Tuning intermolecular pores of resorcin[4]arene-based membranes for enhanced nanofiltration performance

Macrocycles have received increasing attention for the design of thin-film composite (TFC) membranes with unprecedented separation performance due to the well-defined intramolecular cavities. Herein, a macrocycle, tetra-C-ethyl resorcin[4]arene (RA), was synthesized as a novel monomer, to prepare entirely RA-based TFC membranes via interfacial polymerization. The cavities of RA macrocycles thereby act as the homogeneous intramolecular pores of the membrane for water transport and salt rejection. However, the intermolecular pores formed by gaps between RA macrocycles are random, resulting in a wide pore size distribution and average nanofiltration performance. Therefore, a very small amount of crosslinked polyamide was introduced in the gaps to tune the intermolecular pores of the membrane by adding trace amounts of piperazine (PIP) in the aqueous solution. As a result, the optimal membrane has a water permeability as high as 42.3 L m^-2 h^-1 bar^-1 with a Na₂SO₄ rejection of 96.5%, which is much larger than the pristine membrane without incorporating polyamide networks. This facile way of modifying the intermolecular pores gives the membrane an encouraging improvement of the performance in nanofiltration.