Modeling of aqueous poly(oxyethylene) solutions: 1. Atomistic simulations.

The performance of different recently proposed force fields in combination with TIP4P-Ewald (TIP4P-Ew) water in reproducing experimental data of liquid 1,2-dimethoxyethane (DME) and its aqueous solutions for conformer populations, densities of solutions, and self-diffusion coefficients was explored. A modified version of the OPLS force field ("engineered") showed best performance in describing the conformer equilibria, but extremely high interconformational barriers reduce its applicability in dynamical simulations. The TraPPE-united atom force field (TraPPE-UA) by Siepmann et al. (J. Phys. Chem. B 2004, 108, 17596) was found to perform best in reproducing thermodynamic properties, but it showed some deficiency in describing the conformer equilibria. We reparameterized the dihedral potentials to match recent ab initio data by Anderson and Wilson (Mol. Phys. 2005, 103, 89) and could improve significantly the performance of description of conformer populations of DME in water. Subsequently, this modified TraPPE-UA was used in extensive simulations of poly(oxyethylene) oligomers H(CH2OCH2)nH (POEn) with n = 3, 5, 10, 12, 20, 30 repeat units at mass fractions between 3% and 80% at 298 K. Density, radii of gyration, and diffusion coefficients are in very good agreement with available experimental data. We conclude that this force field in combination with the TIP4P-Ew water model is very suitable for simulations of poly(oxyethylene) oligomers in aqueous solution. The application to real polymeric systems on the atomistic level is however hindered by very slow decorrelation of large-scale features and by slow diffusion.