Influence of the dielectric constant on bioapatite adsorption of glycine

The adsorption of glycine onto apatitic calcium phosphates was carried out at 37degreesC in medium with a dielectric constant between those of water and ethanol. These apatites, containing both HPO42- and Co-3(2-) ions, are similar to the mineral part of bone at various stages of its maturation. In this study, the time needed to reach adsorption equilibrium was determined, then the isotherms were established. They are of Freundlich type. The affinity increased with the ethanol content in the medium, in other words, as the dielectric constant decreased. The affinity also was found to depend on the composition of the calcium phosphate studied. It decreased as the HPO42- content decreased, that is, as the CO32- content increased. After adsorption, the soilds were studied by chemical analysis, x-ray diffraction, and IR spectroscopy. It was found that two competitive processes took place with time, the evolution toward a more stoichiometric apatite and the adsorption of glycine. Moreover, this evolution toward a more stoichiometric apatite slowed as the HPO42- content in the solids decreased and the dielectric constant of the medium became lower. IR spectroscopy showed that glycine is adsorbed under its dipolar, zwitterionic, form.