Molecular dynamics study on water self-diffusion in aqueous mixtures of methanol, ethylene glycol and glycerol: investigations from the point of view of hydrogen bonding

Abstract

Molecular dynamics simulations have been performed to investigate the aqueous binary mixtures of alcohols, including methanol, ethylene glycol (EG) and glycerol of molalities ranging from 1 to 5 m at the temperatures of 273, 288 and 298 K, respectively. The primary purpose of this paper is to investigate the mechanism of water self-diffusion in water-alcohol mixtures from the point of view of hydrogen bonding. The effects of temperature and concentration on water self-diffusion coefficient are evaluated quantitatively in this work. Temperature and concentration to some extent affect the hydrogen bonding statistics and dynamics of the binary mixtures. It is shown that the self-diffusion coefficient of water molecules decreases as the concentration increases or the temperature decreases. Moreover, calculations of mean square displacements of water molecules initially with different number n of H-bonds indicate that the water self-diffusion coefficient decreases as n increases. We also studied the aggregation of alcohol molecules by the hydrophobic alkyl groups. The largest cluster size of the alkyl groups clearly increases as the concentration increases, implying the emergence of a closely connected network of water and alcohols. The clusters of water and alcohol that interacted could block the movement of water molecules in binary mixtures. These findings provide insight into the mechanisms of water self-diffusion in aqueous binary mixtures of methanol, EG and glycerol.

Keywords:

• molecular dynamics simulation
• water
• alcohols
• self-diffusion coefficient

Acknowledgements

The support from the National Nature Science Foundation of China (50976017) and NSFC's Key Program Projects (50736001) is greatly appreciated. The authors are grateful to the Computing Center of the Department of Energy and Power Engineering of Dalian University of Technology for providing a parallel computing environment.

Notes

x is the percentage of molar fraction of alcohol solutes; N_s and N_w are the numbers of solute and water molecules, respectively.