Influence of Organic Phosphonates on Hydroxyapatite Crystal Growth Kinetics

Phosphonate additives have important applications both in biomineralization processes and in industrial mineral scale formation. The kinetics of crystal growth of hydroxyapatite (HAP) has been investigated in the presence of seven phosphonate additives: HEDP (hydroxyethylenediphosphonic acid), DETPMP (diethylenetriaminepenta(methylenephosphonic acid)), DHTPMP (dihexyltriaminepenta (methylenephosphonic acid)), AMP (aminotris(methylenephosphonic acid)), HDTMP (hexyldiaminetetra(methylenephosphonic acid)), EDTMP (ethylenediaminetetra(methylenephosphonic acid)), and DETMP (diaminoethoxytetra(methylenephosphonic acid)). The constant composition (CC) method, used in this study, enabled reliable crystal growth rate data to be obtained even when the reactions were appreciably inhibited. The experiments were performed at pH 7.40, ionic strength 0.15 mol L - 1 (maintained with NaCl), and an HAP relative supersaturation, σ, of 5.5. The results indicate that traces of some phosphonates (≤10 - 6 mol L - 1 ) are extremely effective in inhibiting crystal growth. Assuming that the adsorbed additives block discrete growth sites on the crystal surfaces, the kinetic results may be interpreted in terms of a Langmuir adsorption model yielding kinetic affinity constants. The order of inhibitory effectiveness, DETMP > EDTMP > DETPMP > HEDP ≥ DHTPMP > HDTMP ≥ AMP, reflects the ability of the phosphonates to bind to the apatite surfaces. ζ potential measurments of HAP surfaces in the presence of additives provide important corroboratory data for the interpretation of the crystal growth results.