INFLUENCE OF ORGANIC PHOSPHONATES ON HYDROXYAPATITE CRYSTAL-GROWTH KINETICS

Phosphonate additives have important applications both in biomineraliz ation processes and in industrial mineral scale formation. The kinetic s of crystal growth of hydroxyapatite (HAP) has been investigated in t he presence of seven phosphonate additives: HEDP (hydroxyethylenedipho sphonic acid), DETPMP (diethylenetriaminepenta(methylenephosphonic aci d)), DHTPMP (dihexyltriaminepenta (methylenephosphonic acid)), AMP (am inotris(methylenephosphonic acid)), HDTMP (hexyldiaminetetra(methylene phosphonic acid)), EDTMP (ethylenediaminetetra(methylenephosphonic aci d)), and DETMP (diaminoethoxytetra(methylenephosphonic acid)). The con stant composition (CC) method, used in this study, enabled reliable cr ystal growth rate data to be obtained even when the reactions were app reciably inhibited. The experiments were performed at pH 7.40, ionic s trength 0.15 mol L(-1) (maintained with NaCl), and an HAP relative sup ersaturation, sigma, of 5.5. The results indicate that traces of some phosphonates (less than or equal to 10(-6) mol L(-1)) are extremely ef fective in inhibiting crystal growth. Assuming that the adsorbed addit ives block discrete growth sites on the crystal surfaces, the kinetic results may be interpreted in terms of a Langmuir adsorption model yie lding kinetic affinity constants. The order of inhibitory effectivenes s, DETMP > EDTMP > DETPMP > HEDP greater than or equal to DHTPMP > HDT MP greater than or equal to AMP, reflects the ability of the phosphona tes to bind to the apatite surfaces. zeta potential measurements of HA P surfaces in the presence of additives provide important corroborator y data for the interpretation of the crystal growth results.