(208b) Mass Transport through Carbon Nanotube Membranes In Three Different Regimes: Ionic, Liquid and Gas

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 have experimentally studied transport processes over the large spectrum of ionic diffusion to pressure driven liquid and gaseous flow. Central to these studies is a membrane structure consisting of a high density (~ 1010/cm2) of aligned multiwalled carbon nanotubes with inner pore diameters (~ 7 nm) spanning a continuous polystyrene matrix allowing macroscopic measurement of transport through the structure. Several size-exclusion based control experiments including Au nano-crystal retention, separation of large from small molecule, ligand-gated ‘on/off' ionic transport validates a membrane structure with open-ended carbon nanotubes as conduits for molecular transport.

Plasma oxidation step during the fabrication of the membrane introduces carboxylic acid groups at the CNT entrance, which provides electro-static ‘gate-keper' effects on ionic transport. Capacitance measurements of the CNT membrane indicated a charge density of ~ 0.8 x 10-2 C/m2 at neutral pH. The ionic transport experiments are consistent with electrostatic hindrance effects at the entrances to carbon nanotubes (i.e a few nanometers of path length) and not along the entire path length. Diffusive transport of ions of different charge and size through the core of the CNT are therefore close to bulk diffusion expectations and allow estimation of the number of open pores or porosity of the membrane. 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 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 This suggests that the ‘no-slip' boundary condition, is no longer valid between the fluids and the surfaces of the graphitic core.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 flow velocities were close to those reported in biological membrane channels and in agreement with theoretical predictions. Flux of gases like N2, CO2, Ar, H2, CH4 scaled inversely with their molecular weight by an exponent of 0.4 suggesting Knudsen like transport through the CNT pores. However, the magnitude of the fluxes were observed to be one order of magnitude higher than predicted from Knudsen Diffusion calculations This is consistent with enhanced gas transport kinetics, expected for specular reflection inside smooth pores.

The transport studies indicate a membrane structure with exceptionally high mass transport kinetics, limited only by the functional molecules at the entrance to the CNT cores. The membrane structure has the potential to combine high selectivity with high permeability, not obvious with other membrane structures.