The phase behavior of binary mixtures of hydrated dielaidoylphosphatidylethanolamine (DEPE) with two different PEG-lipid conjugates at a molar fraction below 0.2 has been studied by using time-resolved X-ray diffraction, and partial phase diagrams have been constructed. The studied conjugates comprise two saturated hydrocarbon acyl chains 14 carbon atoms long and PEG550 or PEG5000 chains covalently attached to a phosphoethanolamine polar head group, DMPE(PEG550) and DMPE(PEG5000), respectively. When added in small amounts (10–20 mol%) to DEPE aqueous dispersions, both PEG-lipids favor the lamellar liquid crystalline (Lα) phase at the expense of the lamellar gel (Lβ) and the inverted hexagonal (HII) phases. One of the conjugates, DMPE(PEG550), shifts the Lα–HII transition of DEPE to higher temperatures by 2.5°C per mol% PEG-lipid, and induces the spontaneous formation of a cubic phase of space group Im3m in the DEPE dispersions. The cubic phase intrudes between the lamellar liquid crystalline and the inverted hexagonal phases in the DEPE/DMPE(PEG550) phase diagram. Low amounts of the DMPE(PEG5000) conjugate only shift the Lα–HII transition of DEPE to higher temperatures, at 5.2°C per mol% PEG-lipid, but does not promote the formation of additional phases. The respective slopes for the Lβ–Lα transition temperature depression are 10–15 times smaller. At >15 mol% DMPE(PEG550) and at >5 mol% DMPE(PEG5000), the non-lamellar phases are eliminated from the phase diagrams. Structural data on the organization of the pure hydrated PEG-lipid conjugates are also provided, suggesting that these lipids form micelles and lamellae
