Enhancing the hydrophilicity and biofoulant removal ability of a PVDF ultrafiltration membrane via π-π interactions as measured by AFM

Antibiotics are commonly used as inhibitors to terminate protein formation in the biotechnology and pharmaceutical industries; however, due to the large difference in molecular weight between antibiotics and proteins, it is hard to remove them simultaneously through ultrafiltration or nanofiltration. In this work, a novel membrane with nanoscale pores and anti-protein adsorption was developed to reject antibiotics and reduce protein adhesion. Amphiphiles consisting of hydrophobic and hydrophilic moieties were synthesized from a poly(styrene-alt-maleic anhydride) (SMA) copolymer with a 2-hydroxyethyl acrylate (HEA) monomer to enhance the hydrophilicity of polyvinylidene difluoride (PVDF) membranes. A series of SMAs with different polystyrene molar ratios (maleic acid/styrene (m/s) = 1:1, 1:2 and 1:3) was synthesized in this study, and the bonding force between the amphiphiles and PVDF, namely, π-π interactions between styrene molecules and the C–F bonds of PVDF, was evaluated. The π-π interaction between the amphiphiles and the PVDF membrane was determined via atomic force microscopy (AFM) repulsive force measurements for the first time in this work. The repulsive force increased significantly from -5.9 ± 1.0 nN to −18.3 ± 0.8 nN as the m/s ratio increased to 1:3, indicating that the π-π interaction was enhanced as the proportion of polystyrene in the SMA-HEA copolymer increased, resulting in tight attachment. Furthermore, the amphiphile-modified membrane demonstrated better hydrophilicity than the pristine PVDF membrane, and the water contact angle (WCA) decreased considerably from 104° (PVDF) to 55° (m/s = 1:2, P2000-600). The pure water flux recovery rate of P2000-600 improved by approximately 10%, and antibiotic rejections of 92.8%, 99.9% and 97.1% for tetracycline (TC), chloramphenicol (CL) and erythromycin (EM), respectively, were observed during the 7-h filtration process, indicating that P2000-600 can effectively remove antibiotics from water. In addition, P2000-600 also displayed good restriction of bovine serum albumin (BSA) adsorption at 28.4 ± 8.1 μg/cm2 under static adsorption processes and BSA rejection of nearly 90% during the three-cycle filtration.