Concentration Dependence of Water Dynamics in Poly(Ethylene Oxide)/Water Solutions from Molecular Dynamics Simulations

Abstract

We have performed molecular dynamics simulations of aqueous solutions of poly(ethylene oxide) (PEO) in order to investigate the influence of the polymer on water dynamics. Simulations were performed on 12 repeat unit CH 3 -capped PEO chains (530 Da) at 318 K covering a composition range (polymer weight fraction) from 0.17 to 1.0. The simulations employed an ab initio quantum-chemistry-based PEO/water and PEO/PEO force field together with the TIP4P (four-point transferable intermolecular potential) water model. Water translational and rotational diffusion were found to slow monotonically, whereas water-water and waterether hydrogen bond lifetimes increased with increasing polymer concentration. The slowing of water dynamics in PEO/water solutions was associated with PEO-water interactions that are moderated in concentrated solutions by the formation of water clusters. Water translational motion could be ascribed to a combination of free water (water not involved with PEO hydration) that exhibited bulklike water dynamics and bound (hydrating water) whose motion was strongly correlated with that of the PEO molecule. Water rotational motion was found to be strongly correlated with translation motion and exhibited increasing anisotropy with increasing PEO concentration indicative of preferred rotation of the water molecules around their dipole moment vector. At high PEO concentration, water and ether oxygen atoms exhibited well pronounced subdiffusive behavior occurring on a picosecond time scale that disappears upon dilution. The characteristic length scale for the water subdiffusive behavior associated with water caging correlates well with nearestneighbor ether oxygen-ether oxygen distance.

Quasielastic Neutron-Scattering Study of the Local Dynamics of Poly(ethylene glycol) Dimethyl Ether in Aqueous Solution

“…Intermediate scattering functions for a pure PEG-DME melt at 318 K as predicted by simulation. 13 Momentum transfer values from top to bottom are 1, 2, 3, 4, 4.57, 5, 6, 7, 8, 9, 10, 11.39, 17.16, and 21.74 nm -1 .…”

“…13 An increase in the fraction of highly mobile water increases the propensity of fragments of PEG-DME and even whole PEG-DME molecules to move in water without undergoing torsional transitions. This mechanism becomes more and more important with increasing PEG-DME dilution and results in faster PEG-DME motions (Figures 5, 6, and 12) at low W p despite a decreased torsional transition rate of hydrated PEG-DME compared to the one in the melt.…”

“…Our previous neutron-scattering publications have looked at the dynamics of water in aqueous poly(ethylene glycol) (PEG-DME) solutions 7 and in aqueous solutions of deuterated 1,2-dimethoxyethane, 8,9 which is the smallest molecule that is still representative of PEG. Several other publications [10][11][12][13] are concerned with the molecular dynamics simulations of aqueous solutions of PEG-DME. This paper deals with the motion of relatively short PEG-DME molecules (molecular weight approximately 500 u) in aqueous solution as a function of concentration for comparison with the results of recent molecular dynamics simulations.…”

Dynamics of Water Associated with Lithium Ions Distributed in Polyethylene Oxide

“…However, the values from MD simulation is about twice larger than the experimental value [65][66][67][68], where such quantitative discrepancy is generally understood by the fact that the MD simulation is using an effective interaction potential to describe the dynamic properties of the system, which is a simplified model; while the QENS experiments measure the details of the molecular interaction directly in the solution [69]. Borodin and collaborators [64] reported the concentration dependence of water dynamics in PEO/water solution by MD simulation. The diffusion coefficient of interfacial water which was estimated to be 1×10 -5 cm 2 /s at 50 wt% PEO, is consistent with the number we obtained from our MD simulation analysis.…”

“…f is the fraction of interfacial water. In general, the dynamics of bulk water molecules are described by combinations of rotational diffusion and translation diffusion processes [54,64]. However, within the q range and energy range explored by BASiS (0.3 Å -1 < q < 1.1 Å -1 , -115 µeV < E < 115 µeV), the rotational diffusion can be neglected and therefore, the bulk water diffusion process can be approximated by pure translational diffusion, which can be given by Γ 1 = D 1 q 2 , where D 1 is the q-independent diffusion coefficient.…”