Hydrogen Bonding and Dielectric Spectra of Ethylene Glycol-Water Mixtures from Molecular Dynamics Simulations

Abstract

Mixtures of ethylene glycol with water are a prominent example of media with variable viscosity. Classical molecular dynamics simulations at room temperature were performed for mixtures of ethylene glycol (EG) and water with EG mole fractions of x_E = 0.0, 0.1, 0.2, 0.4, 0.6, 0.9, 1.0. The calculated dielectric loss spectra were in qualitative agreement with experiment. We found a slightly overestimated slowdown of the dynamics with increasing EG concentration compared to experimental data. Statistics of the hydrogen bond network and hydrogen bond lifetimes were derived from suitable time correlation functions and also show a slowdown in the dynamics with increasing x_E. A similar picture is predicted for the time scales of EG conformer changes and for molecular reorientation. A slight blue shift was obtained for the power spectra of the molecular center of mass motion. The results were used to give a qualitative interpretation of the origin of three different relaxation times that appear in experimental complex dielectric spectra and of their change with x_E.

Figure 1

Calculated dielectric loss spectra for EG–water mixtures for different EG mole fractions x_E.

Figure 2

Dependence of the orientational lifetimes on the mixing ratio, as calculated from the reorientation correlation function of the EG backbone (dashed line connecting C–C) and the H₂O symmetry axis (dashed line through O).

Figure 3

Lifetimes of the OCCO and HOCC dihedral angles defining the EG conformers.

Figure 4

Distribution of the cosines of O–H–O angles in different H-bonds at x_E = 0.4.

Figure 5

Average number of H-bonds per H₂O molecule, n_HB(H₂O) (left), and per EG molecule, n_HB(EG) (right), and their decomposition into different types. The black bullets give the average number of H-bonds per either one of the two molecules, n_HB(EG, H₂O), (X^A–X^D).

Figure 6

Hydrogen-bond lifetimes defined by S_HB(t_1/2, t*) = 1/2, for strides t* = 0.05 ps (left) and t* = 5 ps (right).

Figure 7

Power spectra of the EG molecules, obtained from the velocity autocorrelation functions of the centers of mass of the EG molecules.

Figure 8

Power spectra of H₂O molecules, obtained from the velocity autocorrelation functions of the centers of mass of the H₂O molecules.