The crystallization of silicalite-1 in aqueous sols has been investigated using ATR/FTIR spectroscopy as a function of sol composition. The existence of oligomeric siliceous species has been verified for all compositions after the end of tetraethoxysilane (TEOS) hydrolysis. On the other hand, more condensed and highly crosslinked siliceous structures have been observed only at the sols of high SiO 2 concentrations. These structures have been considered to be the "primary" nanoparticles reported by other researchers, and their IR spectra revealed that their structures are different from that of the silicalite-1 crystals. The evolution of the nanoparticle structure as a function of crystallization time has been monitored. It has been found that, upon heating of the synthesis sols, the "primary" nanoparticles transform to a new form that has an increased number of SiOSi bonds. These "intermediate" particles do not have the structure of silicalite-1, and they are present for most of the crystallization. Finally, once the silicalite-1 nuclei are formed, they grow in a suspension of these "intermediate" particles.

The present work deals with the influence of pH and temperature on the spontaneous precipitation of calcium phosphate from simulated milk ultrafiltrate (SMUF) solutions. The pH range investigated is 5.7-7.0, and the temperature varied from 55 to 75 °C. It was found that the precipitates were prisms of hydroxyapatite (HAP) with low crystallinity preceded by amorphous calcium phosphate (ACP). Crystallinity improved with solution aging. Microscopic particles in the range of 200 nm were obtained at relatively high supersaturation (pH ) 6.8), whereas at relatively low supersaturation (pH ) 6.0), aggregates in the range of 1 µm were formed. Moreover, it was found that although the SMUF solution is also supersaturated with respect to magnesium phosphate, no magnesium salt was identified in the precipitates, thus precluding the coprecipitation of magnesium phosphate. At the same temperature, the supersaturation was regulated by adjusting the solution pH. Kinetics study of precipitation showed a parabolic dependence of the (initial) rates on the solution supersaturation, suggesting a surface diffusion-controlled mechanism with activation energy equal to 96 kJ mol -1 . The rates of precipitation were significantly reduced in the presence of citrate ions due to the decrease of the solution supersaturation caused by complexation of citrates with Ca 2+ ions. Additionally, a decreasing effect of citrates on the precipitation rate resulted from the adsorption of these species on the formed crystals, blocking some active sites of crystal growth and diminishing the constant of precipitation rate.

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