NanOasis (San Jose, CA) has developed carbon nanotube reverse osmosis membranes for use in desalination, nanofiltration, and ultrafiltration. According to inventors Timothy V. Ratto , Jason K. Holt and Alan W. Szmodis in U.S. Patent 20100025330, comparative research indicates a clear enhancement in permeability for the membranes with nanotubes relative to those without nanotubes and to commercially available membranes also without nanotubes.
Membranes for filtration by size exclusion are formed from open-ended nanotubes embedded in a polymeric matrix. The matrix forms a layer whose thickness is substantially less than the average length of the nanotubes, allowing the nanotubes to be randomly oriented throughout the matrix while providing channels extending through the layer for the selective passage of molecular species or particles based on size.
The inventors discovered that a membrane containing nanotubes embedded in a polymeric matrix such that molecular species selectively pass through the nanotubes on a size-exclusion basis can be fabricated by polymerizing monomers or prepolymers. Polymerizable species in general, from liquid media in which open-ended nanotubes are suspended in random orientations, and sealing the matrix to form a substantially continuous barrier around the nanotubes. The ability of the nanotubes to pass molecular species in a selective manner can be achieved without imposing any particular alignment on the nanotubes while the nanotubes are in suspension in the liquid.
Once the suspension is formed and the polymerizable species are dissolved in the appropriate liquid media, the membrane is formed by contact of the liquids to cause the species to polymerize at the liquid interface(s). While the continuous barrier may serve as a solution/diffusion membrane with reverse-osmosis activity of its own, the size-exclusion action of the nanotubes enhances both the selectivity and the permeability of the membrane.
The membranes have applications in various modes of filtration, including nanofiltration, ultrafiltration, gas separation, and reverse osmosis. The membranes are particularly useful as reverse osmosis membranes, notably for desalination of water.
One advantage is that membranes can be manufactured in relatively large dimensions, suitable for use in high-throughput and high-volume applications. The manufacturing processes are thus not limited to dimensions on the order of a silicon chip such as were those of the prior art methods.
A further advantage is the relatively low cost of the manufacturing procedure and the resulting membranes, since the procedure does not require costly substrates such as silicon nor costly steps or equipment such as those used in chemical vapor deposition. Nor does the manufacturing require specialized methods for fabrication of the nanotubes. The nanotubes can instead be obtained in bulk, synthesized by any known technique, and if the nanotubes are not already open-ended they can be treated by simple and inexpensive means to achieve open ends.
A still further advantage, and one that carbon nanotube membranes have in common with other filtration systems that utilize nanotubes as the filtration medium, is that the filtration behavior of the membrane, notably its molecular weight cutoff (MWCO), can be closely controlled by the selection of nanotubes of the appropriate internal diameter. The MWCO will thus be independent of the polymerization conditions provided that the barrier thus formed is continuous and essentially pore-free.