Thermal Properties of Hexadecane Emulsion Stabilized with Dimyristoylphosphatidylcholine.

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Abstract
Emulsification of hexadecane with dimyristoylphosphatidylcholine (DMPC) in the (Lα+ H2O) phase was investigated as a function of temperature. The particle size of the emulsion prepared under the same condition was about 800 nm in the temperature range from the gel-liquid crystal transition temperature, TM. (23.5°C) to about 28°C, but sharply decreased and reached about 390 nm above the critical temperature, T* (29.0°C), at which a higher order transition occurred in the DMPC bilayer. The changes in emulsion concentration by creaming were observed, as expected from the temperature dependence of the emulsion particle size. The zeta-potential of the emulsion surface was also shifted around. T* from -36 mV to -15 mV. On the other hand, the interfacial tension at the hexadecane-DMPC dispersion interface decreased with increasing temperature up to T*, and then became almost a constant value less than 1 mNm-1 above T*. These novel phenomena were explained by the concept that thermal characters of the DMPC dispersion used as an emulsifier changed not only at TM, but also at T*. Furthermore, by taking into account the amount of emulsifier, the particle size of emulsion, and the molecular area of DMPC in bilayer, it was found that the surface of hexadecane emulsion was covered with the amount of DMPC corresponding to the triple layers of DMPC below T*, but to the monolayer above T*.

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