Atomistic Simulation of Poly(ethylene oxide) In Aqueous Solution: Challenges and Suggestions for Models
We demonstrate that current force field models for poly(ethylene oxide) (PEO), also called poly(ethylene glycol) (PEG), in water solution fail to reproduce solubility and radii of gyration as observed in experiment by leading to chain collapse or excessively outstretched conformations under standard conditions. These difficulties have been resolved by adjusting atomic charges and corresponding dipole moments on the PEO backbone to close agreement with experimental data and consistency with available models for water. We present chemically detailed models for PEO with point charges on every atom in combination with the single point charge model (SPC) for water, and models extended by charged dummy atoms at the oxygen lone pairs in combination with a TIP5P water model. Experimentally known preferences of the O-C-C-O dihedrals for gauche conformations and lower eclipsed torsion barriers (~4 kcal/mol) are included. The new, tentative models reproduce densities, interfacial properties, and radii of gyration in good agreement with experiment, and the parameters are compatible with biomolecular and materials oriented force fields such as CHARMM, GROMACS, CVFF, PCFF (including 12-6 as well as 9-6 Lennard-Jones potentials). Nevertheless, the evaluation of the entire phase diagram for various chain lengths and temperatures [Dormidontova, E. E. Macromolecules 2002, 35, 987] remains as a formidable future challenge.