The phase behavior of binary lipid monolayers as determined by surface chemistry and IR spectroscopy

We have applied both a classical surface thermodynamic analysis and quantitative IR spectroscopy to mixed monolayer films of phosphocholine (PC) and phosphoglycerol (PG) lipids in order to quantitatively describe the phase miscibility of these physiologically important species. The particular PC:PG binary mixtures were chosen to model the major components of mammalian pulmonary lung surfactant. The surface chemistry analysis revealed that saturated PC:PG mixtures were nearly ideal at all mole ratios studied, while the mixtures of saturated PC with unsaturated PG lipids were found to have positive deviations from ideality at all mole Fractions studied, indicating phase-separated binary mixtures. We have also utilized Langmuir-Blodgett transfer methods, ATR-IR, and P-31 NMR spectroscopy as a means of determining the surface composition of the phospholipids at varying surface pressures. Using these IR methods, we have found no evidence for the selective exclusion of any component from the interface, or the so-called ''squeeze-out'' of monolayer components.