Mechanisms of selective ion transport and salt rejection in carbon nanostructures
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Introduction Materials composed of carbon nanopores have been proposed for use in a wide variety of applications, including the desalination of salty water, filtration of contaminated water, gas separations, chemical sensors, nanoscale pumps, energy-conversion devices, components in high-speed chromatography, and many others. These applications take advantage of the extremely rapid flow of molecules that have been measured in carbon nanotubes and graphene.1,2 In addition, most of these applications also require the preferential transport of specific molecules when compared to others. For example, for nanoporous membranes to be used for the desalination of salty water, it is essential that the membranes are able to rapidly pass water while blocking the passage of ions. Similarly, any filtration or separation application relies upon selective passage of specific molecules through pores, or the blockage of others. The feasibility of these applications is supported by experimental reports of divergent rates of transport of differing gases,1,3–7 liquids,2,4 and ions8,9 through carbon nanotubes and graphene nanopores, as well as the rejection of ions10,11 and size-selective transport of aqueous solutes.12 However, utilizing these promising materials requires a careful understanding of how selective transport arises, so that it can be enhanced or targeted toward the molecules of choice. In recent years, there has been a specific focus on understanding how ions can be removed from water or how selective
transport of ions can be achieved using carbon nanotubes and graphene. This is due, in part, to the simplicity of the molecules involved, the similarities with essential biological processes, and the fact that ions can flow through nanoporous materials at rates far exceeding those through conventional materials.8,9 Much of this work has centered on the use of molecular simulations, as they allow for the physical factors involved in transport to be dissected. However, as we will describe, the simplifications and approximations used in the simulations also allow for uncertainties to arise. We explore this literature to help explain the physical mechanisms involved in ion rejection and selectivity in carbon nanotubes and graphene. Not only can this aid in designing the materials to optimize the selectivity of the pores, it can help determine how selection for other molecules can be achieved.
Ion rejection in narrow pores The easiest way to stop an ion from passing through a narrow pore is to make the pore narrower than the size of the ion; that is, to make use of size exclusion (Figure 1a). While molecular sieving is useful for blocking the passage of larger ions such as ammonium, nitrate, or sulfate, it is not as useful for small ions such as Na+ or K+ if water is to simultaneously pass through the pore as well. Simulations have suggested that water will only pass through carbon nanotubes with ≥3.5 Å internal
Ben Corry, Research School of Biology, The Australian National University, Australia; [email protected] doi:10.1557/mrs.
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