Enhanced steric effect and desolvation process on organic solvent nanofiltration: A mechanism study for removing anionic dyes

• The solvation effects of organic anions are investigated in OSN. • Solvation structures exhibit solvent-separated ion pairs and contacting ion pairs. • Pseudo-solvation structure undergoes desolvation due to Na + transmembrane. • Mixed solvents can be used to regulate solvation size for efficient separations. The solvation effect plays a critical role in chemical reactions and ion transport process, but most studies focus on the solvation effect of metal ions, such as Li + , Mg 2+ and so on. Few studies have explored the solvation of organic ions with more complex structures than metal ions. In this work, the solvation effect of organic ions in the organic solvent nanofiltration (OSN) process is investigated, in which methyl orange (MO) is selected as the model organic anions. The solvent-separated ion pairs (SSIP) and contact ion pairs (CIP) are found to form in protic and aprotic solvents, respectively, caused by the differences in solvent polar groups, and the transmembrane mechanism of organic anions in protic and aprotic solvents. Then, the solvation structure of mixed solvent is studied from molecule level, which is further validated by experiment. The addition of water can regulate the solvent environment around MO and weaken adverse effect on rejection performance due to the desolvation process in aprotic solvents. This work is of great significance for achieving high-efficiency separation by adjusting solvation structures, which expands the application scenarios of single membranes.