Domain formation in monolayers

Abstract

For phospholipids at the air/water interface we demonstrate that molecular chirality in some, but not in all, cases influences the domain shapes. In other cases chirality in the head group region can cause a chiral structure considering the tail arrangement. This indicates head group ordering. Minute changes of the molecular structure may change domain morphology from circular to dendritic. This can be related to slight changes of the lattice structure. In case of a dendritic domain the chains are more tilted, the deviation from hexagonal symmetry is more pronounced, and hence the lattice anisotropy is larger. This can be understood also in view of recent simulations considering diffusion-limited aggregation models. In experiments at the oil/water interface we show that the electrostatic repulsion between domains is reduced by exchanging air against oil. Also line tension is reduced which can be understood by means of simple molecular models. The oil may penetrate into domains of the ordered phase if it is aliphatic and exhibits the same chain length as the lipid. In other cases it is excluded from the ordered monolayer regions. Taking aliphatic dioles as the most simple models we show that head group chirality is expressed in domain shapes, and partly also in lattice structure.

Key Words:

• Lipid monolayers
• domain structure
• microstructure
• fluorescence microscopy
• surface diffusion