Kinetics of dissolution of triclinic calcium pyrophosphate dihydrate crystals

Abstract

Under the conditions studied, the rate of dissolution of triclinic calcium pyrophosphate dihydrate crystals can be explained mainly by a polynuclear surface process. Due to the size of the crystals studied, this process is slightly influenced by diffusion of dissolved material away from the crystals. Small nuclei (holes) are formed by fluctuations in the surface. Due to the surface tension, nuclei have to reach a critical size before they can grow larger and intergrow with other nuclei. The lateral growth rate of surface nuclei is found to be mainly controlled by the concentration of calcium ions. The surface tension for dissolution is found to be of the order 30 mJ/m2. From the experimentally determined rate constant, a value of about 1-3×105/s is obtained for a calcium ion to perform a diffusion jump and at the same time partially dehydrate. The value is close to the expected value of 1.6×105/s. Within this model the rate of dissolution can be explained in terms of molecular events on the crystal surface.