Complex formation and other phase transformations mapped in saturated phosphatidylcholine/DSPE-PEG2000 monolayers

Mixtures containing a saturated phosphatidylcholine (PC)-lipid and polyethylene glycol (PEG)-lipid have been widely used in lipid bilayer and monolayer systems, in particular for forming drug delivery stealth liposomes and for stabilizing medical microbubble ultrasound contrast agents. The miscibility and phase behavior of these systems have been incompletely explored and controversial. Here, using complementary experimental methods and analysis, we examine the phase behavior and construct complete surface pressure (pi)-composition phase diagrams for three binary Langmuir monolayer systems of increasing saturated PC-lipid chain-length from n = 14 to n = 18. Specifically, for DMPC, DPPC, and DSPC mixed with DSPE-PEG2000, we examine pi-area per molecule isotherms, pi-isothermal surface compressibility plots, molecular area-composition plots, and microstructure in fluorescent monolayers. We find that all three of these systems resembled different classical binary phase diagrams at 298K with the following notable phase behavior over wide ranges of composition and pi: (1) DMPC/DSPE-PEG2000 forms a single condensed (C) phase containing a stoichiometric complex, DMPC3DSPE-PEG2000, in addition to a eutectic at XDSPE-PEG2000 approximate to 0.66; (2) DPPC/DSPE-PEG2000 forms a single expanded (E) or C phase in addition to a peritectic at XDSPE-PEG2000 approximate to 0.55; and, (3) DSPC/DSPE-PEG2000 displays two-phase coexistence where the two components are almost completely immiscible. The phase diagrams are used to elucidate previously unexplained or conflicting results in the literature and design principles of monolayers and bilayers for medical microbubbles and stealth liposomes. In addition, we suggest that these systems may be ideal models for testing mechanisms of lipid condensation by both experiment and theory.