CNT Membranes with Sub-Nm Pores Showing Salt Rejection and Sharp Size Selectivity at High Ionic Strenghts

Abstract: We report water and gas separation performance for novel carbon nanotube (CNT) membranes consisting of the smallest (0.67 - 1.27 nm inner diameter) single wall nanotubes yet reported for CNT membranes. The CNTs traverse a non-porous, non-water-permeable, flexible polymer film, and show rejection of small salts at high ionic strength, and highly specific size selectivity for sub-1.5 nm solutes. Water flow through the membranes showed significant enhancement over that predicted by Hagen-Poiseuille flow, in agreement with previous CNT membrane studies. Gas selectivity was found to be similar to that predicted by Knudsen flow for non-condensable gases, but the permeabilities were found to be significantly higher than those predicted by Knudsen, also in agreement with previous studies of CNT membranes. CO2 permeation was found to be substantially higher than predicted. The membranes used in this study were fabricated using non-aligned bulk CNTs, orientated during membrane formation to give a membrane with tubes traversing a non-porous polymer film. Membranes of this type will allow for the use of very small diameter nanotubes previously not suitable for use in CNT membranes, offering the potential to perform separations of sub-nm solutes with angstrom-scale selectivities.