MOLECULAR-DYNAMICS SIMULATIONS OF LANGMUIR MONOLAYERS - A STUDY OF STRUCTURE AND THERMODYNAMICS

Molecular dynamics (MD) simulations have been performed on Langmuir mo nolayers 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 in teractions are modeled using an external potential that does not fix t he head group positions. The forces of the skeletal chains involved in tramolecular effects of angle bending, and rotation among quartets of adjacent segments. Several molecular dynamics simulations have been pe rformed on monolayers with densities ranging from 18 to 30 angstrom-2/ molecule. The results show two transitions in the monolayer. The first phase transition is a melting from a triangular lattice state maintai ned by the carbon chains to a fluidlike state with chain diffusion and lattice defects. The second transition is characterized by a change i n molecular conformation, but with no change in lattice defects.