Kinetics and thermodynamics study of proline adsorption and its inhibitory effect on calcium oxalate monohydrate (COM) crystal growth

Abstract Calcium oxalate monohydrate (COM) crystals are the most thermodynamically stable phase of CaOx and a primary constituent of urinary stones. In the present study, crystal growth of COM seed crystals was studied kinetically and thermodynamically in the absence and presence of proline (pro) amino acid using the constant-composition method at physiological conditions. A concentration as low as 9 × 10 − 6 of pro decreased the rates of crystal growth of COM seed crystals by 52.08%. The effect of change in pH, ionic strength, and relative degree of supersaturation, Ω, and temperature on crystal growth of COM in the absence and presence of 10 − 6 mol.dm − 3 was studied. From the results, the reaction followed a surface mechanism. Spectroscopic and microscopic analysis and kinetic results indicated physical adsorption of pro molecules on kinks and terrace sites on the surface of COM seed crystals through electrostatic, H-bonding, and van der Waals forces, thereby disturbing lattice propagation and promoting surface heterogeneity. Thermodynamic parameters, K l , K ads , ∆ G ads , ∆ H ads and ∆ S ads values were 11.00 × 10 4 J/mol, 9.80 × 10 4 J/mol, -39.90 kJ/mol, -1.58 × 10 4 J/mol, and − 77.60 J/mol respectively. The adsorption equilibrium constant, K d , was equal to 4.1 × 10 − 2 l/g on applying the Langmuir model, and K f , was 2.8 × 10 − 1 l/g on applying the Freundlich model, indicating surface heterogeneity. XRD analysis confirms that pro shifts calcium oxalate from COM (peaks at 15.0°, 24.3°, 30.1°) to COD (peaks at 14.3°, 20.0°, 32.1°, 36.7°, 39.8°).