Constant Composition Studies Verify the Utility of the Cabrera−Vermilyea (C-V) Model in Explaining Mechanisms of Calcium Oxalate Monohydrate Crystallization

Abstract

The classic theory of Cabrera and Vermilyea (C-V) postulates that inhibition of crystallization by impurities is the result of pinning of step motion. Although generally accepted, the predictions of the C-V model have not been previously linked to studies of impurity adsorption in macroscopic crystallization systems. Since calcium oxalate monohydrate (COM) is the primary constituent of most human kidney stones, effects of impurities on COM crystallization are biologically relevant. Recent in situ atomic force microscopy (AFM) studies suggest that citrate molecules adsorb to COM step edges on the (−101) face, thereby pinning step motion and decreasing the velocity of the steps. We have now investigated the crystallization kinetics of COM in the presence of citrate at 37 °C and I = 0.15 mol L-1 using the constant composition (CC) method. The dependence of growth rate on both supersaturation and citrate concentration were measured, and the behavior of the COM−citrate system was found to be in reasonable agreement with the predictions of the C-V model, provided that, in addition to pinning of step motion on the (−101) face through the Gibbs−Thomson effect, the kinetic coefficient that relates step speed to supersaturation is assumed to decrease with increasing citrate levels on all faces. The dependence of macroscopic crystal aspect ratio on citrate level in these CC studies also fits this model. Thus, we find that the results provide a mechanistic link between the microscopic AFM measurements of step kinetics and the macroscopic CC data on crystal growth rates and crystal habit.