Solubilization of hydrocarbons into a bilayer assembly of dimyristoylphosphatidylcholine in water

Abstract

Solubilization of octane, hexadecane and perfluorononane into a bilayer assembly of dimyristoylphosphatidylcholine (DMPC) in water has been studied at 25 and 35°C as a function of mole fraction ( X) of these solubilizates. In order to determine the occurrence of solubilization, a change of the gel–liquid crystal transition temperature ( T m) of the DMPC bilayer assembly was monitored by differential scanning calorimetry measurement. Solubilization of octane into the DMPC bilayer assembly was observed both at 25 and 35°C. On the other hand, solubilization of hexadecane took place above X=0.40 at 25°C, while at 35°C solubilization did not occur at any mole fraction. Solubilization of perfluorononane into the DMPC bilayer assembly was not observed either at 25 or 35°C. Saturated solubilization of octane or hexadecane into the DMPC bilayer assembly was achieved at X=0.67, where the mole ratio is 1(DMPC):2(oil), i.e. the ratio of the number of hydrocarbon chains of DMPC to that of oil is 1:1. The heat of gel–liquid crystal transition (Δ H m) of the DMPC bilayers saturated with octane at 25°C is 16.5 J g −1 at T m=14.8°C, and that of the DMPC bilayers saturated with hexadecane at 25°C is 22.0 J g −1 at T m=31.4°C; whereas the Δ H m value for the pure DMPC bilayers is 27.0 J g −1 at T m=23.7°C. X-ray diffraction data indicate that the hydrocarbon solubilizates are located in the palisade layers of the DMPC bilayers. It has been also revealed that the surface of emulsion droplets of octane or hexadecane are covered with multibilayers of DMPC at 25°C, and with a monolayer of DMPC at 35°C. Such differences in solubilization and emulsion stabilization mechanisms between 25 and 35°C are attributable to the higher order transition of the DMPC bilayers at T* (29.0°C).