Structures, intermolecular interactions, and chemical hardness of binary water-organic solvents: a molecular dynamics study.

The evolution of structural properties, thermodynamics and averaged (dynamic) total hardness values as a function of the composition of binary water-organic solvents, was rationalized in view of the intermolecular interactions. The organic solvents considered were ethanol, acetonitrile, and isopropanol at 0.25, 0.5, 0.75, and 1 mass fractions, and the results were obtained using molecular dynamics simulations. The site-to-site radial distribution functions reveal a well-defined peak for the first coordination shell in all solvents. A characteristic peak of the second coordination shell exists in aqueous mixtures of acetonitrile, whereas in the water-alcohol solvents, a second peak develops with the increase in alcohol content. From the computed coordination numbers, averaged hydrogen bonds and their lifetimes, we found that water mixed with acetonitrile largely preserves its structural features and promotes the acetonitrile structuring. Both the water and alcohol structures in their mixtures are disturbed and form hydrogen bonds between molecules of different kinds. The dynamic hardness values are obtained as the average over the total hardness values of 1200 snapshots per solvent type, extracted from the equilibrium dynamics. The dynamic hardness profile has a non-linear evolution with the liquid compositions, similarly to the thermodynamic properties of these non-ideal solvents. Graphical abstract Computed dynamic total hardness, as a function of the cosolvent mass fraction for water-ethanol (EtOH), water-isopropanol (2PrOH) and water-acetonitrile (AN).