sorption sites of the HA surface (9, 10). When there is Ca 2/ The adsorption of citrate on hydroxyapatite (HA) has been in the bulk of the solution, negatively charged calcium citrate studied at 25ЊC, pH 6 and 8, and at 37ЊC, pH 8. The experimental complexes have been reported to be the most likely species results agree with the Langmuir adsorption model at low citrate taking part in the adsorption process (11, 12). concentrations, with similar values of the affinity coefficient K .
In the interactions between citrate and HA, the pH of the
The amount adsorbed Q decreases with the increase of pH and suspension must play a fundamental role because it deterthe rise in temperature. However, the fraction of sites occupied mines the type and number of citrate species able to be on the HA surface is the same at both values of pH. On the other adsorbed on the surface, i.e., Cit 30 , Hcit 20 , and H 2 cit 0 . hand, the desorption is higher at pH 8 and zeta potential values Therefore, in the present work, the interaction between ciof suspensions prepared from HA preadsorbed with citrate are negative and decrease with the rise of pH. The adsorption takes trate and HA has been studied by means of adsorption, deplace by ionic exchange of phosphate by citrate ions at the solidsorption, and electrokinetic experiments, emphasizing the solution interface, caused by a higher affinity of citrate than phosrole played by pH. phate species for the Ca-sites on the HA surface. To explain experimental data we propose a model where citrate species inter-MATERIALS act in different ways: cit 30 interacts weakly in a bidentate manner ( 1 citrate per 2 Ca sites ), whereas the Hcit 20 interaction is Hydroxyapatite stronger ( 1 citrate per 1 Ca site ) than the previous one, possibly due to certain resonance between a monodentate (using 1 -COO 0 Crystals were prepared by precipitation in a continuous group ) and a surface-chelate interaction ( using 2 -COO 0 flow MSMPR reactor, mixing 0.8 mol/L CaCl 2 and 0.48 groups) .