Surface properties of bovine serum albumin – adsorbed oxides: Adsorption, adsorption kinetics and electrokinetic properties

Adsorption, electrokinetic properties and the interactions between biomolecules and clay minerals have recently received much attention, owing to the physicochemical and biological properties of these materials (biocompatibility, bioresorption or chromatography). In this study, the adsorption properties of bovine serum albumin (BSA) onto some oxide minerals such as sepiolite, kaolinite, expanded and unexpanded perlites were studied as a function of concentrations of BSA, sodium phosphate buffer and NaCl concentration, and pH of the solution. Furthermore, the electrokinetic properties of BSA-covered oxide particles were also investigated at the same conditions with adsorption process. The adsorption processes were very slow and attained to the equilibrium within: 10 h for sepiolite, 8 h for kaolinite, and 3 h for perlite samples. Maximum adsorption capacity values (qm) showed a great dependence on pH. It was found that qm–pH curves reached a maximum at around isoelectric point (iep) of BSA. The pH values where the maximum adsorbed mass occurred might be considered as the conditions where electrostatic attraction is the most favourable. Both structural and electrostatic effects must be invoked to explain the diminution of adsorbed BSA on either side of the iep. Structural effects were related to the different conformational states that BSA molecules adopted the changes with pH, whereas electrostatic effects were analysed assuming that BSA molecules behaved as soft particles. This reasoning also allows us to explain the independence of the qm–pH curves of electrolyte concentration. Experimental adsorption data were investigated using Langmuir and Freundlich adsorption isotherm models and found that Freundlich isotherm model gave the best representation of the adsorption equilibrium. In order to investigate the mechanism of adsorption and potential rate controlling step, pseudo-first- and second-order kinetic equations, and intraparticle diffusion model have been used to test the experimental data. The rate constants and the related correlation coefficients were determined in order to assess which model provides the best-fit predicted data with experimental results. Pseudo-first-order kinetic equation provided the best-fit to experimental data.