Perfluoropolyethers: Development of an All-Atom Force Field for Molecular Simulations and Validation with New Experimental Vapor Pressures and Liquid Densities

A force field for perfluoropolyethers (PFPEs) based on the general optimized potentials for liquid simulations all-atom (OPLS-AA) force field has been derived in conjunction with experiments and ab initio quantum mechanical calculations. Vapor pressures and densities of two liquid PFPEs, perfluorodiglyme (CF₃-O-(CF₂-CF₂-O)₂-CF₃) and perfluorotriglyme (CF₃-O-(CF₂-CF₂-O)₃-CF₃), have been measured experimentally to validate the force field and increase our understanding of the physical properties of PFPEs. Force field parameters build upon those for related molecules (e.g., ethers and perfluoroalkanes) in the OPLS-AA force field, with new parameters introduced for interactions specific to PFPEs. Molecular dynamics simulations using the new force field demonstrate excellent agreement with ab initio calculations at the RHF/6-31G∗ level for gas-phase torsional energies (<0.5 kcal mol^-1 error) and molecular structures for several PFPEs, and also accurately reproduce experimentally determined densities (<0.02 g cm^-3 error) and enthalpies of vaporization derived from experimental vapor pressures (<0.3 kcal mol^-1). Additional comparisons between experiment and simulation show that polyethers demonstrate a significant decrease in enthalpy of vaporization upon fluorination unlike related molecules (e.g., alkanes and alcohols). Simulation suggests this phenomenon is a result of reduced cohesion in liquid PFPEs due to a reduction in localized associations between backbone oxygen atoms and neighboring molecules.