Mixing Phase Behavior of 1,3-Dipalmitoyl-2-Oleoyl-sn-Glycerol (POP) and 1,2-Dipalmitoyl-3-Oleoyl-rac-Glycerol (PPO) in n-Dodecane Solution

We examined the mixing phase behavior of 1,3-dipalmitoyl-2-oleoyl-sn-glycerol (POP) and 1,2-dipalmitoyl-3-oleoyl-rac-glycerol (PPO) in organic solutions containing n-dodecane (C-12) as a solvent to confirm whether the molecular compound (MC) crystals of POP/PPO are formed as metastable and most stable forms in the dilute solution by using DSC and synchrotron radiation X-ray diffraction. The POP/PPO mixtures were prepared at different concentration ratios of POP and PPO with a 10 % increment in two solutions having weight concentration ratios of (POP+PPO):C-12 of 50:50 (50 % solution) and 20:80 (20 % solution). We found that MC crystals formed at a ratio of POP/PPO = 50/50 having the melting point of 23.1 and 20.6 degrees C in 50 and 20 % solution systems in stable states of incubated samples, respectively. The metastable (beta') and stable (beta) forms of MC of POP/PPO = 50/50 having a double-chain-length structure were confirmed in kinetic behavior and thermodynamically stable states attained after long time incubation, in contrast to the triple-chain-length structure of stable forms of POP (beta) and PPO (beta'). In addition, we observed the formation of MC crystals of POP/PPO in diluted solutions having n-dodecane concentrations up to 98 % and conclude that molecular interactions between POP and PPO molecules to form MC crystals are not altered by solvent molecules in diluted solutions.