Preparation and characterization of negatively charged organic–inorganic hybrid ultrafiltration membranes for protein separation

• Negatively charged organic–inorganic hybrid ultrafiltration membranes.
• The membranes have high bound water capacity.
• The membranes exhibited minimal protein adsorption.
• The highest transmission of model proteins was at their isoelectric points.

Negatively charged organic–inorganic hybrid ultrafiltration membranes with adjustable charge density were fabricated from blends of water soluble poly vinyl alcohol and N-O-sulfonic acid benzyl chitosan (NSBC) in combination with tetraethyl orthosilicate (TEOS) silica precursor by the sol–gel method and precipitation in a mixture of methanol and water (80 wt%:20 wt%) solvent. The porous hybrid membrane morphologies, structures and surface properties were characterized comprehensively using scanning electron microscopy, Fourier transform infrared spectroscopy in the attenuated total reflection mode, as well as contact angle measurements. The results confirmed that the fabricated membranes were porous, hydrophilic and mildly charged in nature. The water flux and flux recovery ratio (i.e. protein fouling resistance) of the membranes were highly dependent on the fraction of NSBC. The protein transmission as a function of pH and the fraction of NSBC was studied for two model proteins (bovine serum albumin; BSA and lysozyme; LYZ) and found to be controlled by size exclusion and the fraction of NSBC, and therefore the membrane charge density. The highest transmission of proteins at their isoelectric points was obtained for the membrane containing 40 wt% of NSBC. The best separation of LYZ from BSA in the model mixture solution was obtained at pH = 11 when membrane C-40 was used in ultrafiltration of protein solution.