Interactions of phenytoin with lipids in mixed Langmuir monolayers

The behaviour of dipalmitoyl phosphatidylcholine (DPPC)/cholesterol monolayers on pure water and on water with phenytoin was studied by performing Langmuir monolayer technique experiments. Surface pressure–area isotherms were measured for pure DPPC, cholesterol films and their mixtures. The changes of the limiting molecular area, collapse pressure and maximum compressional modulus show that phenytoin can modify the structure of lipid aggregates. The introduction of phenytoin to the subphase reduces the deviations of mean molecular areas from additivity rule observed for mixed DPPC/cholesterol monolayers. Thermodynamic stability of the mixed monolayers, as compared to that of pure monolayers, was determined by analysis of the excess Gibbs free energy of mixing. Results indicate an increase of thermodynamic stability with increasing surface pressure and decrease of thermodynamic stability in the presence of phenytoin in the system. The existence of a minimum indicates that the mixed monolayers are thermodynamically more stable than the monolayers with separation between individual components, especially around xDPPC = 0.6, on both subphases.