Calcium Oxalate Crystal Growth in Silica Hydrogels. Influence of pH and Additives
Khor, Pi Jeng
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Calcium oxalate is one of the major constituents of kidney and gallbladder stones formed in the human body. In-vivo, these are mineralized in a complex organic matrix consisting of proteins and polysaccharides. The main objective of this study was to investigate the crystallization of calcium oxalate in an environment that resembles such complex natural environment in-vitro. We employed a double-diffusion experimental set-up and silica hydrogels as the crystallization media. The hydrogel is inside a U-shape tube and is brought in contact with two reservoirs on the opposite sides of the tube containing calcium and oxalate ions, respectively. The double-diffusion set-up allows for counter diffusion of the reactant ions across the gel column and precipitation of calcium oxalate inside the gels. We evaluated the effects of the gel density of hydrogels, through controlling their pH, on the nucleation, growth and morphological evolution of calcium oxalate. The crystals obtained were characterized by scanning electron microscopy (SEM), Fourier Transform infrared (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The sizes of crystals grown in silica hydrogels were pH-dependent. Calcium oxalate monohydrate, COM, was the only polymorph produced. The aggregates were found composite in nature including both calcium oxalate and silica. The role of anionic macromolecules in controlling crystallization was also studied. Poly(sodium 4-styrene sulfonate), (PSS) was incorporated into silica hydrogels and was found to change the morphology of the crystals from rosette-like calcium oxalate monohydrate, COM to spherical shape calcium oxalate dihydrate, COD. Further, the precipitation profile in the hydrogel-based double-diffusion system was estimated in the context of fundamental diffusion phenomena based on calcium and oxalate diffusion.