Mixed monolayers of dipalmitoyl phosphatidylcholine and ethyl palmitate at the air/water interface

The behaviour of monolayers containing dipalmitoyl phosphatidylcholine and ethyl palmitate and their mixtures at different molar fraction, using surface pressure-molecular area results, was investigated. The negative deviation from additivity of the mean molecular areas as a function of the mixture composition indicates the miscibility. The miscibility was confirmed by applying the two-dimensional phase rule, since the collapse pressure values vary with the composition of the mixtures. Also the free energy of mixing ΔG_mix and the excess free energy of mixing ΔG^E_mix were determined. The negative values of ΔG_mix and ΔG^E_mix indicate that the mixed monolayers are thermodynamically more stable compared to the pure ones and that the compounds in the two dimensional state experience mainly attractive interactions.