A theoretical study of model lipid monolayers

Studies of pure phospholipid monolayers or various well defined lipid mixtures have greatly contributed to the current knowledge of the relationship between monolayer composition and its properties and to understand how their physicochemical properties, e.g. refraction, polarization, are controlled by structural variations at the molecular level. Therefore, an attempt was made to investigate model lipid molecules adsorbed on the air/water interface. Semi-empirical (AM1) quantum chemical calculations were performed for several clusters and were compared with experimental data. The optimized acidic molecules show a marked tendency to group forming domains. An increase in the number of surfactant molecules in the cluster leads to a decrease in the effective headgroup area. On the other hand, the area grows with the increase of the degree of ionization. Both relations are in accordance with the experimental trends and the electrostatic theory. The employed theoretical approach proves to be applicable to the study of monolayers, thus providing a reliable description at molecular level of the traditionally phenomenological investigations in the field.