Osmotic water transport through carbon nanotube membranes

TLDR

The study uses molecular dynamics simulations to investigate osmotically driven water transport through hexagonally packed carbon nanotube membranes. The authors simulate two semipermeable carbon nanotube membranes separating pure water and salt solution, driving water flow by osmotic pressure and analyzing the resulting nanoscale flow characteristics and confined water monolayers. The simulations reveal that nanoscale water flow is stochastic, frictionless, and limited by pore entry/exit barriers, achieving high rates comparable to aquaporin‑1 that are length‑independent, and can be quantitatively described by a 1D continuous‑time random walk, with long‑time drainage of the pure‑water compartment interrupted by structured solvation layers.

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.