Modeling of aqueous poly(oxyethylene) solutions. 2. Mesoscale simulations.

We extend our work on aqueous solutions of poly(oxyethylene) oligomers H-(CH2-O-CH2)n -H (POEn). On the basis of atomistic simulations of trimer and decamer solutions (first part of this series of papers), different sets of coarse-grained implicit-solvent potentials have been constructed using the iterative Boltzmann inversion technique. The comparison of structures obtained from coarse-grained simulations (gyration radii, end-to-end distances, radial distribution functions) with atomistic reference simulations and experiments shows that the state-specific potentials are transferable both to a wide concentration range, if the same molecule size is considered, and to at least 2 orders of magnitude larger molecules (in terms of molecular mass). Comparing the performance of different mesoscale potentials, we find different applicability ranges in terms of molecule sizes. The experimental gyration radii for chains comprising up to 1500 monomers are reproduced almost quantitatively by the decamer-fitted potentials with dihedral interactions included. The trimer-fitted potentials reproduce experimental chain dimensions of up to some hundred monomers but seem to become metastable beyond a certain chain length, as we evidenced some chain collapses. Relaxation of large-scale features is 1-2 orders of magnitude faster in the mesoscale simulations than in the atomistic simulations. The diffusion behavior in dependence of concentration is captured correctly when the decamer potential is applied to the decamer itself. For all other chain lengths, we find that time mapping from coarse-grained to atomistic trajectories has to be determined separately for each concentration. Overall, diffusion is 1-2 orders of magnitude faster on the mesoscale, depending considerably on the Lowe-Andersen thermostat parameters. The CG simulations provide an overall speed-up of about 3 orders of magnitude.