Growth of hydroxyapatite on silica gels in the presence of organic additives: kinetics and mechanism

Synthetic hydroxyapatite was grown on surfaces of silica gels by immersing silica monoliths in a simulated body fluid at 37 degreesC. The gels were pr epared by the sol-gel method and the drying process was controlled by using different additives (mono- and di-ethylene glycol, formamide and glycerin) to obtain large monoliths. The additives affect the growth of the apatite by changing the kinetic constants of chemical reactions on the silica surfa ce. Surface areas and pore size distributions were determined, energy dispe rsion and FTIR spectra obtained, and scanning electron microscopy performed . The formation of the hydroxyapatite competes with the formation of crysta lline calcium carbonate, but the results allow optimization of conditions f or the growth of the former as a function of the type and concentration of the additive. For these optimal conditions, the mechanism and the order of the reaction were determined.