PHASE-BEHAVIOR OF MIXED SYSTEMS OF SOS AND OSO

The phase behavior of mixed systems of SOS (sn-1,3-distearoyl-sn-2-oleoylglycerol) and OSO (sn-1,3-dioleoyl-sn-2-stearoylglycerol) was studied with X-ray diffractometry and thermal analysis by using pure samples (>98.0%). A molecular compound was formed at the mixture of equal weight ratio, giving rise to two monotectic phases of SOS/compound and compound/OSO in a juxtapositional way. The stable form of the compound, beta(c), has an interlamellar distance of 44.72 angstrom of double chain length structure, which is in contrast to the triple chain length structure of the stable forms of SOS and OSO having the same interlamellar distance of 65 angstrom. Beta(c) was formed through specific molecular interactions between stearoyl and oleoyl chains connected to glycerol groups: stearoyl chains at the 1,3-positions of SOS and at the 2-position of OSO are packed together in one leaflet of the double chain length lamellae, whereas another leaflet contains oleoyl chains of the two molecules. In the monotectic region of SOS/compound, two crystals are present below the melting point of beta(c), 36-degrees-C. Above that temperature, the stable crystals of SOS equilibrated with mixed liquid are present. In the monotectic region of compound/OSO, beta(c) is present with mixed liquid above the melting point of the stable form of OSO, 25-degrees-C, below which two crystal fractions of the compound and OSO are present. Beta(c) was expected to have a congruent melting point, according to its unique melting and crystallizing behavior. The compound formation having the double chain length structure was also observed in the mixtures of POP/OSO and POS/OSO, in which P is a palmitoyl chain. A molecular model of the compound crystal is proposed, based on the molecular structures of SOS, OSO, and principal monounsaturated fatty acids.