Computer simulation study of ion-water and water-water hydrogen bonds in sulfuric acid solutions at low temperatures

Abstract

Molecular dynamics simulations of sulfuric acid solutions at several temperatures (230, 250, 273 and 298 K) and concentrations (10, 20, 30 and 40 wt%) have been carried out. The dissociation of the acid has been explicitly considered, in accordance with the available experimental measurements. So, the simulated systems are made up of the following molecular constituent species: water molecules, hydronium cations, bisulfate anions and sulfate dianions. Calculations have been conducted using a reliable force field, which allows us to determine density and viscosity values, which are in a reasonably good agreement with the available experimental data in the temperature and concentration ranges considered in this work. Simulations have also been employed to determine the radial distribution functions, which involve water molecules, and the diffusion coefficients of the constituent species. Moreover, a hydrogen bond analysis, involving only water molecules, has also been fulfilled. To this end, the mean number of hydrogen bonds between water molecules and between ions and water molecules, the percentages of molecular species hydrogen bonded with a given number of water molecules, and the continuous and interrupted lifetimes have been calculated. We observe that most properties are mainly sensitive to the concentration. We also notice that water molecules bonded to the anions are more labile than those bonded to other water molecules or to hydronium ions, the labilities increasing when the temperature rises. Moreover, the characteristic tetrahedral structure of water vanishes when the concentration or the temperatures increases. In the first case because water molecules tend to bond to a larger number of ions. In the second because the mean number of hydrogen bonds between water molecules and their lifetimes decrease. Finally, we also estimate the activation energies from the interrupted lifetimes, the viscosity and the diffusion coefficients. We note that these energies are very similar and they increase when the concentration rises.