The equilibria of phosphatidylcholine–fatty acid and phosphatidylcholine–amine in monolayers at the air/water interface

Monolayers of phosphatidylcholine, fatty acid and amine and binary mixtures phosphatidylcholine–fatty acid or phosphatidylcholine–amine were investigated at the air/water interface. Phosphatidylcholine (lecithin, PC), stearic acid (SA), palmitic acid (PA), decanoic acid (DA) and decylamine (DE) were used to the experiment. The surface tension values of pure and mixed monolayers were used to calculate π–A isotherms. The surface tension measurements were carried out at 22 °C using an improved Teflon trough and a Nima 9000 tensiometer. The Teflon trough was filled with a subphase of triple-distilled water. Known amounts of lipid dissolved in 1-chloropropane were placed at the surface using a syringe.The interactions between lecithin and fatty acid as well as phosphatidylcholine and amine result in significant deviations from the additivity rule. An equilibrium theory to describe the behaviour of monolayer components at the air/water interface was developed in order to obtain the stability constants of PC–SA, PC–PA, PC–DA and PC–DE complexes. We considered the equilibrium between the individual components and the complex and established that lecithin and fatty acid as well as phosphatidylcholine and amine formed highly stable 1:1 complexes.