Controlled and uncontrolled crystallization of calcium oxalate monohydrate in the presence of citric acid

The nucleation and growth of calcium oxalate monohydrate (COM) crystals beneath stearic acid (SA) monolayers in the presence of citric acid (H3cit) was examined at different surface pressures. The results were compared with those of crystals grown in bulk solution. In bulk solution, the morphology of the precipitated COM crystals transformed from the (101) face being most exposed to the (010) face being exposed when the concentration of H3cit increased from 0.01 to 0.30 mmol L^-1. This resulted in a morphological change of the COM crystals from elongated hexagonal to rhombic. At the SA monolayers, the (101) faces of the COM crystals were remarkably stabilized independent of either the change in H3cit concentration from 0.01 to 0.30 mmol L^-1 or the change in the surface pressure of the SA monolayer from 1 to 20 mN m^-1. This result was due to the fact that the (101) face of the COM crystals is a Ca2+-rich face, and the carboxylic groups in the headgroups of the SA monolayers and H3cit are negatively-charged. There is a strong interaction between the (101) face of the COM crystals and the negatively-charged carboxylic groups. However, the size of the COM crystals grown at low surface pressures is larger and the crystals more ordered than those grown at high surface pressures. This indicates that the monolayers at low surface pressure have a greater dynamic freedom and compressibility than those at high pressure. This allows the monolayers to reorganize themselves in order to optimize the geometrical and stereochemical fit and then to accommodate the nucleating or growing crystals. H3cit can inhibit both the nucleation and growth of COM crystals in aqueous solution and at monolayers. Furthermore, H3cit can reduce the order of COM crystals grown at SA monolayers because H3cit disturbs the ordered array of the first layer of COM nuclei at the monolayer surface.