Powder Hydroxyapatite Dissolution Kinetics and a Study of Effects of Dodecylamine and Fluoride.

Recently a model was presented for describing the acid dissolution rate of hydroxyapatite (HAP) based upon experiments with compressed HAP disks. The model featured dissolution from two sites on the HAP crystal and diffusion of species in the pores of the matrix as well as diffusion across the aqueous boundary layer. The powder dissolution method which involves essentially 100% surface-controlled kinetics under appropriate conditions has been developed and the experiments with the HAP suspensions clearly validate the original two-site model. The present work shows that dissolution from site #2 is well-described by a first-order expression and for all different HAP preparations, a driving force for dissolution corresponding to a single K(,HAP) value of 10('-130(+OR-)1) was found to be consistent with experiments over wide ranges of partial saturation and calcium/phosphate ratios in the solution. The adsorption of dodecylamine hydrochloride (DAC) onto HAP crystal surface and its influence on the dissolution rate behavior of HAP crystal suspensions have also been studied. The sigmoidal nature of adsorption isotherms indicated that there was considerable cooperativity among the DAC molecules in the adsorbed state leading to patchwise adsorption of "isl and s" which could completely stop the dissolution from the HAP surface beneath them. This mechanism is consistent with the observation that there was an almost quantitative, inverse relationship between the adsorption and the dissolution rate of HAP in an acidic medium. The studies of the influence of fluoride (F) alone and acting together with DAC on the HAP dissolution showed that F and DAC inhibit the dissolution in an additive fashion by different modes of action: F acts by changing the apparent driving force that governs the dissolution kinetics and DAC acts by physically blocking the HAP surface. The existence of a residual component for the dissolution rate at high concentrations that is about 15% of the maximum rate suggests that there are two or more dissolution sites on the HAP crystal. The one associated with the predominant surface is strongly inhibited by both F and DAC whereas the other site(s) is insensitive to F either intrinsically or, in the presence of DAC, because of possible antagonism.PhDPharmaceutical sciencesUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159356/1/8314255.pd