Selective attachment of monovalent background electrolyte ions and growth inhibitors to polar steps on sulfates as studied by molecular simulations and AFM observations

Molecular simulations and in situ atomic force microscopy (AFM) experiments are used to demonstrate the influence of the ionic strength of the solution and the mechanism of growth inhibitors on growth and dissolution of barite with some comparative experiments on celestite. Growth and dissolution rates, as determined from monolayer step edge velocities, increase with increasing background electrolyte (NaCl) concentration. The electrolyte effect is analyzed in terms of changes of the bulk solution. physicochemical properties of the near-surface region and the mechanisms on the sulfate surfaces at a molecular scale. Bulk solution effects are mainly based on changes in the activity coefficient of dissolved species whereas increased growth and dissolution rates are partly due to a decreased interfacial tension between barite and supersaturated aqueous solution at high ionic strength. We have also found possible mechanisms from molecular simulations that explain the rate changes and also the significant changes in morphologies of etch pits and growth islands when monovalent background electrolyte ions are added. In solutions with high ionic strength, the relative stability of polar [010] steps is increased in comparison to the dipole-free <120> direction, evident as growth islands and etch pits. which are elongated in the [010] direction under growth and dissolution conditions, respectively, This indicates a specific interaction between the background electrolyte and certain sites on the mineral surface. The increased relative stability of steps parallel to [010] relative to those parallel to <120> could be explained by the formation of stabilized -Na-SO4-Na-SO4- or Cl-Ba-Cl-Ba-Cl- chains along the step, which are less polar than terraces bounded by either SO42-.