Carbon nanotube membranes: From flow enhancement to permeability

Abstract

Abstract: Since their discovery, carbon nanotubes have been considered as a potential material for filtration applications due to low tortuosity, smooth structure and the possibility of fine tuning their diameter. Measurements of fluid flow in nanotubes, with diameters ranging from 0.6 to 100 nm dramatically raised interest in them, with very high water flow rates promising to deliver orders-of-magnitude higher performance compared to other membranes. This promise was based on reports of flow enhancement, defined as a ratio of the measured flow compared to a no-slip Poiseuille flow, ranging from 10 to 100,000 with the underlying assumption that commercial membranes would exhibit the no-slip behavior. The concept of flow enhancement, though, is of little help for actual filtration applications where one is interested in a membrane׳s performance in terms of selectivity and permeability. In this work, the flow enhancement and permeability of UF carbon nanotube–anodic alumina membranes (CNT–AAM) with a large range of diameters is reported. Using a recently developed model, it is shown that the permeability is directly related to the solid–liquid molecular interactions between the liquid and the nanotubes. Finally, the performance of these CNT membranes and others in the literature has been analyzed in terms of permeability, comparing them to commercial membranes in the RO, NF and UF ranges. Results show that in fact, carbon nanotube membranes have a higher pure water permeability than commercial polymer membranes.