Surface Properties of HAp Particles Obtained by Hydrothermal Decomposition of Urea and Calcium-EDTA Chelates

In this paper surface properties of calcium-hydroxyapatite, synthes ized by modified hydrothermal reaction of urea and Ca(EDTA) 2– in phosphate solutions, as well as the dependence of calcium hydroxyapatite dispersion stability on pH were determined. T he specific surface area of calcium-hydroxyapatite was experimentally found to be 67 m /g. The point of zero charge (PZC) of indifferent KCl electrolyte of 6.8 ± 0.1 was determined by the batch equilibration method. From the dependence of surface charge density on pH and electrolyte concentrati on, the intrinsic equilibrium constant of surface groups p a2 K was determined to be 8.3 ± 0.1, while p int a1 K , calculated from the p int a2 K and point of zero charge values, is 5.3 ± 0.1. Hy droxyapatite suspension stability was investigated, in the pH range from 7 to12, it was r evealed that the most stable dispersion at pH 10. Introduction Hydroxyapatite is one of the main constituent compo unds of bones and teeth. Knowledge of the hydroxyapatite surface chemistry is very important from the point of view not only of bone tissue formation but also of HAP solubility, due to its in teraction with the surrounding fluid. Solubility of apatites is directly connected with various illness es uch as decalcination of bones (osteoporosis) and dental caries[1-2]. Behavior of the aqueous dispersion of hydroxyapatit e depends on the interfacial chemistry of the solid phase and dispersive medium. Surface charge o f hydroxyapatite in aqueous systems plays one of the main roles in determination of its colloidal and sorption properties [3]. The studies of the point of zero charge (pH PZC) and surface charge characteristics of apatites in dicate that H + and OH are potential-determinig ions and that the surface charge development at the apatite-water interface could be described by amphoteric dissociation react ions of surface functional groups [4,5]. In this paper, behavior and surface properties such as specific surface area, presence of particular surface groups, point of zero charge, surface charg e density, and intrinsic equilibrium constants of carbonate HAP obtained by hydrothermal decompositio n of urea and Ca-chelate, were reported. Methods and Materials Synthesis of carbonate hydroxyapatite was performed by a modified method of Fujishiro et al.[6-8], and was described earlier [9]. Specific surface area of HAP was determined by the B.E.T. (Brunauer – Emett – Teller) method, the sample was previously degassed in vacuum for 2 hour s at 150°C. The point of zero charge of HAP was determined by t he batch equilibration method described by one of the authors[10]. The adopted technique is as follows: KCl of different ionic strengths, 0.1 and 0.01 mol/ dm, was used as an electrolyte. First set of measurements included 12 samples with 25 cm 3 KCl of 0.1 mol/dm ionic strength. The initial pH (pHi) values, from 3 to 10, were adjusted by adding sm all volumes of 0.1 mol/dm 3 HCl or KOH to Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 437-440 doi:10.4028/www.scientific.net/KEM.240-242.437 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,21:20:44) the electrolyte solution. Then, 0.1g of HAP, previously washed by deionized w ater to prevent surface impurities and their eventual effects, was added to each s ample. Equilibration, under constant stirring, was carried out for 24 h at room temperature. Dis pers ons were then filtered through a filter paper, and the final solution pH values (pH f) were determined. The point of zero charge was obtained from the dependence of pH f on pHi , as the pH value where plateau appears on the curve pHf vs. pHi. For the second set of measurements, the procedure was repeated but thi s time KCl of 0.01 mol/dm ionic strength was used to determine its indifference. Determination of the surface charge density and surface group ionization constants. The surface charge density of HAP was determined by the potentiome tric titration method. The method is based on the observation of the difference in titrant (base) consumption, during titration, between the suspension and the blank, which provides information on the partaking of the solid phase in titration, i.e., adsorption of OH – ions on the particle surface. Three series of titration were performed with different concentrations of KCl solutions (0.1, 0.01, and 0.001 mol/dm ). 0.4g HAP was suspended in 200 cm 3 KCl and the obtained diserpsion was titrated, in the pH range from 6.8 (point of zero charge of HAP) to 10.5, with KOH solution of a 0.1 mol/dm 3 concentration. In the same way the blank, i.e., KCl solution of the same concentration but wit hout powder, was titrated. Titration was performed in a PVC vessel, at room temperature, the KOH solution (titrant) was added every 5 minuts and the pH value measured afterwards. During titr ation nitrogen was inroduced, at constant stirring, to prevent CO 2 absorption from air, and consequently change in pH values of the blank. Surface charge density ( σo, μC/cm ) was calculated by equation: σo = 100⋅∆v⋅c⋅F/S⋅A⋅V (1) where: ∆v – difference between the titrant volumes of the suspension and the blank, at the same pH value (cm), c – concentration of KOH (titrant) (mol/ dm ), F – Faraday's constant 96500 [C], S – specific surface area of the solid phase (m /g), A – concentration of HAP in the suspension g/dm 3 , and V – volume of the dispersion and the blank (cm ), respectively. Degree of the surface ionization was calculated using the calcul ated surface charge and supposed maximal surface charge density values, max o σ . Using experimentally determined and calculated parameter values, p 2 a Q was calculated by equation