Surface properties of alkaline earth apatites

Abstract

In contrast to other investigations of double layer systems, dissolution and ion exchange processes were found to play a more important role in this work. Both from an experimental and interpretative vantage point this statement has been corroborated. Experimental data were obtained exclusively by electrophoretic measurements because the high solubility of the synthesized apatites prohibited direct determination of the adsorption density of the p.d. ions. The observed magnitude of the e.m. in solutions of constant ionic strength (∼10−2 M) and of a given electrolyte concentration was found to increase in the reverse order as the solubility of the alkaline earth apatites: CaFA>CaHA>SrHA>BaA. Thermodynamic arguments suggest that the double layer on these apatites, which may be classed as tri-ionic crystals (three distinct lattice ions), is characterized by the existence of a series of p.z.c.'s lying on a line on the equilibrium “solution” surface. The latter is constructed by plotting on three mutually perpendicular axes the concentration (activity) of the three p.d. ionic species. In absence of specific adsorption, di-ionic crystals are characterized by a single p.z.c. but the continuous shift in p.z.c. produced in such solids by specific adsorption may be likened to the predicted p.z.c. curve for apatites. The predicted multiple p.z.c.'s were verified experimentally. As many as three independent p.z.c.'s were established for the hydroxyapatites of calcium and strontium. Only the experimental procedure prevented the observation of more points. The experimental p.z.c.'s are given in Table 5.TABLE 5. POINTS OF ZERO CHARGE OF ALKALINE EARTH APATITIESSolidp.z.c.Empty CellEmpty CellEmpty CellIaIIbIIIcCaFApF=4.63pCa=4.38pHPO4≃5.22CaHApH=7.15pH=7.0pCa=4.38pHPO4=4.19pHPO4=4.48SrHApH=8.00pH=9.80pSr=4.19pHPO4=4.0pHPO4≃4.4BaApH=8.57pH=11.88pH=3.43aExperimentally determined by addition of K2HPO4 to suspensions of CaHA, SrHA and BaA.bExperimentally determined by addition of KOH to suspensions.cBy extrapolation. TABLE 5. POINTS OF ZERO CHARGE OF ALKALINE EARTH APATITIES Experimentally determined by addition of K2HPO4 to suspensions of CaHA, SrHA and BaA. Experimentally determined by addition of KOH to suspensions. By extrapolation. On applying suitable corrections for electrophoretic retardation and orientation to the Smoluchowski equation the zeta potential of the cyclindrical-shaped apatite particles was calculated. With few exceptions ξ never exceeded a value of 40 mV over a hundred-fold concentration range of p.d. ions. The surface charge density of the particles was calculated to vary in magnitude from about − 2 to +2 μC cm−2. The fluoride ion and the calcium ion were observed to play a special role in controlling the electrophoretic behavior in those apatites for which these ions are not lattice ions. An explanation of these findings must take into account ion exchange reactions involving at least a monolayer of the solid surface.