Emulsion describes a heterogeneous liquid system in which one liquid is initimately dispersed in another (immiscible) in the form of droplets, and is divided broadly into two types: oil-in-water (O/W) emulsion and water-in-oil (W/O) emulsion. Emulsions are used in many fields; cosmetics, 1) foods, 2-4) medicines 5) and so on. It is thus desirable to prepare emulsions with good stability. Surfactants are used as an emulsifier and/or dispersing agent, and the properties of the emulsion depend on the kind of surfactant. Meanwhile, the effect of surfactant on human skin must be taken into account. Amino acid-type surfactants are biodegradable and have little irritating action on the skin. [6][7][8] We reported the surface active properties of N-[3-lauryloxy-2-hydroxypropyl]-L-arginine L-glutamate (C12HEA-Glu), a new amino acid-type surfactant. 9) Furthermore, the emulsifying potency of C12HEA-Glu in an O/W emulsion system was examined previously 10) : the emulsifying potency of C12HEA-Glu was greater than that of popular amphoteric and nonionic surfactants; the stability of emulsions containing oleic acid (OA), (C12HEA-Glu/OA/water system), was greatest.Good stability of O/W emulsions is attained by choosing a proper oil (dispersed phase) and surfactant (emulsifier).11) On the contrary, stabilization of W/O emulsions is rather difficult compared with O/W emulsions. 12) One emulsifying technique for preparation of W/O emulsions is a liquid-crystal emulsification. In a previous report, 10) a possible complex formation between OA and C12HEA-Glu on the droplet surface was suggested, which contributes to stabilization of O/W emulsion system.In this study, formation of liquid crystals between C12HEA-Glu and OA was examined by microscopy under polarized light and infrared (IR)-spectroscopy, and the ternary phase diagram for C12HEA-Glu/OA/water system was established. W/O emulsions consisting of C12HEA-Glu, OA, liquid paraffin (LP) and water were prepared, and the effects of the amount of water (dispersed phase) and weight ratio of OA to C12HEA-Glu on the stability of W/O emulsions were investigated.
ExperimentalMaterials C12HEA-Glu was used as an amino acid-type surfactant. The synthesis and purification of C12HEA-Glu were described elsewhere. 13) OA (99% purity) obtained from Nippon Oil and Fats Co., Ltd. and LP obtained from Wako Pure Chemical Ind., Ltd. were used as supplied. Distilled water for injection was purchased from Otsuka Pharmaceutical Co.Phase Diagram The ternary phase diagram for C12HEA-Glu/OA/water system was obtained as follows: 3-4 g mixtures were placed in test tubes and heated at 90-100°C for 1 week; after equilibrium had been established, the mixtures were observed at 25°C under an inverted microscope.Opticalmicroscopy Liquid crystals in the C12HEA-Glu/OA/water system were observed under an inverted microscope with a transmitted light differential interference contrast attachment (IMT-2, Olympus Optical Co., Ltd.). Liquid crystalline phase was confirmed under polarized light.pH Measurement The pH of aq...