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

Alexander Kaiser,Renat Nazmutdinov,Marcel Ritter,Michael Probst

doi:10.1021/acs.jpcb.6b05236

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 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.

Highlights

Water and ethylene glycol (EG) are perfectly miscible and the viscosity of EG is much larger than that of pure water
From the viewpoint of modern quantum mechanical theories of electron transfer in condensed media,[6] classical solvent modes contribute to the activation (Franck−Condon) energy barrier, while their characteristic relaxation times are responsible in the adiabatic limit for solvent dynamical effects
In a previous article,[7] we provided a comprehensive analysis of Hbonding in liquid EG based on molecular dynamics (MD) simulations with a critical look on the differences between various force fields

Summary

INTRODUCTION

Water and ethylene glycol (EG) are perfectly miscible and the viscosity of EG is much larger than that of pure water. EG−water mixtures are a prominent example of media with controllable viscosity This makes them useful for investigations of solvent dynamics effects on electron transfer reactions (i.e., the dependence of the mechanism of elementary act on solvent viscosity, saddle point avoidance)[1,2] that are sometimes the dominant contributions to the reaction rate. Dielectric spectra of EG−water solutions have been reported in the classical region of frequencies (ħω ≪ kB).[1] The imaginary part of the complex dielectric spectra was fitted to a Debye model with three characteristic relaxation times. From the viewpoint of modern quantum mechanical theories of electron transfer in condensed media,[6] classical solvent modes contribute to the activation (Franck−Condon) energy barrier, while their characteristic relaxation times are responsible in the adiabatic (strong coupling) limit for solvent dynamical effects.

COMPUTATIONAL DETAILS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES