Page 380 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 365

Case a is a template formed from particles suspended in ethanol. In the
case of ethanol, the particles experience a net repulsive electrostatic force
in the bulk as they approach the glass surface on which they were
deposited. Thus, these “nonsticky” particles form a compact deposit. In
contrast, particles suspended in an aqueous solution of 1.5 M ionic
strength (b) form a dendritic template.
The voids in these resulting templates are filled with a polymeric or
inorganic material and upon etching (or buring) of the particles, a porous
material with a three-dimensional structure is formed (Figure 9.7). The
use of nanoparticles in the templating process allows for a high degree
of control over chamber and pore size. The interior of the templated
object can be subsequently functionalized or functionality can be intro-
duced through the choice of material used to make the membrane. A high
degree of control over both the structure and internal functionality of
these membranes might be exploited to perform highly controlled reac-
tions, with each chamber of the membrane serving as a reactor.
Membrane selectivity can be modified with respect to both size exclu-
sion and chemical affinity.
Alternatively, asymmetric templates can be formed via the sequential
deposition of particle layers of nanoparticles from Langmuir-Blodgett
films. A layer of small particles is first deposited onto the support, fol-
lowed by a layer of larger particles. After casting the membrane around
this template, a membrane with a more complex structure composed of
distinct layers is formed (Figure 9.8).
CNTs have also been used as membrane templates. Beginning with an
array of aligned carbon nanotubes, spaces are filled between the tubes as
described earlier for the case of aligned CNT membranes. Using silicon

Figure 9.7 Cross section of a templated membrane formed from
“nonsticky” particles forming honeycomb like structure.