Water Flows Through Nanotubes
at an Impossibly Rapid Rate
by John Emsley
Chemistry
Top Ten Papers
Rank
Papers
Cites
Mar-Apr 08
Rank
Jan-Feb 08
1
J.K. Holt, et al., "Fast
mass transport through
sub-2-nanometer carbon nanotubes,"
Science, 312(5776):
1034-7, 19 May 2006. [Lawrence
Livermore Natl. Lab., Livermore,
CA; U. Calif., Berkeley] *043UX
16
†
2
L. Venkataraman, et al.,
"Dependence of single-molecule
junction conductance on molecular
conformation," Nature,
442(7105): 904-7, 24 August 2006.
[Columbia U., New York, NY] *076LT
16
†
3
S. Stankovich, et al.,
"Graphene-based composite
materials," Nature,
442(7100): 282-6, 20 July 2006.
[Northwestern U., Evanston, IL;
Purdue U., West Lafayette, IN]
*064WT
15
†
4
P. Jurecka, et al.,
"Density functional theory
augmented with an empirical
dispersion term. Interaction
energies and geometries of 80
noncovalent complexes compared with
ab initio quantum
mechanics calculations," J.
Comput. Chem., 28(2): 555-69,
30 January 2007. [U. Calgary,
Canada; Acad. Sci. Czech Rep.,
Prague] *122PK
15
†
5
D. Enders, et al.,
"Control of four stereocentres in a
triple cascade organocatalytic
reaction," Nature,
441(7095): 861-3, 15 June 2006.
[Aachen U., Germany] *052SL
14
8
6
P. Schreiner, et al.,
"Many density functional theory
approaches fail to give reliable
large hydrocarbon isomer energy
differences," Organic
Lett., 8(17): 3635-8, 17
August 2006. [U. Giessen, Germany;
Princeton U., NJ; U. Gottingen,
Germany] *072EJ
14
†
7
N. Tian, et al.,
"Synthesis of tetrahexahedral
platinum nanocrystals with
high-index facets and high
electro-oxidation activity,"
Science, 316(5825): 732-5,
4 May 2007. [Xiamen U., China;
Georgia Inst. Tech., Atlanta]
*163RR
14
†
8
S.Z. Luo, et al.,
"Functionalized chiral ionic
liquids as highly efficient
asymmetric organocatalysts for
Michael addition to nitroolefins,"
Angew. Chem.-Int. Ed.,
45(19): 3093-7, 5 May 2006.
[Chinese Acad. Sci., Beijing;
Nankai U., Tianjin, China] *042MY
13
†
9
M. Rueping, A.P. Antonchick, T.
Theissmann, "A highly
enantioselective Brønsted
acid catalyzed cascade reaction:
Organocatalytic transfer
hydrogenation of quinolines and
their application in the synthesis
of alkaloids," Angew. Chem.
Int. Ed., 45(22): 3683-6, 26
May 2006. [U. Frankfurt, Germany]
*050DP
13
†
10
J.E. Green, et al., "A
160-kilobit molecular electronic
memory patterned at 1011
bits per square centimetre,"
Nature, 445(7126): 414-7,
25 January 2007. [Caltech,
Pasadena; U. Calif., Los Angeles;
Ohio St. U., Columbus] *128WD
Paper #1 reports the discovery that gas and water flow through
carbon nanotubes at much faster rates than theories predict. The
research may be the basis for new separation techniques, and could have
applications for the desalination of seawater, which might one day help
to solve the global shortage of drinking water. The research has even
wider implications because "nanofluidics" is the key to transfer of
molecules across membranes.
Paper #1 comes from a group headed by physicist Olgica Bakajin and
chemist Aleksandr Noy based at the Lawrence Livermore National
Laboratory (LLNL) in California. The team developed membranes with
carbon nanotube pores of diameter less than 2 nanometers. They used a
catalytic chemical deposition process to grow double-walled nanotubes
on the surface of a silicon chip and sealed the gaps between them with
silicon nitride generated by low-pressure chemical vapor deposition.
That the membranes really were free of gaps was proved by their
impermeability to both liquids and gases. The ends of the tubes were
then re-opened using argon ion etching to strip away the surface layers
of silicon nitride.
The nanotubes were between 2 and 3 microns in length, and there were
250 billion of them per square centimeter. They had diameters of 1.3 to
2.0 nanometers (nm), the upper limit shown by their not allowing 2 nm
gold particles to pass through. Transmission electron microscopy showed
that the inner diameter of the tubes was about the width of six water
molecules.
What was remarkable about the new membranes was the speed with which
gas and water molecules could flow through them. This was more than 100
times faster than the Knudsen model for diffusion of gases, and more
than 1,000 times faster than theory would predict for the hydrodynamic
flow of water. However, the flow of water was in accord with molecular
dynamic calculations, which predict a flow of 12 water molecules per
nanosecond through a 1nm hole. The rapid flow of water is promoted by
"wires" of hydrogen-bonded molecules pulling themselves down a tube
with an almost frictionless surface. Bakajin and Noy report their
nanofluidics research in more detail in Nano Today (see A.
Noy, et al., 2[6]: 22-9, 2008).
These researchers are in fact the first to provide unambiguous
experimental evidence of the remarkable flow of gas and water
through carbon nanotube pores. Their new devices might well be
important in two key areas: as models for cell membranes, which by
their very nature must permit only selected molecules to pass through
them; and as a method for desalinating seawater.
In this latter respect the LLNL team has been working on the transport
of ions through nanotubes and have reported on this in the online
edition of PNAS, (10.1073/pnas.0710437105). They placed
negatively charged groups around the openings of the nanotubes and
report that these restrict access by common ions like sodium. Ion
exclusion was as high as 98% for highly charged ions. This ion blocking
might well be the key to future desalination plants, which currently
work on the basis of reverse osmosis, a process requiring enormous
water pressures to make it effective. Lower-pressure desalination would
have worldwide benefits.
The other area in which their discovery might well have a role is cell
membrane simulation, but if that is to be effective, the carbon
nanotubes will have to operate in a fluid medium. However, suspending
carbon nanotubes in a liquid phase has so far defeated researchers, but
now Bakajin and Noy have breached this barrier. They coated the
nanotubes with an inorganic shell which allowed them to be transferred
to the liquid, where the coating could be removed. What is equally
remarkable is that the nanotubes still display their original
electronic properties in the new medium—see A.B. Artyukhin,
et al., International Journal of Nanotechnology,
5(4-5): 488-96, 2008. For earlier work in this area consult: A.B.
Artyukhin, et al., Nano Letters, 6(9): 2080-5, 2006;
and S.C.J. Huang, et al., Nano Letters, 7(11):
3355-9, 2007.
Bakajin and Noy are optimistic about the future: "We believe that we
can utilize the enhanced transport properties of carbon nanotube pores
to create the world’s best membranes for new generations of
separation technologies." I believe they may well be
right.
Dr. John Emsley is based at the Department of Chemistry,
Cambridge University, U.K.
Keywords: carbon nanotubes, Olgica Bakajin, Aleksandr Noy, carbon
nanotube pores, water desalination, fast mass transport.