Water filtration performance of a lyotropic liquid crystal polymer membrane with uniform, sub-1-nm pores

Abstract

The water filtration performance of a supported, nanostructured, lyotropic (i.e., surfactant) liquid crystal (LLC) polymer membrane containing uniform, sub-1-nm pores was examined in more detail using more extensive aqueous filtration tests and a more structurally representative nanopore model. This nanoporous membrane material is based on the radical cross-linking of an ionic, LLC monomer that forms a type I bicontinuous cubic (Q I) phase with a 3D interconnected, annulus-like, water pore network. Previously, initial water transport studies using a limited number of solutes demonstrated initial proof-of-concept that this LLC polymer membrane can reject small molecules based on molecular size discrimination. In this study, more comprehensive transport experiments using aqueous solutions containing a wider range of different size neutral organic molecules and salt ions were undertaken. The results of these experiments showed that the LLC polymer membrane has a rejection performance in between that of a commercial, high-performance reverse osmosis (RO) membrane (Dow SW30HR) and that of a conventional porous nanofiltration (NF) membrane (Dow NF-270) with non-uniform pore sizes. Specifically, the LLC membrane was found to reject inorganic salt ions much better than a NF membrane, and on a level comparable to a RO membrane. It does not reject small neutral organic solutes as well as a RO membrane, but it was found to reject organic solutes better than a conventional NF membrane. Also, the organic solute rejection behavior was fitted using two different Donnan-steric pore models (DSPM) with different pore shapes (i.e., cylindrical and straight slit). It was determined that the Q I-phase LLC membrane has an effective pore radius between 0.29 nm and 0.45 nm. This modeling also suggested that the LLC nanopores are indeed uniform in size. Initial chemical stability and fouling experiments suggest this nanoporous LLC polymer membrane is also very resistant to chlorine degradation and protein fouling. This combination of properties makes this nanoporous LLC polymer a very promising membrane material for water desalination and other NF processes.