Routes for Fabrication of Monoliths

The sol-gel process and the prepared monoliths can be studied using a variety of characterization techniques. There are some techniques that are more appropriate for monolith characterization such as UV-visible-NlR spectroscopy, refractive index measurements, density, XPS, porosity, small-angle scattering (SAXS), mechanical properties, and NMR. Reference [39] discusses these techniques extensively, which is beyond the scope of this chapter. 

---

Xerogel monoliths are semiporous monoliths (usually 30-70% porosity). Yoldas [40] in the 1970s was the first to demonstrate fabrication of monoliths via a sol-gel process. Since then numerous routes for fabrication of xerogel silica monoliths and nonsilica monoliths were suggested and demonstrated. In this section a few examples of xerogel monolith febrication will be presented. 

In this chapter we review the principles of monolith fabrication, routes for febrication of monoUths such as xerogel monoliths, nanocomposite monoliths, ORMOSIL monohths, hybrid monoliths, and aerogel monohths. In addition, we will present several characterization methods of monohths, encapsulation or doping in monohths, and examples of apphcations of monohths. 

Fabrication procedures of silica monoliths by a one-step catalyst procedure are presented in Ref [20]. A basic procedure to prepare a silica xerogel monolith is by following the stoichiometry ratio (H2O Si) R = 4, while using a similar molar amount of alcohol as a common solvent for achieving homogeneous solution. In fabrication of sol-gel monoliths, the catalyst used has crucial influence. The common route for preparing silica monoliths is to use a recipe based on a one-step catalyst reaction. 

Wang, H. Zhu, Y. Yan, X. Lin, J., Fabrication of molecularly imprinted hybrid monoliths via a room temperature ionic liquid-mediated nonhydrolytic sol-gel route for chiral separation of zolmitriptan by capillary electrochromotography. Electrophoresis 2008, 29, 952-959. 

Moreover, in-situ copolymerization approaches of polymerizable chiral cin-chonan carbamate selectors have also been shown to be viable straightforward routes to enantioselective separation media. In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of functional monomers and crosslinker in presence of porogenic solvents [80-85]. They have been utilized mainly for CEC (and will be described in detail later) while they turned out to be less suitable for HPLC application because of a low crosslinking degree. 

---