THE INHIBITION OF CALCIUM-OXALATE MONOHYDRATE CRYSTAL-GROWTH BY MALEIC-ACID COPOLYMERS

The crystallization of calcium oxalate monohydrate (COM) was investiga ted at conditions of constant supersaturation both in the absence and in the presence of synthetic maleic acid copolymers at 37C, 0.15 M NaC l. The dependence of the rates of COM crystallization in the absence o f inhibitors was found to be second order at low and first order at hi gher supersaturations suggesting a surface diffusion controlled mechan ism. The presence of all copolymers tested at concentration levels up to 5 ppm retarded the rates of COM crystal growth up to 90%. The decre ase of the COM crystal growth rates by the polymers depended on the na ture of the comonomer polymerized with maleic acid and the order of in hibition was found to be vinyl acetate > N-vinylpyrrolidone > styrene. Taking into consideration kinetics data published in the literature c oncerning the inhibition of COM crystal growth, it is suggested that m olecular weight also plays a role, with more inhibition at higher mole cular weights, The morphology of the COM crystals grown was unaffected yet the crystals growing at lower rates in the presence of the copoly mers were larger and their size more uniform. It is concluded that mal eic acid copolymers are strong inhibitors of the crystallization of CO M, the inhibitory activity being more pronounced in the case of the li near copolymers.