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 20degreesC. 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 10-30degreesC. At 10 and 20degreesC mixed monolayers form condensed states from low pi all over the composition range. At 30degreesC, the liquid-expanded (LE)-liquid-condensed (LC) phase transition occurs at increasing pi with X-DPPC. The shape and size of condensed domains change with X-DPPC 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 G(E) data in a limited range of compositions at 30degreesC. (C) 2004 Elsevier Ireland Ltd. All rights reserved.