Differences in adsorption of anionic surfactant AOT by calcium oxalate: Effect of crystal size and crystalline phase

Abstract

The adsorption of anionic surfactant sodium diisooctyl sulfosuccinate(AOT) onto calcium oxalate monohydrate(COM) and dihydrate(COD) with sizes of 50, 100 nm, 1, 3 and 10 µm was comparatively studied to simulate the interaction between urinary crystallites and urine components. The adsorption quantity of different concentrations of AOT onto COD and COM with different sizes was detected using a UV-Vis spectrophotometer. The crystalline phase transition of COM and COD before and after adsorption was analyzed by X-ray powder diffraction and Fourier transform infrared spectrometry. The zeta potential of the crystal surface after adsorption of different concentrations of AOT was measured using a zeta potential analyzer. The adsorption quantity of AOT on COM and COD with different sizes was ranked in the following order: 50 nm>100 nm>1 µm>3 µm>10 µm. The adsorption quantity of COM was greater than that of COD with the same size because the density of the positive charges on the COM surface was higher than that on COD surface. With the increase of AOT concentration, the adsorption curves of the large-sized COM and COD(3 and 10 µm) were S-type, whereas the adsorption curves of the small-sized COM and COD(50 nm, 100 nm and 1 µm) were linear. The adsorption capacities of small-sized COM and COD were much greater than those of the 3 and 10 µm crystals. On the basis of the above results, we proposed a molecular model to summarize the absorption of AOT onto COM and COD crystals. Small crystals exhibit a large specific surface area and high surface energy. Thus, their adsorption capacities are stronger than those of large crystals. Overall, this study implied that small crystals can easily absorb anionic molecules in urine and may easily adhere to a negatively charged cell surface, thereby increasing the severity of cell injury.