Lipid-block copolymer immiscibility

We have investigated the binary system of a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), EO5PO68EO5, in water. The ternary system with the same polymer, water and soybean phosphatidylcholine (lecithin) has also been studied. Small-angle X-ray scattering (SAXS), H-2 NMR and differential scanning calorimetry (DSC) were used to characterize these systems. The phase diagram of the binary system is presented together with the characteristic parameters in the lamellar and hexagonal phases. In the ternary system, it was found that the lecithin and the block copolymer are essentially immiscible, forming separate phases. In a differential scanning calorimetry experiment it was found that the presence of the block copolymer did not affect the melting temperature of dipalmitoyl phosphatidyl choline. Again indicating immiscibility. The alkane hexadecane is a bad solvent for polypropylene oxide at room temperature. We conclude that it is the difference in hydrophobicity (or polarity) of the hydrophobic parts of the lecithin (lipid) and the block copolymer that explains their immiscibility.