Additive mediated polymorphic transformations of calcium oxalate in aqueous solutions
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Kidney stone formation is a disease which poses a great threat to human health. The widespread occurrence of such a painful disorder and the necessary medical treatment ranging from consultation to surgery make it worthwhile to investigate ways to inhibit the formation of kidney stones. The objective of this thesis is to study the effects of additives on the crystallization of calcium oxalate, the primary mineral constituent of kidney stones. We have employed anionic and cationic molecules which can control the final morphology and polymorph of calcium oxalate crystals produced during a precipitation reaction. The effects of the various additives on the nucleation and crystal growth of calcium oxalate were evaluated by a combination of Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Thermogravimetric Analysis (TGA) methods. The study involved the use of a negatively charged polyelectrolyte, sodium polystyrene sulfonate (PSS) as a model polymer. PSS was found to affect the morphology of the produced crystals and to preferentially promote the crystallization of calcium oxalate dihydrate (COD) over the thermodynamically stable calcium oxalate monohydrate (COM). The effects were evaluated at different calcium to oxalate concentration ratios in the mother liquid. At sufficiently high PSS concentrations, the crystallization of calcium oxalate was completely inhibited. The stability of the produced COD crystals left in the initial reaction mixtures was also deduced. The PSS effects on the nucleation and growth of calcium oxalate were remarkably reduced or completely eliminated when a cationic surfactant molecule, cetyltrimethylammonium bromide (CTAB) was introduced in the system. CTAB forms complexes with PSS, thus making PSS unavailable to interact with nucleating or growing crystals. The study indicates that in addition to the investigation of the role of certain inhibitors on the calcium oxalate nucleation and growth, competitive interactions between these inhibitors are also important and should be taken into consideration.