Surface modification of hydroxyapatite. Part II. Silica

Nanophase hydroxyapatite (HAP) particles were coated with varying amounts of silica (5–75 wt%) via the hydrolysis of tetraethyl orthosilicate. The nanocomposite particles were characterized by transmission electron spectroscopy, X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), BET N2 gas adsorption, sedimentation time studies, acid dissolution, and zeta potential (ζ). A sequential decrease in infrared spectral features characteristic of HAP was accompanied by an increase in features characteristic of silica as revealed by DRIFTS. The specific surface area of the silica-coated HAP particles showed a non-systematic increase. In comparison to the uncoated HAP (50 m2/g), silica coatings of 5, 25, 50 and 75 wt% yielded specific surface areas of 55, 93, 70, and 138 m2/g, respectively. This behavior can be explained based on a heterocoagulation coating mechanism in which silica clusters of approximately 14 nm in diameter adsorb onto the HAP particle surface. The decrease in specific surface area at 50 wt% silica corresponded to the attainment of a complete surface coating. This conclusion was substantiated by the observed resistance of these particles to dissolution in 1 HCl. However, the acid treatment transformed the silica-coated HAP core particles to CaCl2•Ca(H2PO4)2•2H2O (calcium chloride phosphate hydrate) based on XRD analysis.