Differences in Adsorption of Anionic, Cationic and Nonionic Surfactants on Calcium Oxalate Crystals: Effect of Crystal Size

Abstract

The adsorption differences of surfactants with different types of charges onto
nano/micron calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD)
crystals were investigated to explore the effects of urinary molecules with different types of
charges on formation of urinary stones. These surfactants include anionic surfactant sodium
dodecyl sulfate (SDS), cationic surfactant cetyltrimethyl ammonium bromide (CTAB), and
nonionic surfactant nonylphenol ethoxylate (NP-40). The sizes of COM and COD crystals are
100 nm and 10 μm, respectively. The adsorption capacity of different surfactants on
nano/micron COM and COD crystals and the Zeta potential on crystal surface after adsorption were measured. The settling velocity of the crystal suspensions after adding different surfactants was compared. The adsorption quantity of three surfactants on COM and COD was ranked in the following order: SDS > CTAB > NP-40. The adsorption quantity of COM was greater than that of COD with the same size. It indicated that the adsorption capacity is related to the crystal structure of the crystal surface. The absolute value of Zeta potential on surface of COM and COD increased after adsorption of surfactants. The crystal settling velocity was negatively correlated with the amount of surfactants adsorbed and the Zeta potential of the crystals. Surfactants can interact with the calcium sites or oxalate sites on the crystal surface. There are different interactions between different surfactants and nano/micron crystals. We proposed a molecular model of crystal adsorption with different surfactants. Larger adsorption quantity of surfactants can slower the sedimentation rate of crystals and thus stabilizes crystal suspension, finally inhibits the formation of calcium oxalate stones.