Effects of alkali on the polyester membranes based on cyclic polyphenols for nanofiltration

To meet separation need under various conditions, a great variety of membranes have been fabricated via interfacial polymerization (IP), especially for the polyester membranes with intrinsic micro-porosity. However, the aqueous phase used for making those polyester membranes often requires alkaline environment, resulting in a quite different procedure compared with conventional polyamide membranes, which may alter IP process that have been widely studied. Here, the KOH, NaOH and LiOH were used respectively to create alkaline environment whereby the impact of alkali on polyester membranes were investigated systematically. It was found that the alkali not only showed influence on the IP process, but it also affected diffusion of monomers in aqueous phase across the water/hexane interface. When the concentration of alkali or pH of the aqueous phase were the same, the membranes using KOH as base were found to have the lowest permeability while the membranes using NaOH showed the highest permeability, and the membranes using LiOH showed the moderate permeability. The mechanism of the permeance trend was further investigated in detail, and it was found that the monomer containing hydroxyl not only could act as IP monomers, but also could play role as surfactant, thus affected its own diffusion to organic phase. This study paves the way for a wider integration of these macrocyclic polyphenols in the field of high-performance nanofiltration (NF) membranes, and also offers a new approach to optimize the performance of NF membranes based on polyester.