Cucurbit[6]uril builds a water channel for improving the water/salt selectivity of polybenzimidazole

Polybenzimidazole (PBI) has garnered significant interest in desalination due to its exceptional chemical stability and mechanical strength , but suffers from poor water permeability. In this work, different contents of hydroxylated Cucurbit [6]uril (CB [6] were added to PBI to build a “water transport channel”. The effects of CB [6] addition on the chemical structure and surface morphology were analyzed by FTIR, SEM, and Zeta potential , respectively. Based on the solution-diffusion model, the effect of CB [6] on the water/salt transport properties of PBI membranes was thoroughly investigated. The results show that appropriate CB [6] doping can enhance water sorption of PBI through the special “outer hydrophilic and inner hydrophobic cavity” structure, and boost water diffusion with the channeling effect of the hydrophobic CB [6] cavity. While the complexation between the carbonyl group in CB [6] and Na + ions slightly increased salt sorption, the specific cavity structure of CB [6] can significantly decrease salt diffusion by blocking salt ions and increasing transport path tortuosity. The cavity effect of CB [6] was further confirmed by hybridizing CB [6] with biphenylsulfonyl-substituted PBI when the cavity can be occupied. Therefore, the introduction of CB [6] improves the water/salt diffusion selectivity and then the permeability selectivity of PBI. Notably, the PBI@CB [6] membrane with a 0.25 wt% CB [6] loading exhibited a 40-fold increase in water permeability coefficient compared to pure PBI, while experiencing only a slight decrease in salt permeability coefficient, water/salt selectivity almost exceeding the empirical upper bound line of polymers. This work provides a theoretical foundation for the development of high-performance desalination membranes based on macrocyclic molecules such as CB [6].