Page 372 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 357

the extent of aggregation in the colloidal sol, a gel of desired properties
can be produced. The aggregation of colloidal particles in the sol may be
controlled by adjusting the solution chemistry to influence the diffuse
layer interactions between particles, by adding stabilizing agents such
as surfactants, or through ultrasonification. Knowing that the proper-
ties of the gel will influence the permeability of the future membrane, it
is clear that the gelation step is extremely important. This gel is then
deposited, typically by a slip-cast procedure, on an underlying porous sup-
port. In variations on this procedure, functionalized surfaces may also
be used to achieve a more ordered deposit or micelles can be used to direct
film formation in specific geometries through self-assembly. In conven-
tional sol-gel, the excess liquid is removed by drying and the final ceramic
is formed by firing the resulting gel at higher temperatures. Drying and
firing conditions have been shown to be very important in the structural
development of the membranes, with higher drying rates resulting in
more dense membrane films.
The sol-gel approach of reacting small inorganic molecules to form
oligomeric and polymeric nanoparticles has several limitations such as
difficulties in controlling the reaction conditions, and the stoichiometries,
solubility, and processability of the resulting gel. It would thus be desir-
able to prepare nanoparticles in a one-pot bench-top synthesis from
readily available, and commercially viable, starting materials, which
would provide control over the products. One strategy for producing
nanostructured membranes involves an environmentally benign alter-
native to the sol-gel process for ceramic membrane formation. Metal
nanoparticles such as alumoxanes [19] and ferroxanes [20] can be pro-
duced based upon the reaction of boehmite, [Al(O)(OH)] , (or lepidi-
n
crocite in the case of the ferroxanes) with carboxylic acids [21]. The
physical properties of such metal-oxanes are highly dependent on the
identity of the alkyl substituents, R, and range from insoluble crys-
talline powders to powders that readily form solutions or gels in hydro-
carbon solvents and/or water. Thus, a high degree of control over the
nanoparticle precursors is possible. Metal-oxanes have been found to be
stable over periods of at least years. Whereas the choice of solvents in
sol-gel synthesis is limited, the solubility of the carboxylate metal-
oxanes is dependent on the identity of the carboxylic acid residue, which
is almost unrestricted. The solubility of the metal-oxanes may therefore
be readily controlled so as to make them compatible with a coreactant.
Furthermore, the incorporation of metals into the metal-oxane core
structure allows for atomic scale mixing of metals and formation of
metastable phases. In the case of the aluminum-based alumoxanes, the
low price of boehmite ($ 0.5 per kilogram) and the availability of an
almost infinite range of carboxylic acids make these species ideal as pre-
cursors for ternary and doped aluminum oxides.