Molecular dynamics simulations of the interaction between the surfaces of polar solids and aqueous solutions

Molecular dynamics ( MD) simulations were performed on the interaction of two solid surfaces, namely the (00.1) hematite and (10.4) calcite surfaces, in contact with aqueous electrolyte solutions containing different concentrations of dissolved NaCl. The structure and a number of properties of the interface were investigated. The size and amount of statistics needed for convergence of these calculations required the use of high performance computers. The two surfaces show different bonding mechanisms with the water, but both result in a distinctive layering of the water, which in turn modifies a range of surface behaviour including diffusivity and charge distribution. We find that the resulting charge distribution from the solvent has a greater control of the disposition of dissolved ions than either surface charge or ionic strength, within reasonable limits. Thus we see a characteristic double layer at neutral surfaces and the charge distribution oscillates into the bulk. Finally, preliminary work on calculating the free energy of dissolution of ions from the surface to the aqueous solution suggests that the presence of dissolved ions makes a small but significant reduction to the dissolution free energies.