESR spectra, DSC,and zeta-Potential.As a result, it was confirmed from these experiments that in the lamellar liquid crystals of the (L_<alpha>+H_2O) phase a higher order phase transition of bilayr occurred at the temperatures several degrees higher than their gel-liquid crystral transition temperatures (Tm ; 23.5ﾟC for DMPC and 23.7ﾟC for DMPG). The higher order phase transition for these lipids were observed at 29.0ﾟC for DMPC dispersion and at 31.7ﾟC for DMPG dispersion. The physical and surface-chemical properties of these phospholipids were found to d … More iffer incredibly below and above T^<**>.Subsequently, the emulsification of hexadecane with DMPC dispersion was investigated below and above T^<**> temperature. It was found that the particle size, zeta-potential, and thermal stability of emulsion were changed in paralell with the variation of the thermal properties of DMPC dispersion at T^<**>. Especially, a new finding was that when hexadecane wa emulsified with DMPC at a temperature between Tm and T^<**>, the emulsion surface was covered and stabilized with the triple layrs of DMPC,but above T^<**> the surface of emulsion was composed of a monolayr of DMPC as a usual surfactant. This observation was supported by the precise study on the coalescence kinetics for the DMPC stabilized hexadecane emulsion. The conclusion was that the activation energy for coalescence of the emulsion taking place below T^<**> was twice lager than that for coalescence above T^<**>. This may explain why the emulsion formed with DMPC above T^<**> became less stable. Less