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 ( π– A) measurements and by direct visualisation of monolayers by Brewster angle microscopy (BAM). The π– 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 °C. Monolayers of the three systems reach the closest packing of alkyl chains in the 0.4−0.6 range of X DPPC. 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 X DPPC and temperature in the range 10–30 °C. At 10 and 20 °C mixed monolayers form condensed states from low π all over the composition range. At 30 °C, the liquid-expanded (LE)−liquid-condensed (LC) phase transition occurs at increasing π with X DPPC. The shape and size of condensed domains change with X DPPC and π. 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 G E data in a limited range of compositions at 30 °C.