Adsorption of citrate ions on hydroxyapatite synthetized by various methods

The specific adsorption of citric acid ions at hydroxyapatite interface was investigated by the means of radioisotope method (¹⁴C) as a function of citric acid ions concentration, NaCl concentration and pH. Application of the hydroxyapatite has become wide in the biomaterial field as the Ca₁₀(OH)₂(PO₄)₆ possess biocompatibility with human hard tissue. Hydroxyapatite was synthesized using three different methods. The physical properties of the resulting powder were characterized by DTA/TG, XRD, AFM and SEM microscopy. Physicochemical qualities characterizing the electrical double layer of the hydroxyapatite/NaCl solution interface were determined. The zeta potential and the adsorption of citric acid molecule were studied as a function of pH. The point of zero charge and the isoelectric point of samples were determined. Electrical double layer parameters of hydroxyapatite/NaCl interface are influenced by a synthesis method. The points pH_pzc and pH_IEP for sample 1 are pH_pzc 7.5 and pH_IEP 3; for sample 2 pH_pzc 7.05 and pH_IEP 3, for smaple 3 pH_pzc 6.7 and pH_IEP 3. Temperature has weak influence both on pure substance and with citric acid adsorbed, as derivatographic analysis has shown, and characterization of hydroxyapatite structure may be carried out by this thermal analysis. Two phenomena are responsible for citric acid adsorption: phosphate group's replacement at hydroxyapatite surface by citric ions parallel to intraspherical complexes formation.