Unraveling the role of ion valence in nonsolvent induced phase separation: A salt coagulation Bath strategy for tailoring SPES/PES membrane structure

Precisely controlling pore structure and surface chemistry in ionic polymer membranes during nonsolvent induced phase separation (NIPS) remains challenging. Here, SPES/PES blend membranes were fabricated by systematically varying salt type and concentration in the coagulation bath, showing that cation-anion valence matching, besides salt concentration, also regulates membrane structure during NIPS. Experimental results showed that, relative to pure water, salt-containing baths slowed phase separation and yielded denser membranes, with a stronger effect at higher salt concentrations. Valence-matched salts produced smaller surface pores than valence-mismatched salts (Na 2 SO 4 and MgCl 2 , 22.79 and 19.62 nm, respectively), whether compared at equal concentration (0.1 M: NaCl 16.74 nm, MgSO 4 13.73 nm) or equal ionic strength ( I  = 0.3: NaCl 11.26 nm, MgSO 4 15.14 nm). This trend may be related to stronger SPES chain aggregation in valence-matched salt solutions. Salt baths suppressed SPES loss into the coagulation bath, thereby enriching surface sulfonate groups and improving hydrophilicity. The membrane prepared in 0.1 M MgSO 4 exhibited excellent separation performance (>99% BSA, 96% Lyz, and 97% MB rejection) and enhanced mechanical strength (5.97 MPa). This pore-forming-agent-free strategy provides a simple route to high-performance functionalized membranes.