Force field parameter estimation of functional perfluoropolyether lubricants

The head disk interface in a hard disk drive can be considered to be one of the hierarchical multiscale systems, which require the hybridization of multiscale modeling methods with coarse-graining procedure. However, the fundamental force field parameters are required to enable the coarse-graining procedure from atomistic/molecular scale to mesoscale models. In this paper, we investigate beyond molecular level and perform ab initio calculations to obtain the force field parameters. Intramolecular force field parameters for Zdol and Ztetraol were evaluated with truncated PFPE molecules to allow for feasible quantum calculations while still maintaining the characteristic chemical structure of the end groups. Using the harmonic approximation to the bond and angle potentials, the parameters were derived from the Hessian matrix, and the dihedral force constants are fit to the torsional energy profiles generated by a series of constrained molecular geometry optimization

Force Field Parameter Estimation of Functional Perfluoropolyether Lubricants

The head disk interface in a hard disk drive can be considered to be one of the hierarchical multiscale systems, which require the hybridization of multiscale modeling methods with coarse-graining procedure. However, the fundamental force field parameters are required to enable the coarse-graining procedure from atomistic/molecular scale to mesoscale models. In this paper, we investigate beyond molecular level and perform ab initio calculations to obtain the force field parameters. Intramolecular force field parameters for Zdol and Ztetraol were evaluated with truncated PFPE molecules to allow for feasible quantum calculations while still maintaining the characteristic chemical structure of the end groups. Using the harmonic approximation to the bond and angle potentials, the parameters were derived from the Hessian matrix, and the dihedral force constants are fit to the torsional energy profiles generated by a series of constrained molecular geometry optimization

Molecular Simulations of the Thermophysical Properties of Polyethylene Glycol Siloxane (PEGS) Solvent for Precombustion CO₂ Capture

The thermophysical properties for neat polyethylene glycol siloxane solvent (PEGS) along with CO₂, H₂, H₂O, and H₂S gas absorption in PEGS at 298–373 K were investigated via molecular simulations. The predicted neat PEGS density, heat capacity, surface tension, and CO₂ and H₂ solubilities in PEGS solvent agree reasonably well with the experimental data, with typical differences of 0.8–20%, while the predicted PEGS solvent viscosity is 1.7–2.5 times larger than the experimental data. Gas solubility in PEGS at 298 K decreases in the following order, H₂O (31000) > H₂S (230) > CO₂ (33) > H₂ (1), which follows the same order as the gas–PEGS interaction. In contrast, gas diffusivity in PEGS at 298 K decreases in an opposite way, H₂ (1) > CO₂ (0.22) ≈ H₂S (0.12) > H₂O (0.018). The numbers in parentheses are the corresponding values relative to H₂. Compared to the widely studied poly­(dimethylsiloxane) (PDMS) solvent, PEGS is more hydrophilic due to its stronger interaction with H₂O and fewer branched −CH₃ groups, which in turn leads to fewer hydrophobic pockets. The CO₂/H₂ solubility selectivity in PEGS is larger than that in PDMS due to a stronger interaction with CO₂ in PEGS. Finally, it was found that CO₂ absorption in PEGS could significantly improve the CO₂–PEGS solution dynamics by 5–6 times, resulting in a decrease in solution viscosity and increase in diffusivity. These CO₂ absorption effects are due to solution volume expansion upon CO₂ absorption compared to the neat PEGS solvent volume and the possibility that CO₂ acts as a “lubricant” to decrease the solvent–solvent interaction