Environmental Engineering Reference
In-Depth Information
Drying
Pyrolysis
Carbon xerogel
(nonporous glassy
carbon)
Resin xerogel
RF resin nanoparticle
SCHEME 2.1
Formation of carbon xerogels by conventional drying of RF resin.
be dried using supercritical liquids (aerogels) [88] or low-surface-tension solvents (ambi-
gel) [89]. In both cases, the small surface tension forces prevent the collapse of gel pores.
Another way involves sublimation of the solvent from the pores by lyophilization of the
gels (cryogels) [90]. All these procedures are slow and cumbersome. Therefore, the use of
additives, which maintain porosity under drying in air, will be advantageous.
2.2.1.1 Use of Cationic Surfactant (CTAB) Micelles as Nanoparticle Stabilizers
Ordered mesoporous inorganic materials can be produced with a tailored pore size dis-
tribution by sol-gel techniques and using a variety of micelles of surfactant molecules
as templates [91]. They are produced in various compositions, such as oxides (e.g., silica,
alumina, titania, zirconia, mixed oxides), by condensation of inorganic species around the
arrays of self-assembled aggregates of surfactant molecules in water [92]. The synthesis of
RF organic sol-gel occurs by a condensation reaction mechanism, that is analogous to the
synthesis of inorganic oxides. Accordingly, Bell and Dietz [93] reported the preparation of
porous RF resins, claiming that surfactant micelles act as templates that are eliminated
during carbonization. The material could be air-dried without signiicant contraction,
avoiding the use of organic solvents or supercritical luids [94]. We [95] have suggested
that the actual mechanism involves the stabilization of resin nanoparticles (negatively
charged) by cationic micelles (positively charged) (see Scheme 2.2).
Drying
Pyrolysis
Nanoporous
carbon
(nPC-CTAB)
Nanoporous
RF resin
CTAB micelles
SCHEME 2.2
Formation of a nanoporous carbon by stabilization of the RF resin using cationic micelles.
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