The effect of cation:anion ratio in solution on the mechanism of barite growth at constant supersaturation: Role of the desolvation process on the growth kinetics

The mechanism of barite growth has been investigated in a fluid cell of an Atomic Force Microscope by passing solutions of constant supersaturation (Ω) but variable ion activity ratio (r=a_Ba/a_SO4) over a barite substrate.The observed dependence of step-spreading velocity on solution stoichiometry can be explained by considering non-equivalent attachment frequency factors for the cation and anion. We show that the potential for two-dimensional nucleation changes under a constant thermodynamic driving force due to the kinetics of barium integration into the surface, and that the growth mode changes from preexisting step advancement to island spreading as the cation/anion activity ratio increases. Scanning electron microscopy studies of crystals grown in bulk solutions support our findings that matching the ion ratio in the fluid to that of the crystal lattice does not result in maximum growth and nucleation rates. Significantly more rapid rates correspond to solution stoichiometries where Ba²⁺] is in excess with respect to ].Experiments performed in dilute aqueous solutions of methanol show that even 0.02 molar fraction of organic cosolvent in the growth solution significantly accelerates step growth velocity and nucleation rates (while keeping Ω the same as in the reference solution in water). Our observations suggest that the effect of methanol on barite growth results first of all from reduction of the barrier that prevents the Ba²⁺ from reaching the surface and corroborate the hypothesis that desolvation of the cation and of the surface is the rate limiting kinetic process for two-dimensional nucleation and for crystal growth.