Influence of Crystallite Size On the Surface-properties of Calcium-deficient Hydroxyapatites

The surface properties of calcium-deficient hydroxyapatites (HAps) prepared under different synthesis conditions (e.g. by precipitation at pH 10, or by hydrolysis of dicalcium phosphate at pH 6.5) were studied. All the products tested were pure, well crystallized calcium-deficient HAps. The samples prepared under slightly acidic conditions exhibit larger crystallite sizes and higher calcium deficiency compared with those prepared under alkaline conditions. Electrophoretic mobilities were measured as a function of pH in 1.0 x 10(-3) M KNO3 aqueous solutions. The isoelectric points (iep) of HAp appear to depend strongly on the synthesis conditions; iep values of 4 were found for samples prepared under more acidic conditions, while values ranging from 5.5 to 7.2 were found for solids precipitated from alkaline solutions. The adsorption of hen egg-white lysozyme was measured at ambient temperature, in 1.0 x 10(-3) M KNO3 aqueous solutions buffered at pH 6.8 with 2.0 x 10(-3) M phosphate buffer. The parameters of the Langmuir-type adsorption isotherms were found to depend significantly on the specific surface of the samples; the monolayer coverage goes through a maximum of 40 x 10(-9) mol m-2 at S(BET) = 80 m2 g-1, while the affinity constant culminates at 355 mM-1 at the same value of S(BET). The decay of both monolayer coverage and affinity constant beyond S(BET) = 80 m2 g-1 can be ascribed partly to the lower accessibility of smaller pores to lysozyme molecules, particularly when electrostatic repulsion exists between the lysozyme molecules and the HAp surface. It is thought that specific bonds between lysozyme and the hydroxyapatite surface must exist, which are very sensitive to the nature and the relative amounts of crystallographic planes exposed on the solid surface.