Temperature Dependence of Water Dynamics in Poly(Ethylene Oxide)/Water Solutions from Molecular Dynamics Simulations and Quasielastic Neutron Scattering Experiments

摘要

The dynamics of water in aqueous solutions of poly(ethylene oxide) (PEO) were studied by performing molecular dynamics (MD) simulations and quasielastic neutron scattering (QNS). The simulations and experiments were carried out on PEO/water solutions for the composition EO:O_w (ether oxygen:water oxygen) = 1:2.3 in the temperature range from 298 to 410 K. To selectively measure the motion of the water, perdeuterated PEO (d-PEO) was used for the QNS experiments. Intermediate scattering functions derived from the MD simulations were found to be in good agreement with those from QNS experiments. The QNS and MD dynamic structure factors were analyzed using the random jump diffusion (RJD) model, a model frequently applied in analysis of water QNS data, yielding rotational and translational diffusion coefficients for water in the d-PEO/H_2O solutions. Analysis of MD results indicates that, although the translational self-diffusion coefficient can be extracted from the RJD fits to the data rather accurately, the rotational diffusion coefficient has an uncertainty of a factor of 3. The water rotational relaxation was found to be anisotropic, with the rotation in the plane perpendicular to the water dipole vector being 2-2.5 times faster than the water dipole vector relaxation. The Sears model if isotropic rotation provided a poor description of the water rotational dynamics at 298 and 368 K, with better agreement at 410 K. The increased anisotropy of water rotation in PEO/water solutions compared to that of pure water is attributed to PEO/water electrostatic interactions.