Biomineralization mechanisms: a kinetics and interfacial energy approach

摘要

The calcium phosphates and oxalates are among the most frequently encountered biomineral phases and numerous kinetics studies have been made of their crystallization and dissolution in supersaturated and undersaturated solutions, respectively. These have focused mainly on parameters such as solution composition, ionic strength, pH, temperature, and solid surface characteristics. There is considerable interest in extending such studies to solutions more closely simulating the biological milieu. The constant composition method is especially useful for investigating the mechanisms of these reactions, and in the present work, the interfacial tensions between water and each of these surfaces have been calculated from measured contact angles using surface tension component theory. Values for the calcium phosphate phases such as dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), hydroxyapatite (HAP), and fluorapatite (FAP) may be compared with data calculated from dissolution kinetics experiments invoking different reaction mechanisms. Agreement between the directly measured interfacial energies and those calculated from the kinetics experiments provides valuable corroborative information about individual growth and dissolution mechanisms. For the calcium phosphates, the much smaller interfacial tensions of OCP and DCPD in contact with water as compared with those of HAP and FAP support the suggestion that the former phases are precursors in HAP and FAP biomineralization. The ability of a surface to nucleate mineral phases is closely related to the magnitude of the interfacial energies. Constant composition studies have also shown that HAP is an effective nucleator of calcium oxalate monohydrate, both of which are frequently observed in renal stones.