On the Effect of Cu(II) Impurity on the Growth Kinetics of Ammonium Oxalate Monohydrate Crystals from Aqueous Solutions

Abstract

The effect of concentration of Cu(II) ions and solution supersaturation on growth rates of different faces of ammonium oxalate monohydrate (AO) single crystals is studied at 30 °C and supersaturations up to 10%. It was observed that the growth rates of the (001) face of the crystals decrease while those of the (100) and (010) faces increase with an increase in the concentration of the impurity. Instead of analysing the dependence of face growth rates R on impurity concentration ci for different supersaturations σ in terms of Cabrera-Vermilyea and Kubota-Mullin models, the dependence of R on σ for different ci were analysed using spiral growth theory involving cooperating dislocation source and multiple nucleation model. It was found that both models describe the experimental data on growth kinetics satisfactorily. Analysis of the data revealed that an increase in the face growth rate with impurity concentration is associated with a decrease in the free energy of the growing face (thermodynamic effect) while a decrease in face growth rate with an increase in the concentration of the impurity is a kinetic effect involving a reduction in the value of the kinetic coefficient β for the motion of steps on the surface. It was also found that usually both effects take place simultaneously but one of them dominates during the growth of a face. The dependences of surface free energy and kinetic coefficient on impurity concentration follow Freundlich adsorption isotherm.