Article pubs.acs.org/JPCC
Simulation of Water Transport Through Functionalized Single-Walled Carbon Nanotubes (SWCNTs) Zak E. Hughes,† Cameron J. Shearer,§,∥ Joe Shapter,§ and Julian D. Gale*,‡ †
School of Biomedical Sciences and ‡Nanochemistry Research Institute/Department of Chemistry, Curtin University, GPO Box U1987 Perth, WA 6845, Australia § Flinders Centre for Nanoscale Science and Technology, Flinders University, Sturt Road, Bedford Park, Adelaide 5001, South Australia ∥ Institüt für Physikalische Chemie, Universität Münster, Münster 48149, Germany ABSTRACT: Carbon nanotubes have attracted interest as possible membranes for desalination based on the observation of fast diffusion of water in simulations of long or infinitely periodic systems. When carbon nanotubes are finite they have often been simulated in force-field studies as having unsaturated dangling bonds for convenience, even though this is chemically unrealistic. In the present work, the influence of realistic terminations on the diffusion of water through the nanotubes is examined through computer simulation as well as the nature of the interface with saline solution at seawater concentrations. Termination of the cleaved nanotubes by hydrogen with a range of functional groups is explored including hydroxyl, carboxylic acid, and carboxylate anions with sodium counter cations. Realistic structures are found to lead to a reduced alignment of the nanotubes within the membrane layer and therefore a broader interfacial region. Diffusion of water within the finite nanotubes is slower than observed in the infinite limit and is, in general, further reduced as the polarity of the end functional groups increases. The largest impact occurs for carboxylate termination in contact with saline solution, where the ionic atmosphere of sodium ions retards water diffusion across the interface.
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INTRODUCTION There are three basic categories of water purification: membrane technologies (including reverse osmosis (RO) membranes), thermal processes (including various types of distillation), and chemical approaches (including aggregation and flocculation). A membrane is a thin film of porous material that allows water molecules to pass through it but simultaneously prevents the passage of larger, undesirable molecules such as viruses, bacteria, and salts. Membranes can be made from polymeric materials such as cellulose, acetate, and nylon or from nonpolymeric materials such as ceramics, metals, and composites.1 In general, water flow in membrane technologies is either pressure or electrically driven. There have been many studies that seek to improve the fundamental understanding of such technologies and thereby improve their efficiency. Improvements can be achieved in the form of either greater selectivity or a reduction in the excess energy required to overcome the thermodynamic cost of extracting water from saline solution. Whereas the current generation of RO membranes consists predominately of supported polymers, there is a quest for alternative materials that may ultimately deliver superior performance. Here there has been interest in carbon nanotubes, both as additives to polymers and as membranes in their own right. © 2012 American Chemical Society
Whereas experimental studies clearly predominate in the study of desalination, theoretical and computational methods can also provide useful insight into the atomic detail of interactions between water and membranes. The earliest calculations regarding water transport in nanotubes involved extensive molecular dynamics (MD) simulations on the “simplest” nonpolar pore: a carbon nanotube.2,3 This was achieved by building an uncapped single-walled carbon nanotube (SWCNT) of 1.34 nm in length by 0.81 nm in diameter (based on the distance between carbon centers) and simulating its dynamics while solvated in a water reservoir for 66 ns. It was found that the nanotube was quickly filled and the water molecules in the pore formed a quasi-1D wire connected by hydrogen bonds more or less aligned with the tube axis. Water molecules were found to not only enter the CNT but also to exit from the other end. Overall, 17 water molecules per nanosecond were calculated to pass through the tube (51 × 10−14 cm3 s−1); this is of comparable flow to water through the trans-membrane protein aquaporin-1.4 This fast flow is attributed to the frictionless, smooth, inner wall of the CNT. Received: August 2, 2012 Revised: November 7, 2012 Published: November 7, 2012 24943
dx.doi.org/10.1021/jp307679h | J. Phys. Chem. C 2012, 116, 24943−24953
The Journal of Physical Chemistry C
Article
the three- to five-fold improvement for a CNT MD simulation over a poly(methyl methacrylate) membrane.20 These results indicate that low diameter (C−C distance of
