Molecular dynamics studies of Langmuir monolayers of F(CF2)20F

We report the results of molecular dynamics simulations of Langmuir monolayers of F(CF2)20F using both a united atom model and a recently developed anisotropic united atom model of the molecular force field. A comparison of our simulation results to experimental measures of the structure of Langmuir monolayers and lamellar crystals of F(CF2)20F indicates that the anisotropic united atom model provides the better representation. Simulations using the new model at 275 K generate an azimuthally disordered rotator phase monolayer, while simulations at 150 K generate a monolayer phase with azimuthal order. The relative intensities of peaks in the in-plane structure function can be used to distinguish between the azimuthally ordered and disordered phases. Our simulations also predict, in agreement with experiment, that at large average areas per molecule the monolayer breaks up into islands that have a close-packed structure at the center but are disordered at the periphery.