Thermodynamic characterization of mixed monolayers of phosphatidylcholine and arachidic acid on different subphases

The mixed monolayers of phosphatidylcholine (PC) and arachidic acid (AA) are essentially important for studying biomembranes. The mixing behavior, molecular interactions and stability of PC and AA on three subphases of pure water, NaCl solution and CdCl2 solution, have been analyzed thermodynamically. It is found out that Na+ and Cd2+ can affect the formation of the monolayers of PC, AA and their mixtures sensitively. PC monolayers and some PC-rich mixed monolayers may undergo a phase transition on a subphase of NaCl or CdCl2 due to the orientation change of the PC molecules, but similar transition disappears in all monolayers on a pure water subphase. In general, the mixing of PC and AA is non-ideal on the three subphases and the molecular interactions between PC and AA are less attractive than those of the ideally mixed monolayer. At constant composition, non-ideality follows a descending rule in line order of pure water, NaCl and CdCl2 subphase. On the contrary, the attractive molecular interactions and stability of the monolayers vary in a reverse way. The maximums of these properties depend on the surface pressure and composition, indicating that the mixing behavior of PC and AA is influenced by the monolayer state, as well as the subphase.