Persistence of phase coexistence in disaturated phosphatidylcholine monolayers at high surface pressures

Prior reports that the coexistence of the liquid-expanded (LE) and liquid-c ondensed (LC) phases in phospholipid monolayers terminates in a critical po int have been compromised by experimental difficulties with Langmuir trough s at high surface pressures and temperatures. The studies reported here use d the continuous interface of a captive bubble to minimize these problems d uring measurements of the phase behavior for monolayers containing the phos phatidylcholines with the four different possible combinations of palmitoyl and/or myristoyl acyl residues. Isothermal compression produced surface pr essure-area curves for dipalmitoyl phosphatidylcholine (DPPC) that were ind istinguishable from previously published data obtained with Langmuir trough s. During isobaric heating, a steep increase in molecular area correspondin g to the main LC-LE phase transition persisted for all four compounds to 45 mN/m, at which collapse of the LE phase first occurred. No other discontin uities to suggest: other phase transitions were apparent. Isobars for DPPC at higher pressures wet-a complicated by collapse of the monolayer, but con tinued to show evidence up to 65 mN/m for at least the onset of the LC-LE t ransition. The persistence of the main phase transition to high surface pre ssures suggests that a critical point for these monolayers of disaturated p hospholipids is either nonexistent or inaccessible at an air-water interfac e.