PHASE-TRANSITIONS IN LANGMUIR MONOLAYERS - A MOLECULAR-DYNAMICS STUDY

Molecular dynamics (MD) simulations have been performed on Langmuir monolayers of linear chain surfactants at the air-water interface. Two configurations of the solid phase have been studied: a rectangular unit cell monolayer and a hexagonal unit cell monolayer. Structure factor results and other order parameters indicate no significant shape dependence. The results indicate two transitions in the monolayer as a function of density: (1) a continuous transition characterized by a change in chain conformation and (2) a first-order phase transition characterized by a sharp rise in lattice defects and chain diffusion. The high-density phase is a crystal lattice with insignificant gauche defects, and with nearly upright molecules. The middle phase is characterized by kink defects, uniform tilt, and bond orientational order, but with no lattice defects or diffusion. The low-density phase is characterized by chain diffusion and lattice defects.