The Behavior of Water in Monodisperse Polyethylene Glycols Determined by Molecular Dynamic Simulations

Abstract

A molecular dynamics simulation study is presented for mixture systems containing either di-, tetra-, or pentaethylene glycol and 20,000 parts per million (ppm) of water in the determination of whether water clusters or bridging hydrogen bonds form in such systems. An analysis of the validity of the all-atom Optimized Potential for Liquid Simulations (OPLS/AA) force field utilized for the polyethylene glycols (PEGs) is also presented. Results from densities, self-diffusion coefficients, hydrogen bonding numbers, radial distribution functions, and simulation trajectory snapshots revealed that water formed bridging hydrogen bonds rather than clustered aggregates, thereby acting as a glue between the PEG molecules and causing extensive structuring and a slowdown in the system dynamics. The role of PEG-PEG cross-links and hydrogen bonding between hydroxyl groups and water in reducing water movement was proposed to be the basis for the formation of water bridges rather than clusters. Densities were reproduced well by the OPLS/AA force field, although self-diffusion coefficients and shear viscosities were severely underestimated and overestimated, respectively. Despite the poor reproduction of PEG dynamical properties by the force field, simulations were found to follow Stokes-Einstein behavior, suggesting that a significant reparameterization of the OPLS/AA force field for PEGs is needed.