Molecular Dynamics Study of Aqueous Solution of Polyethylene Oxide: Critical Test of Force Field Models

Five differently proposed force fields are compared on their ability to reproduce experimental data of dilute aqueous solutions of polyethylene oxide (PEO). Solution densities, radii of gyration, radial distribution functions, self-diffusion coefficients, and also solvation free energies are evaluated in using atomistic Molecular Dynamics simulations. The chosen force fields are the three ones from Bedrov et al. [J. Phys. Chem. B 1998, 102, 996], including the original force field with the generic combining rule (GEN), one with empirical combination rule (EMP) and quantum chemistry based force field (QC). The other two force fields are the one from Cordeiro et al. [Macromolecules 2010, 43, 1583] (CZMP) and the CHARMM force field. The CZMP force field performs best in reproducing both thermodynamic and dynamic properties of the PEO dilute solution. The QC force field describes the chain dimension and hydrophilic behavior of PEO oligomers reasonably well and also the power-law exponent matches well with experimental results, but it overestimates the interaction strength between PEO and water resulting in slightly higher densities and slower mobilities for PEO oligomers in dilute solution. The CHARMM force field reproduces the densities and self-diffusion coefficients for PEO oligomers quite well, but it gives PEO a semi-hydrophobic character in dilute solution. The original force field, together with the EMP combining rule gives only the thermodynamic properties of PEO dilute solutions in a reasonable agreement with experimental data, but describes neither the thermodynamic nor the dynamic properties of PEO oligomers properly when it is used with the generic combination rule. Simulation results also reveal that CZMP, QC and CHARMM can qualitatively reproduce the solvation free energies for the ether unit in dilute aqueous solution. The influence of different water models on phase behavior of dilute PEO aqueous solution is indistinguishable in our simulations.