Conformer Selection Upon Dilution with Water: The Fascinating Case of Liquid Ethylene Glycol Studied via Molecular Dynamics Simulations

The aqueous solution of ethylene glycol (EG) is a binary liquid mixture that displays rich conformational and structural behaviour, which has not yet been adequately explored through atomistic molecular dynamics simulations. Herein, employing an accurate force field for EG, several physical properties of this solution are calculated to be in quantitative agreement with experimental data. While 79 % of molecules in neat liquid EG exist with their central OCCO dihedral in the gauche state, this fraction increases to 89 % in the dilute aqueous solution, largely in response to the increase in the static dielectric constant of the solution from that of neat liquid EG. The increase in gauche conformers increases the mean dipole moment of EG molecules in the solution which is additionally contributed by specific conformational states of the two terminal HOCC dihedral angles.

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

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 xE = 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 xE. 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 xE.