Molecular dynamics simulations of Langmuir monolayers: A study of structure and thermodynamics

Abstract

Molecular dynamics (MD) simulations have been performed on Langmuir monolayers of single chain surfactants at the air–water interface using a new anisotropic united atom model (AUA) for chain–chain interactions and a dipolar potential for head–head repulsions. Water–surfactant interactions are modeled using an external potential that does not fix the head group positions. The forces of the skeletal chains involved intramolecular effects of angle bending, and rotation among quartets of adjacent segments. Several molecular dynamics simulations have been performed on monolayers with densities ranging from 18 to 30 Å2/molecule. The results show two transitions in the monolayer. The first phase transition is a melting from a triangular lattice state maintained by the carbon chains to a fluidlike state with chain diffusion and lattice defects. The second transition is characterized by a change in molecular conformation, but with no change in lattice defects.