Effect of surface charge and roughness on ultrafiltration membranes performance and polyelectrolyte nanofiltration layer assembly

In this study, negatively charged Polyethersulfone (PES)/Polyimide (PI) ultrafiltration membranes were fabricated in order to enhance membrane performance. Moreover, the surface properties such as surface charge and roughness were studied as essential properties which can enhance Polyethyleneimine (PEI) nanofiltration layer assembly. Due to the possibility and observation of phase segregation, for the first time, PES/PI blend was investigated and approved both thermodynamically and experimentally via DSC and FESEM analysis. Highly charged surface of fabricated ultrafiltration (UF) blend membranes were able to reject BSA more than 95% due to smaller pores, surface roughness, and electrostatic forces according to the AFM and zeta potential analyses. Fouling parameters depicted lower fouling with increasing the PI content. Further investigations provided evidence that the presence of PI greatly affected membrane morphology, phase separation behavior, and pore size distribution. According to zeta potential and AFM analysis, the presence of negative charge along with increased roughness of blend membrane surface drastically enhanced the formation of positively charged hyperbranched polyethyleneimine NF layer. As seen in FESEM images, using the blend membranes as substrate enhanced the NF layer assembly by increasing the interfacial affinity resulting in more efficient layer assembly of the NF layer. The NF membranes exhibited high flux (52 LMH/bar) at low pressure (2 bar) in salt removal filtration tests along with increased performance in salts rejection in order of MgSO4 (72%) > MgCl2 (62%) >= NaCl (61.4%).