Page 376 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 361

modeling first suggested that flow through pores composed of carbon
nanotubes might not have the same limitations as those observed for
other nanometer-sized pores [30]. Simulations indicate that water should
be able to flow much faster through hydrophobic CNTs due to the for-
mation of ordered hydrogen bonds. In the confined space of a nanotube,
water is present in ordered crystalline domains. When the nanotube wall
interacts significantly with the water, such as in the case of a small silica
channel where silinol groups may anchor water molecules to the wall of
the channel, the ordered water in the pore is thought to be less mobile.
In contrast, the hydrophobic surface in the interior of a defect-free carbon
nanotube appears to allow for a nearly frictionless flow that has been com-
pared with the flow through the protein channel aquaporin-1 [30].
Visualization of water within CNTs confirms the lack of interaction
between water molecules and the interior surface of CNTs [31].
Although these theoretical results suggest that CNTs are very prom-
ising materials for water filtration membranes, there are many chal-
lenges to be overcome in aligning CNTs and fabricating such a
membrane. The problem of aligning membranes was first approached
by filtering suspensions of SWNTs in a strong magnetic field [32]. A more
promising approach is to grow arrays of CNTs on a substrate where
nanoparticle catalysts for CNT growth have been arranged in a distinct
pattern that defines the number and spacing of the resulting CNTs.
The diameter of the nanotube is controlled by the size of the nanopar-
ticle catalyst initially arranged on the substrate [33]. A working mem-
brane of aligned CNTs requires that the spaces between CNTs be filled
with a material that seals the membrane to flow between CNTs, allow-
ing flow only through the interior of the CNTs. Among the approaches
taken to accomplish this has been spin-coating the CNT arrays with a
polymer solution [34] or filling the interstices of the aligned CNT with
silicon nitride [35]. The permeate flux of water measured across a mem-
brane of aligned multiwall CNTs with 7nm-diameter pores imbedded in
a polystyrene matrix has been reported to be four to five orders of mag-
nitude greater than that predicted by Eq. 30 [36]. However, smaller
diameter CNTs may exhibit a bamboo-like structure [37] that impedes
fluid flow [35]. Similar to these CNT-based membranes, fullerene-based
onto the surface of
membranes have also been made by grafting C 60
track-etched membranes [38].
These fullerene-based membranes can be thought of as composite
membranes in that they are composed of the fullerenes and at least
one other material. The properties of these composite membranes reflect
the sum of the properties of the components of the membrane. As such,
they resemble conventional thin film composite membranes such
as those used in RO. RO composite membranes rely on a thin layer of
material (typically a polyamide) on the surface or skin of the membrane