(35c) Carbon Nanotube Membranes

Carbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. Towards this goal, a composite membrane structure containing vertically aligned carbon nanotubes passing across a polystyrene matrix film have been fabricated. Fabrication steps, material characterization and ionic diffusion transport properties are described. Plasma oxidation during the fabrication process introduces carboxylic acid groups on the CNT tips that are modified using carbodiimide mediated coupling between carboxylic acid on the CNTs and accessible amine groups of the functional molecule. The entrances to CNT's cores were thus functionalized with aliphatic amines of different lengths, charged dye molecule and an aliphatic amine elongated by spacers containing poly-peptides and the simultaneaous permeation of two differently sized but equally charged molecules was studied. The relative selectivity of ruthenium bi-pyridine [Ru-(bipy)3+2] and methyl viologen [MV+2] was seen to vary from 1.9 to 3.6 as a function of tip-functionalization chemistry Anionic charged functional groups are seen to sharply increase flux of cationic permeates. This effect is reduced at higher solution ionic strength consistent with shorter Debye screening length screening attractive charge at the CNT core entrance. Using a hindered diffusion to model observed selectivities was consistent only with a geometry of only CNT tip functionalization, not along the length of CNT core. Bio-chemical gating of CNTs is also seen by tethering desthiobiotin to CNT tips with the reversible binding to streptavidin. The complete ATP cycle (phosphylation/dephosphylation) can be performed on CNT tips with corresponding modulation of flux across CNT membrane. Strong electrostatic effects of binding protein are seen with enhanced cationic flux seen for the relativel open anioic protein binding at CNT tip entrance. The functional density of tethered charge molecules can be substantially increased by the use of electrochemical grafting of diazonium salts. Functionality can be forced to occur at the CNT tip entrances by fast fluid flow of an inert solvent through the core during electrochemical functionalization. The selectivity between Ru(bi-pyridine)32+ and methyl viologen 2+ flux is found to be as high as 23 with -130mV bias applied to the membrane with tethered anionic dye molecule. Changes in the flux and selectivity support a model where charged tethered molecules at the tips are drawn into the CNT core at positive bias hindering/gating flux across the membrane. Application towards controlled transdermal drug delivery are discussed. In general, the transport mechanisms through CNT membrane are a) ionic diffusion is near bulk expectation with no enhancement from CNT b) gas flow is enhanced by ~1-2 order of magnitude due to specular reflection off of flat graphitic surface c) and pressure driven flux of a variety of solvents (H2O, hexane, decane ethanol, methanol) are 4-5 ORDERS OF MAGNITUDE FASTER than conventional Newtonian flow due to atomically flat graphite planes inducing nearly ideal slip conditions. This workshop will focus on the current state of the art in the reported literature on Carbon Nanotube membranes, fundamental transport mechanisms, and chemical modification design strategies for separation applications.