Carbon Nanotubes: Electroosmotic Flow Control in Membranes
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Carbon nanotubes are currently of great interest in both fundamental and applied sciences. Potential applications for these tubular nanostructures include use in nanoelectronics, hydrogen storage, and battery and field-emitting display technologies. We and others have shown that carbon nanotubes can be prepared via the template method by chemical vapor deposition (CVD) of carbon along the pore walls of nanoporous alumina membranes. These membranes have cylindrical pores with monodisperse diameters, and a corresponding cylindrical nanotube is deposited within each pore. Carbon nanotubes with outside diameters as small as 20 nm have been prepared via this method. Using an alternative template material and method, Joo et al. have prepared carbon nanotubes with outside diameters of 9 nm.
At a recent workshop, it was suggested that carbon nanotubes might be used as chemically selective filters. This membrane separations application requires alignment of the tubes into a defect-free, mechanically strong membrane, where each tube spans the complete thickness of the membrane and the tube mouths are open at the faces of the membrane. Although Sun and Crooks have recently developed an elegant procedure for doing this with a single carbon nanotube, template synthesis inherently provides nanotube-containing membranes of this type, with up to 1011 nanotubes cm− 2 of membrane surface area. Having large numbers of nanotubes per square centimeter of membrane area is essential to enhancing flux across the membrane—a critical issue in membrane separation processes. These highly porous carbon nanotube membranes (CNMs) have monodisperse pore dimensions, which can be controlled by template synthesis. Template-prepared CNMs have a number of advantageous properties such as versatile surface chemistry and good electrochemical properties, which make them an interesting membrane material.
Flux can also be enhanced by augmenting diffusive transport across the membrane with electrophoresis, or pressure-driven or electroosmotic flow (EOF). Sun and Crooks demonstrated diffusive, pressure-driven, and electrophoretic transport of colloidal particles across their single CNMs. To our knowledge, we have performed the only investigation of EOF in CNMs. Electroosmotic flow was driven across template-prepared CNMs by allowing the membrane to separate two electrolyte solutions and by using an electrode in each solution to pass a constant ionic current through the nanotubes. According to Bath et al. EOF was investigated by measuring the flux of a probe molecule across the membrane. The as-synthesized CNMs have anionic surface charge and, as a result, EOF is in the direction of cation migration across the membrane. Electroosmotic flow in the direction of diffusion enhances permeant flux, whereas EOF in the opposite direction of diffusion lowers flux. Measurements of the rate of EOF as a function of applied transmembrane current provided the zeta (ζ) potential. The effect of pH on ζ provided the pKa for the surface acidic sites responsible for this anionic charge; the acidic site density was also determined. An electrochemical derivatization method was used to attach carboxylate groups to the nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate. Electrochemical derivatization was also used to attach cationic ammonium sites to the nanotube walls to yield CNMs that show EOF in the opposite direction of the as-synthesized or carboxylated membranes.