Supersaturation evolution and first precipitate location in crystal growth in gels; application to barium and strontium carbonates

Abstract

Supersaturation is not enough to interpret nucleation behavior in crystal growth in gels. The explanation for the first precipitate location needs complementary crystalization criteria. The experimental testing of both mass-transfer and pH evolution is the background to establish rigorous hypotheses. From these data, the actual distribution of free ions, ionic associations and supersaturation throughout the gel collumn may be found. In the case of nucleation of strontianite and witherite crystals, although the critical supersaturation conditions remain necessary, a second concept, the "equality range" of the reagent concentrations, has to be introdud to account for observations. With these dual conditions, the behaviour of the described gel-diffusion system is correctly reproduced. The significance of pH evolution is also experimentauy shown. Nucleation density and growth mechanisms are finally explained on the base to supersaturation rate and kinetic roughness.