Molecular Dynamics Study of the Green Solvent Polyethylene Glycol with Water Impurities

Abstract

Polyethylene glycol (PEG) is one of the environmentally benign solvent options for green chemistry. It readily absorbs water when exposed to the atmosphere. The Molecular Dynamics (MD) simulations of PEG200, a commercial mixture of low molecular weight polyethyelene glycol oligomers, as well as di-, tetra-, and hexaethylene glycol are presented to study the effect of added water impurities up to a weight fraction of 0.020, which covers the typical range of water impurities due to water absorption from the atmosphere. Each system was simulated a total of four times using different combinations of two force fields for the water (SPC/E and TIP4P/2005) and two force fields for the PEG and oligomer (OPLS-AA and modified OPLS-AA). The observed trends in the effects of water addition were qualitatively quite robust with respect to these force field combinations and showed that the water does not aggregate but forms hydrogen bonds at most between two water molecules. In general, the added water causes overall either no or very small and nuanced effects in the simulation results. Specifically, the obtained water RDFs are mostly identical regardless of the water content. The added water reduces oligomer hydrogen bonding interactions overall as it competes and forms hydrogen bonds with the oligomers. The loss of intramolecular oligomer hydrogen bonding is in part compensated by oligomers switching from inter- to intramolecular hydrogen bonding. The interplay of the competing hydrogen bonding interactions leads to the presence of shallow extrema with respect to the water weight fraction dependencies for densities, viscosities, and self-diffusion coefficients, in contrast to experimental measurements, which show monotonous dependencies. However, these trends are very small in magnitude and thus confirm the experimentally observed insensitivity of these physical properties to the presence of water impurities.

Keywords: density; ethylene glycol oligomers; hydrogen bonding; polyethylene glycol; radial distribution functions; self diffusion; viscosity; water impurity.

Figure 1

(a) Triethylene glycol as an example of ethylene glycol oligomers that make up PEG200, H-[O-CH₂-CH₂]_n-OH, shown with the distance between the oxygen atoms of the terminal hydroxy end groups used in this report as a measure for the end-to-end distance, and the changes in the hydroxy partial charges in the modified OPLS forcefield; (b) an illustration of the proper dihedral angle referred to as (HO)-C-C-O in this report for which its potential energy was reduced in the modified OPLS forcefield; (c) a summary of possible inter- and intramolecular hydrogen bonding interactions in the studied water containing ethylene glycol oligomer mixtures.

Figure 2

Snapshot of a MD simulation of the PEG200 with w_water = 0.020 using the OPLS force field for the PEG200 and the SPC/E forcefield for the water showing (a) all molecules, (b) a water molecule interacting with hexaethylene glycol, and (c) just the water molecules, (oxygen = red, hydrogen = white, carbon = teal).

Figure 3

Radial distribution functions obtained with the TIP4P/2005 and OPLS forcefields of the water oxygen (a) with water oxygen and (b) with hydroxy (black) and with ether oxygen (red) of diethylene glycol, each from four different water mass fractions of 0.001 (omitted in (a)) to avoid clutter due to large data noise), 0.005, 0.010, and 0.020. Aside from different noise level, the respective radial distribution functions are indistinguishable.

Figure 4

Radial distribution functions of the water oxygen with (a) water oxygen as well as (b) oligomer hydroxy oxygen (black) and ether oxygen (red) for 0.02 mass fraction of water in hexaethylene glycol obtained with the water force fields TIP4P/2005 (solid) and SPC/E (dotted) in combination with the OPLS forcefield for hexaethylene glycol. The respective radial distribution functions are essentially indistinguishable.

Figure 5

Radial distribution functions of the water oxygen with (a) water oxygen as well as (b) oligomer hydroxy oxygen (black) and ether oxygen (red) for 0.02 mass fraction of water in hexaethylene glycol obtained with the water force fields TIP4P/2005 in combination with the OPLS forcefield (solid) and the modified OPLS force field (dotted).

Figure 6

Radial distribution functions of the water oxygen–oligomer hydroxy oxygen (black) and water oxygen–oligomer ether oxygen (red) for 0.02 mass fraction of water in the tetraethylene glycol (solid line) and in the PEG200 (dotted line) obtained with the water force fields SPC/E in combination with the OPLS forcefield.

Figure 7

Number of the hydrogen bonds per number of the water molecules for the intermolecular hydrogen bonding of the water with hydroxy groups (squares), ether groups (circles), other water molecules (triangle up), and the total of all these water hydrogen bonding interactions (triangle down) as a function of the water mass fractions, w_water, at 328 K for diethylene glycol (a), tetraethylene glycol (b), hexaethylene glycol (c), and PEG200 (d) obtained from the simulations using the SPC/E force field for the water and the OPLS (black symbols) or the modified OPLS force field (red symbols) for the oligomers.

Figure 8

Number of hydrogen bonds per number of the oligomer molecules for intramolecular hydrogen bonding between (a) the two hydroxy groups and (b) the hydroxy and ether groups and for intermolecular hydrogen bonding between (c) the two hydroxy groups and (d) hydroxy and ether groups as a function of the water mass fractions, w_water, at 328 K for the diethylene glycol (squares), tetraethylene glycol (circles), and hexaethylene glycol (triangle up), obtained from the simulations using the SPC/E force field for water and the OPLS (black symbols) or the modified OPLS force field (red symbols) for the oligomers.

Figure 9

(a) End-to-end distances and (b) the radii of gyration as a function of the water mass fractions, w_water, at 328 K for the diethylene glycol (squares), tetraethylene glycol (circles), and hexaethylene glycol, (triangle up), obtained from the simulations using the SPC/E force field for water and the OPLS (black symbols) or the modified OPLS force field (red symbols) for the oligomers.

Figure 10

(a) Densities, (b) viscosities, and self-diffusion coefficients of the (c) oligomers and (d) water as a function of the water mass fractions, w_water, at 328 K for the diethylene glycol (squares), tetraethylene glycol (circles), hexaethylene glycol (triangle up), and PEG 200 (triangle down) obtained from the simulations using the SPC/E force field for water and the OPLS (black symbols) or the modified OPLS force field (red symbols) for the oligomers.