ADSORPTION OF PEROXYDIPHOSPHATE ONTO HYDROXYAPATITE - KINETICS, THERMODYNAMICS, AND CRYSTAL-GROWTH INHIBITION

Peroxydiphosphate ion, P2O84-, PDP, a potential agent to control miner alization in biological systems, adsorbs readily onto hydroxyapatite, HA, the prototype of the mineral in bones and teeth. The apatite cryst als, however, catalyze the hydrolysis of PDP, which is faster at 37 de grees C than at 4 degrees C. Hydrolysis at the lower temperature is a first order reaction; at 37 degrees C the process is more complex and may be limited by surface diffusion. Orthophosphate in solution repres ses the hydrolysis of PDP. Adsorption isotherms were determined in the presence of orthophosphate at the two temperatures mentioned. The exp erimental data were best described by a model involving two adsorption sites at which PDP adsorbs in competition with orthophosphate ions. T he adsorption bond at one of the sites is more than 10 times stronger but its capacity is from 1/4 (at 37 degrees) to 1/8 (at 4 degrees) the capacity of the site displaying the weaker bond. The inhibition of cr ystal growth by PDP is related only to its adsorption onto one of the adsorption sites, which represents only 18% of the maximal PDP adsorpt ion. It is inferred that crystal growth occurs (or is initiated) on a relatively small fraction of the crystal surface. Calculation of the t hermodynamic functions for adsorption shows that the reaction is endot hermic and, therefore, that adsorption of PDP is entropically driven; the gain in entropy is probably associated with changes in the locatio n of water molecules on the adsorbate and the adsorbent. The reported thermodynamic properties of the two adsorption sites are distinctly di fferent. (C) 1994 Academic Press, Inc.