Osmotic water transport through carbon nanotube membranes

Abstract

We use molecular dynamics simulations to study osmotically driven transport of water molecules through hexagonally packed carbon nanotube membranes. Our simulation setup comprises two such semipermeable membranes separating compartments of pure water and salt solution. The osmotic force drives water flow from the pure-water to the salt-solution compartment. Monitoring the flow at molecular resolution reveals several distinct features of nanoscale flows. In particular, thermal fluctuations become significant at the nanoscopic length scales, and as a result, the flow is stochastic in nature. Further, the flow appears frictionless and is limited primarily by the barriers at the entry and exit of the nanotube pore. The observed flow rates are high (5.8 water molecules per nanosecond and nanotube), comparable to those through the transmembrane protein aquaporin-1, and are practically independent of the length of the nanotube, in contrast to predictions of macroscopic hydrodynamics. All of these distinct characteristics of nanoscopic water flow can be modeled quantitatively by a 1D continuous-time random walk. At long times, the pure-water compartment is drained, and the net flow of water is interrupted by the formation of structured solvation layers of water sandwiched between two nanotube membranes. Structural and thermodynamic aspects of confined water monolayers are studied.

Diameter-dependent ion transport through the interior of isolated single-walled carbon nanotubes

“…Examples include silicon-based nanopores [2][3][4][5][6][7] , which achieve selectivity primarily through size-exclusion of macromolecules, and biological nanopores [8][9][10][11][12] , which manipulate their shape or present a specific pore opening to allow the passage of only specific ions such as potassium 8 , calcium 9 or sodium 9 . Singlewalled carbon nanotubes (SWNTs) are promising candidates for nanopore studies [13][14][15][16][17][18][19][20][21] as they can match the well-defined diameter and high aspect ratios of traditional silicon nanopores with sub-2 nm diameters and hydrophobic interior that allows for ion selectivity. These properties potentially allow for interesting desalination 22 and DNA-sequencing applications 13 , as well as the ability to probe basic fluid structure properties at the smallest of possible scales.…”

Tail Ordering Due to Headgroup Hydrogen Bonding Interactions in Surfactant Monolayers at the Water−Oil Interface

“…It allows for extensive hydrogen bonding between the triacontanoic acid headgroups, as illustrated by a molecular dynamics simulation at the water-hexane interface (Figure 3). The hydrogenbonded headgroups are arranged in linear rows, similar to the linear H-bonded wires formed by water molecules in hydrophobic environments (e.g., inside carbon nanotubes 15 ). The linear hydrogen-bonded arrangements were formed in different simulations started with different initial structural arrangements.…”

Transport properties and induced voltage in the structure of water-filled single-walled boron-nitrogen nanotubes

“…Thus, water flux and velocity could be measured. 8,18,19 The experimental condition was simulated by applying a constant force on a layer of bulk water molecules. Consequently a pressure gradient was induced between the two ends of a SWBNNT.…”