Interactions in mixed monolayers between dioleoyl-l-phosphatidylcholine and all-trans retinal

The surface pressure isotherms of all-trans retinal and dioleoyl-l-phosphatidylcholine at the nitrogen-water interface were studied at 20.5°C. The extrapolated limiting areas per molecule are 49 and 83 Ų molecule^-1, respectively. The properties of seven mixtures of all-trans retinal and dioleoyl-l-phosphatidylcholine, covering the whole range of molar fractions were also measured. The results show that an almost constant collapse pressure is observed at 44 ± 1 mN m^-1 for all the mixtures studied. Furthermore, an apparent collapse pressure is also observed at 16, 18, 22, 26, 32, and 35 mN m^-1 at a molar fraction of dioleoyl-l-phosphatidylcholine of 0.100, 0.200, 0.400, 0.500, 0.600, and 0.800, respectively. An analysis of the results in terms of the surface phase rule shows that below the apparent collapse pressure, the components are completely miscible. Negative deviations from ideality were observed in the region of miscibility for all the systems, as shown by the negative deviations to the additivity rule and the negative excess free energies of mixing calculated for all the systems at 5, 7, 10, and 13 mN m^-1. Furthermore, it is shown that above the apparent collapse pressure, all-trans retinal is rejected from the monolayer, the rejection being total at the collapse pressure.