Phase behavior of a binary mixture of 1,3-dipalmitoyl-2-oleoyl-sn-glycerol and 1,3-dioleoyl-2-palmitoyl-sn-glycerol in n-dodecane solution.

The phase behavior of a binary mixture of triacylglycerol (TAG), POP (1,3-dipalmitoyl-2-oleoyl-sn-glycerol), and OPO (1,3-dioleoyl-2-palmitoyl-sn-glycerol) in an organic solvent, n-dodecane, was examined by DSC and synchrotron radiation X-ray diffraction (SR-XRD). In our previous study on POP-OPO mixtures in neat liquid, the formation of molecular compound (MC) crystals of POP/OPO = 50/50 was observed (Minato et al. J. Am. Oil Chem. Soc. 1997, 74, 1213-1220). The purpose of the present study was to determine whether MC is formed in the POP-OPO mixture system in diluted solution when n-dodecane (C(12)) is added to the mixture as a solvent. Two solution systems having weight concentration ratios of POP and OPO to n-dodecane ((POP+OPO):C(12)) of 50:50 (50% solution) and 20:80 (20% solution) were prepared. We found that MC formed at a ratio of POP/OPO = 50/50 in both 50% solution and 20% solution in the stable and metastable states. Differences between the neat liquid and solution systems appeared in two results. In the first, the melting temperatures of MC crystals were lowered from 32.0 degrees C (neat liquid) to 20.4 degrees C (50% solution) and 18.0 degrees C (20% solution). In the second, the beta polymorph of the MC was always observed in the two solutions under the conditions examined in the present study, whereas alpha and beta polymorphs were observed in the neat liquid. These differences were caused by thermodynamic solvent effects. However, we conclude that aliphatic chain-chain interactions through palmitic and oleic acid moieties forming the MC of POP-OPO having a double chain length structure are not disturbed by solute-solvent interactions.