Effect of hydrogen bonding on self-diffusion in methanol/water liquid mixtures: A molecular dynamics simulation study

Abstract

Self-diffusion motion is strongly dependent on hydrogen bonding and temperature. In this work, the methanol/water mixtures of molalities from 1 to 5m have been studied by molecular dynamics simulation. A definition of hydrogen bonding state is proposed to detailedly analyze the effects of the type and number of hydrogen bonds on the water self-diffusion. It is shown that most water molecules are in the hydrogen bonding state fkk, which means that one water molecule simultaneously hydrogen bonded to k water molecules with one hydrogen bond, respectively. Methanol prefers to produce isolated water molecules by the insertion of its hydrophobic group into the hydration shell of water. Besides, calculation of mean square displacements of water in different hydrogen bonding states shows that one water molecule with more hydrogen bonds diffuses more slowly. Pair energy of the hydrogen bonded molecules is also calculated to compare the attractive interactions of different types of hydrogen bonds. The hydrogen bonds between methanol and water present stronger attraction than that between water molecules. It indicates that increasing the concentration of methanol is conductive to restricting the water self-diffusion. Cluster analysis reveals that methanol cluster is more stable than water cluster in the binary mixtures. Thus high concentration of methanol enhances the blockage of the methanol cluster to water movement. These findings will lead us to further understand the mechanisms of water self-diffusion in the methanol/water mixture.