Improved United-Atom Models for Perfluorinated Self-Assembled Monolayers

Several united-atom (UA) force fields for perfluorinated self-assembled monolayer (FSAM) surfaces are proposed. These UA models of FSAM are based on a preceding force field, and the modifications done in this work involved the type of potential function and parameters used to represent the nonbonded interactions among the united atoms of the FSAM chains, which have been shown to play a key role in the energy transfer that takes place in collisions of gases with self-assembled monolayers. Specifically, the proposed UA models employ two-body Buckingham terms, with parameters obtained by fits to averaged nonbonded interaction energies computed with a well-tested all-atom (AA) force field. The performances of the UA force fields are assessed by comparisons of relevant properties for collisions of CO₂ with FSAM, such as the percentage of collision energy transferred to the surface and the rotational and translational energy distributions of the scattered CO₂ molecules.