Carbon Nanotube Nanofluidics

Abstract

Carbon nanotubes (CNT), with diameters in the nanometer range and atomically smooth surfaces, offer a unique system for studying molecular transport and nanofluidics. Although the idea that water can occupy such confined hydrophobic channels is somewhat counterintuitive, experimental evidence has confirmed that water can indeed occupy these channels (Naguib, Ye et al. 2004; Kolesnikov, Loong et al. 2006). Water transport through molecularscale hydrophobic channels is also important because of the similarity of this system to transmembrane protein pores such as aquaporins (Agre, Borgnia et al. 2001; Agre 2004; Agre 2006). In recent years, numerous simulations (Hummer, Rasaiah et al. 2001; Kalra 2003) of water transport through single-walled carbon nanotubes (SWNT) have suggested not only that water occupies these channels, but also that fast molecular transport takes place, far in excess of what continuum hydrodynamic theories would predict if applied on this length scale. Molecular dynamics (MD) simulations attribute this enhancement to the molecular-level smoothness of the nanotube interior surface and to molecular ordering phenomena that may occur on confined length scales in the 1to 2-nm range (Hummer, Rasaiah et al. 2001; Skoulidas, Ackerman et al. 2002; Kalra 2003). For similar reasons, simulations of gas transport through SWNTs (Lai, Bonilla et al. 2003) predict flux enhancements of several orders of magnitude relative to other similarly sized nanoporous materials. Membrane-based gas separations, such as those using zeolites (Hinds, Chopra et al. 2004), provide precise separation and size exclusion, although often at the expense of throughput or flux. It may be possible to use SWNT to create a membrane that offers both high selectivity and high flux. To investigate molecular transport on this length scale, we need to fabricate a carbon nanotube membrane that has a pore size of the order of 1 nm. Researchers have recently fabricated multi-walled carbon nanotube (MWNT) membranes with larger pore diameters (6 to 7 nm) by encapsulation of vertically aligned arrays of MWNTs (Hinds, Chopra et al. 2004; Holt, Noy et al. 2004) and by templated growth within nanochannel alumina (Li, Papadopoulos et al. 1999). Enhanced water transport through these larger MWNTs has recently been reported (MajumderMainak, ChopraNitin et al. 2005). Quantifying transport through an individual tube in a MWNT membrane is difficult, however, because MWNTs are prone to blockages, in particular by “bamboo” structures and catalyst particles that can migrate to and obstruct the nanotube interior (Cui, Zhou et al. 2000; Maruyama, Einarsson et al. 2005; Wang, Gupta et al. 2005). The consequence of such blockages is a marked reduction