Osmotic water transport through carbon nanotube membranes
PNAS vol.100 no. 18 (2003) 10175?10180
Amrit Kalra*†, Shekhar Garde†, and Gerhard Hummer*
*Laboratory of Chemical Physics, Building 5, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520
†The Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180
The paper investigated the osmosis of water through carbon nanotube (CNT) membranes using molecular dynamics (MD) simulations. It also studied the structural and thermodynamic aspects of water molecules confined inside the nanotubes.
The MD system consists of two sets of CNT membranes that separate a pure water compartment and two salt water compartments having a concentration of 5.8 M. Carbon atoms comprising the (6,6) armchair-type CNT were sp2 hybridized. CNTs have a diameter of 8.1 ? while length used was 13.4 and 27 ?. Water molecules were modeled using TIP3P. Na+ and Cl? were used as salt ions.
One of the quantities calculated for the system was the free energy G(z) that is dependent on the separation distance of the membrane layers, also known as the potential of mean force (PMF). This quantified the stability of layered water structures. From the density profiles of water perpendicular to the membrane surface, the layering of water molecules was observed.
Water flow rate obtained for each nanotube was 5.8 water molecules per nanosecond, which is high and comparable to the biological membrane protein, aquaporin-1. This result was found to be independent of the length of nanotube, as the water flow was observed to be almost friction-less.
The authors also observed that for long simulation durations, the pure water compartment would be drained and the net flow of water would be interrupted by the formation of structured solvation layers of water sandwiched between two nanotube membranes. During this phenomenon, they observed that the separation distance between the two layers of membranes decreased with time.
Contribution and application:
The authors of this study found out a scenario that is favorable for reverse osmosis using two CNT membrane layers: when a narrow gap of approximately 3 ? is maintained between the two membranes, reverse osmosis will happen and water molecules will fill up this gap. The study also gave details on how to understand and interpret free energy profiles that could be obtained from MD simulations.
By: Hannah Ebro