Biomineralization mechanisms: a kinetics and interfacial energy approach

The calcium phosphates and oxalates are among the most frequently encounter ed biomineral phases and numerous kinetics studies have been made of their crystallization and dissolution in supersaturated and undersaturated soluti ons, respectively. These have focused mainly on parameters such as solution composition, ionic strength, pH, temperature, and solid surface characteri stics. There is considerable interest in extending such studies to solution s more closely simulating the biological milieu. The constant composition m ethod is especially useful for investigating the mechanisms of these reacti ons, and in the present work, the interfacial tensions between water and ea ch of these surfaces have been calculated from measured contact angles usin g 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 cal culated from dissolution kinetics experiments invoking different reaction m echanisms. Agreement between the directly measured interfacial energies and those calculated from the kinetics experiments provides valuable corrobora tive information about individual growth and dissolution mechanisms. For th e 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 su ggestion that the former phases are precursors in HAP and FAP biomineraliza tion. The ability of a surface to nucleate mineral phases is closely relate d to the magnitude of the interfacial energies. Constant composition studie s have also shown that HAP is an effective nucleator of calcium oxalate mon ohydrate, both of which are frequently observed in renal stones. (C) 2000 E lsevier Science B.V. All rights reserved.