Design of Loose Nanofiltration Membranes by Tailoring Hydrophilicity and Molecular Mass of Deep Eutectic Solvent Additives: Thermodynamics and Kinetics of Phase Inversion

The effectiveness of the membranes is largely influenced by the characteristics of the additives. This study introduces a novel approach to the fabrication of loose nanofiltration (LNF) membranes by utilizing deep eutectic solvents (DESs) as innovative pore formers. For the first time, we systematically investigated the combined effects of DES hydrophilicity and molecular mass on membrane morphology during the phase inversion process. Four distinct polysulfone (PSF)-based LNF membranes with the same nature and different hydrophilicity and molecular mass were synthesized to elucidate the thermodynamic and kinetic parameters influencing casting solution behavior. Our analysis reveals that an increase in DES molecular mass correlates with elevated thermodynamic parameters, while viscosity increases contribute to enhanced kinetic hindrance during membrane formation. Furthermore, the study demonstrates that higher hydrophilicity in DES additives not only reduces thermodynamic instability but also encourages the formation of finger-like pores throughout the membrane’s structure. The synthesized membranes exhibited superior separation performance, achieving over 86% rejection of Congo red dye and less than 3% rejection of sodium ions. These findings present a promising strategy for designing membranes with the desired performance for salt–dye separation, mitigating water waste, and improving sustainability.