Water-water and ion-water hydrogen bonding in sulfuric acid solutions

Abstract

Molecular dynamics simulations of sulfuric acid solutions at room temperature and at concentrations up to 40 wt% have been performed. The dissociation of the acid in water has been implicitly taken into account by assuming that the simulated systems are made up of the following molecular constituent species: water molecules, hydronium cations, bisulfate anions and sulfate dianions. Calculations have been carried out using a reliable force field which provides a good agreement between the experimental and the simulation densities and viscosity coefficients. The density and several structural and dynamical quantities, such as the radial distribution functions, the shear viscosity and the diffusion coefficients, have been calculated. The results show a noticeable good agreement with the available experimental data. A study of the hydrogen bonding which involve water molecules has also been performed. Then, the mean number of hydrogen bonds between water molecules and between ions and water molecules, and the percentages of these hydrogen bonds per water molecule or ion have been computed. Moreover, an analysis of the formation and rupture of the hydrogen bonds has also been made. To this end, the continuous and the interrupted hydrogen bond lifetimes have been calculated. Our findings suggest that the water molecules hydrogen bonded to the anions are more labile than those bonded to other water molecules or to hydronium ions. Finally, the role played by the acid concentration on these quantities has also been analyzed. Then, for instance, an increase of the lability of the water molecules, which are hydrogen bonded to other water molecules, has been observed when the concentration rises.