Lipid monolayers to understand and control properties of fluid interfaces

Abstract

Langmuir monolayers are probably the best defined interfacial systems since they enable variation of many parameters independently and since many techniques have been established in recent years to characterize them at the molecular level. In the first part of the paper the progress in the physical understanding of the polymorphism is given. Ordered mesophases are encountered, distinguished by aliphatic chain tilt, azimuth and the freedom of chain rotation about the long axis. The orientation of polar molecules at the interface in addition is responsible for long range electrostatic interactions causing peculiar domain shapes and superlattices. Having largely understood the polymorphism one can use structural studies to understand interactions of lipids with adjacent molecules. Samples presented are phospholipid molecules electrostatically interacting with polyelectrolytes, enzymatically attacked by phospholipases and partly solubilized by hydrocarbons. It is shown that these interactions specifically affect the monolayer structure, changing chain tilt and partly increasing the order. In addition it is shown that enzymatic reactions at interfaces are varied via structure and composition of the monolayer.