Impact of pH on sodium caseinate binding and structural changes on montmorillonite surface.

The immobilization of proteins onto clay surfaces has proven beneficial for pharmaceutical and environmental applications. This study examines the adsorption of sodium caseinate (Cas), an amphiphilic protein widely used in pharmaceutical formulations, onto sodium montmorillonite (Mt). Adsorption isotherms and kinetics were examined at two pHs, above and below Cas isoelectric point (IEP). Dynamic light scattering (DLS), zeta potential, Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD) were used to characterize the complexes formed. Results showed that at pH 3, Cas reversed Mt surface charge from negative to positive due to electrostatic attraction, intercalating into the basal spacing. At pH 7, despite repulsion expected, different mechanisms occurred, including protein orientation on the external surface and Cas self-assembly on Mt, increasing the negative zeta potential. Fluorescence quenching and isothermal titration calorimetry (ITC) revealed binding parameters and thermodynamic interactions, identifying electrostatic, hydrogen, and hydrophobic forces at both pH levels. This study contributes to understanding protein immobilization mechanisms and key factors affecting protein-clay adsorption.