Computer simulation study of ion-water and water-water hydrogen bonds in methanesulfonic acid solutions at room temperature

Abstract

Classical molecular dynamics simulations of aqueous methanesulfonic acid solutions have been conducted at room temperature in the entire composition range. The dissociation of the acid has been considered according to the available experimental data. Then, the systems are constituted by the following molecular species: water and methanesulfonic acid molecules, hydronium cations, and mesylate anions. The simulations have been carried out employing a reliable force field, which provides density values that show an excellent agreement with the available experimental data. The shear viscosity, the diffusion coefficients of the molecular species, and the radial distribution functions, which involve water molecules, have also been computed. We have observed that water molecules diffuse faster than the other species at all concentrations. Moreover, the shear viscosity and the diffusion coefficients exhibit a noticeable unimodal concentration dependence with extrema located at 90 wt% (0.628 mol fraction). A detailed hydrogen bond analysis, concerning water molecules, has also been made. At low dilutions, water exhibits its well-known hydrogen bond tetrahedral structure, which vanishes as the acid concentration increases. The most labile water molecules are those bonded to the anions. At large concentrations, the lability of the water molecules, bonded to other water molecules, increases, and that of the water molecules, bonded to the cations, decreases. The interrupted lifetimes also show a unimodal dependence with maxima located at 90 wt%. This behavior could be related to the appearance of the methanesulfonic acid hydrogen bond network, which emerges at large weight percentage values because the acid almost completely dissociates up to concentrations of 70 wt% (0.304 mole fraction).