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

Abstract

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 (CF-O-(CF-CF-O)-CF) and perfluorotriglyme (CF-O-(CF-CF-O)-CF), 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 error) and molecular structures for several PFPEs, and also accurately reproduce experimentally determined densities (<0.02 g cm error) and enthalpies of vaporization derived from experimental vapor pressures (<0.3 kcal mol). 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.

Tuning the Reactivity of Perfluoropolyether-Functionalized Aluminum Nanoparticles by the Reaction Interface Fuel-Oxidizer Ratio

“…Compared with traditional fluorine-containing oxidizers, perfluoropolyether (PFPE) has attracted interest from several different fields because of its unique combination of advantageous physical and chemical properties (i.e., it has an excellent thermal and oxidation stability, enabling it to be used in applications requiring a wider temperature range) [ 34 , 35 ]. Nonetheless, the use of PFPE in micro-sized/nano-sized Al particles has rarely been reported thus far [ 19 , 36 , 37 ].…”

Sub-layer rationale of anomalous layer-shrinkage from atomistic simulations of a fluorinated mesogen

“…All MD simulations were performed at a fully atomistic level within GROMACS 5.1.2, 60-65 using the General Amber Forcefield 55 with modified parameters better optimised for simulation of liquid crystal systems 56 as well as specific nonbonded parameters that were reported to more accurately capture the behaviour of perfluoropolyether compounds. 59 The lowest energy structure of 3M 8422 was established through optimisation at the B3LYP/6-31G(d) level 66,67 within Gaussian 09. 68 Partial atomic charges within the nonperfluorinated sub-units were calculated from the optimised geometry using the RESP method, 69 while partial atomic charges within the perfluoropolyether chain were modified to align with previous studies of such systems, 59 as partial charges generated through the RESP method were found to significantly underestimate the values.…”

“…59 The lowest energy structure of 3M 8422 was established through optimisation at the B3LYP/6-31G(d) level 66,67 within Gaussian 09. 68 Partial atomic charges within the nonperfluorinated sub-units were calculated from the optimised geometry using the RESP method, 69 while partial atomic charges within the perfluoropolyether chain were modified to align with previous studies of such systems, 59 as partial charges generated through the RESP method were found to significantly underestimate the values. Topology files for MD simulations were generated using Antechamber 70 within the Amber18 software package, 71 and were converted into a GROMACS readable file format using Acpype.…”

“…MD simulations of compounds that include structural fragments such as perfluorinated chains have previously proved difficult due to the lack of suitable parameters available in default forcefields, but recent extension of some default forcefields through parameterisation has led to more flexibility in relation to possible simulation targets. Recent work on an all-atom forcefield for perfluoropolyether compounds has yielded non-bonded parameters, 59 which have the potential to be utilised in the simulation of compounds exhibiting de Vries behaviour such as 3M 8422, shown in Fig. 1.…”

“…With replacement of the default non-bonded GAFF parameters assigned to the perfluoropolyether chain with those optimised by J. Black, et al, 59 alongside specific parameterisation of through-bond parameters, it was hoped that the polyperfluoroether chain could be simulated with sufficient accuracy to replicate de Vries behaviour successfully. Hence, the focus of this investigation is the assignment of optimised parameters to the GAFF forcefield and subsequent simulation of the achiral semi-perfluorinated compound 3M 8422, 19 as well as an extensive study of the resulting phase behaviour at the SmA-SmC transition through the application of atomistic MD simulations.…”