Heterogeneous nucleation of calcium phosphate salts at a solid/liquid interface examined by scanning angle reflectometry

Mineralization of calcium phosphate salts at a solid/liquid silica interfac e was examined by means of scanning angle reflectometry (SAR). A critical s upersaturation of 7.60 +/- 0.15 mM was found to be the lowest calcium phosp hate concentration at pH 6.85 at which the reflectivity at the Brewster ang le started to vary. The analyses of the signals by mean of the homogeneous isotropic layer model led to layer thicknesses of about 3.5 mu m also chara cterized by very low mean refractive index increments. These observations w ere completed by complementary experiments. Scanning and transmission elect ron microscopy led to similar layer thicknesses with crystallites dispersed in the whole layer confirming thus SAR results. X-ray analyses showed a pr esence of brushite (DCPD) at the silica interface. A pH of 5.44 was estimat ed for the acidic silica interface for which DCPD is nucleated after additi on of 7.60 +/- 0.15 mM calcium and phosphate. However, at this pH value, th e solution is largely supersaturated with respect to hydroxyapatite (HAP th eoretical saturating concentration: 1.37 mM). A faster growth of DCPD cryst als compared to HAP crystals is thus assumed to explain our observations. ( C) 1999 Elsevier Science B.V. All rights reserved.