Phase behaviour and morphology of binary mixtures of DPPC with stearonitrile, stearic acid, and octadecanol at the air-water interface

The behaviour of dipalmitoylphosphatidylcholine (DPPC), mixed with stearonitrile (SN), was investigated at the air-water interface by surface pressure-area (pi-A) measurements and by direct visualisation of monolayers by Brewster angle microscopy (BAM). The pi-A-X diagram of system DPPC/SN was compared with the corresponding diagrams of systems DPPC/stearic acid (SA) and DPPC/octadecanol (OD) at 20 degrees C. Monolayers of the three systems reach the closest packing of alkyl chains in the 0.4-0.6 range of XDPPC. Thermodynamic analysis indicates miscibility in the three binary systems with negative deviations from the ideal behaviour. Morphological features of system DPPC/SN change significantly with XDPPC and temperature in the range 10-30 degrees C. At 10 and 20 degrees C mixed monolayers form condensed states from low pi all over the composition range. At 30 degrees C, the liquid-expanded (LE)--liquid-condensed (LC) phase transition occurs at increasing pi with XDPPC. The shape and size of condensed domains change with XDPPC and pi. Contrarily to the behaviour of pure components, mixed monolayers of DPPC/SN exhibit orientational order in the 0.2-0.6 mol fraction range of DPPC. BAM observation confirmed the partial miscibility indicated by GE data in a limited range of compositions at 30 degrees C.