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In this work we studied the effect of organic compounds, from plant and animal kingdom, on crystal growth of calcium carbonate. All the experiment were done by the constant composition technique. Chitin, a nitrogen – containing polysaccharide which is a major component of arthropods, was found to be a substrate favoring the deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25 °C. The crystallization was studied at constant solution supersaturation thus making it possible for relatively large amounts of the overgrowth to be formed and be identified exclusively as calcite. The induction periods varied markedly with supersaturation. The apparent order found from kinetics data was n=2, thus suggesting a surface diffusion controlled mechanism.Elastin, a highly cross – linked protein polymer found in arterial walls was found to be a substrate favoring deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25 °C. The apparent ...
In this work we studied the effect of organic compounds, from plant and animal kingdom, on crystal growth of calcium carbonate. All the experiment were done by the constant composition technique. Chitin, a nitrogen – containing polysaccharide which is a major component of arthropods, was found to be a substrate favoring the deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25 °C. The crystallization was studied at constant solution supersaturation thus making it possible for relatively large amounts of the overgrowth to be formed and be identified exclusively as calcite. The induction periods varied markedly with supersaturation. The apparent order found from kinetics data was n=2, thus suggesting a surface diffusion controlled mechanism.Elastin, a highly cross – linked protein polymer found in arterial walls was found to be a substrate favoring deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25 °C. The apparent order for the calcite crystallization reaction found to be 2.0±0.1, suggesting a surface diffusion controlled mechanism. The crystallization was studied by the constant composition technique thus making it possible for relatively large amounts of the overgrowth phase to be formed and be identified exclusively as calcite. The induction periods varied markedly with supersaturation. Analysis of the initial rates of the reaction as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 25 mJm-2 for the surface energy of the growing phase and a three – ion cluster forming the critical nucleus. Recent studies in the bibliography showed that calcium carbonate was the major constituent (77.8%) in gall stones, and the polymorph calcite was at 62.5% of the cases examined. The kinetics of crystallization of calcite on sodium cholate has been studied using the constant composition technique. Analysis of the initial rates as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 33 mJm-2 for the surface energy of the growing phase and a five – ion cluster forming the critical nucleus. The apparent order for the calcite crystallization was found to be 4.5±0.7 indicative of a surface nucleation mechanism. The formation of calcite may be initiated through the interaction of Ca2+ ions and the negative end of the C=O bond of the sodium cholate molecule. The kinetics of spontaneous precipitation of vaterite (CaCO3) from an aqueous solution in the presence of chondroitin sulfates (CSA, CSB, CSC) was investigated by the constant composition method. The presence of chondroitin sulfate in the supersaturated solution resulted in a reduction of the crystal growth rate by 23-65%. Induction times preceding vaterite precipitations were inversely proportional to the solution’s supersaturation and a surface energy of 52 mJm-2 was calculated according to the classical nucleation theory. Chondroitin sulfate influences the particle size distribution of the vaterite crystals formed and stabilizing the mineral phase, preventing the transformation to calcite. The apparent order found from kinetics data was n>2, thus suggesting a surface nucleation mechanism. The kinetics of vaterite (CaCO3) crystallization on calcite in the presence of glutamic acid was investigated by the constant composition method. The presence of glutamic acid in the supersaturated solutions stabilizes this calcium carbonate polymorph. The number of ions forming the critical nucleus was found to be four and a surface energy of 12 mJm-2 was estimated. The apparent growth order was found to be 1.2±0.2 typical for a surface diffusion – controlled spiral growth process. The kinetics of vaterite (CaCO3) crystallization on calcite in the presence of aminoacids alanine, glycine, lysine as well as on lysine, polytyrosine and polymethionine was investigated by the constant composition method. The presence of the above mentioned aminoacids stabilizes this calcium carbonate polymorph. The number of ions forming the critical nucleus were n*=2, 4, 2, the surface energy 7, 10, 8 mJm-2 as well as the apparent order was 1,0±0,1, 1,0±0,2 και 1,0±0,2 for the forming phase was estimated, for lysine, glycine and alanine respectively. A mollusk cell, the xiphoid from cuttlefish was found to be a substrate favoring the deposition of aragonite crystals from stable supersaturated solutions at pH 8.50 and at 25 °C. The crystallization was studied at constant solution composition, thus making it possible for a relatively large amount of the overgrowths to be formed and to be identified exclusively as aragonite crystals. The apparent order fount from kinetics data was n=4.1±0.4, thus suggesting a polynuclear mechanism. A surface energy of 24±3 mJm-2 was calculated for the growing phase and a five – ion cluster forming the critical nucleus, according to the classical nucleation theory.Kinetics of spontaneous precipitation of vaterite (CaCO3) from aqueous solution in the presence of ethanol, isopropanole and diethylene glycol was investigated by the constant composition method. The presence of the above mentioned additives resulted in acceleration of the crystal growth rate. Initial rates of vaterite precipitation were proportional to the solution supersaturation and a surface energy of 43 mJm-2 was calculated according to the classical nucleation theory. These additives influence the morphology of the vaterite crystals formed and stabilize this mineral phase by preventing the transformation to calcite. The apparent order found from kinetics data was n>2, thus suggesting a surface nucleation mechanism. The effect of sodium alginate in supersaturated solution of calcium carbonate investigated under plethostatic conditions. The rates of crystal growth measured in the presence of sodium alginate at concentrations as low as 0.83x10-7 mol dm-3 were drastically reduced. Kinetic analysis according to a Langmuir – type adsorption isotherm led to the calculation of an affinity constant Kaff = 999.8x10-4 mol dm-3. The apparent order found from kinetic data was 3.0±0.2 suggesting a surface nucleation mechanism
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