(110b) Nano-Scale Transport Phenomena in the Ionic, Solvent and Gaseous Regimes through Carbon Nanotube Membranes

Transport phenomena through the hollow conduits of carbon nanotubes (CNT) has been the subject of intense theoretical research due to the smooth graphitic structure of CNTs, the large van der Waals distances normal to the CNT wall and their similarity to biological membrane channels. We experimentally study fundamental transport processes over the large spectrum of ionic diffusion to pressure driven liquid and gaseous flow. Dramatic enhancement of pressure driven flows is seen through the atomically flat CNT cores. A membrane structure consisting of a high density (~ 10¹⁰/cm²) of aligned multiwalled carbon nanotubes with inner pore diameters (~ 7 nm) spanning across a continuous polystyrene matrix was successfully fabricated. This structure provided a platform to experimentally measure ionic, liquid and gas transport through the inner cores of carbon nanotubes in macroscopic quantities.

Diffusive transport measurements of ions of different charge and size through the core of the CNT are close to bulk diffusion expectations, contrary to hindered diffusion considerations in small pores. Hindered diffusion effects are observed only at the entrances to carbon nanotubes with charged functionality.

Polar liquids such as water, ethanol, iso-propyl alcohol and non-polar liquids such as hexane and decane were observed to have pressure driven flow velocities four to five orders of magnitude higher than that predicted from Newtonian flow using the Hagen-Poiseuille equation. This suggests that the ‘no-slip' boundary condition, is no longer valid between the fluids and the surfaces of the graphitic core. The observed enhanced flow velocities were close to those reported in biological membrane channels and in agreement with theoretical predictions.

Enhanced gas transport kinetics, as expected by the specular reflection correction for Knudsen Diffusivity inside smooth pores, was observed. Gases like N2, CO2, Ar, H2, CH4 were observed to be an order of magnitude higher than predicted from Knudsen Diffusion calculations.

The transport studies indicate a bio-mimetic membrane structure with exceptionally high mass transport kinetics, limited only by the functional molecules at the entrance to the CNT cores, thus providing a basis for ‘gate-keeper' selective transport.