Silica gel monoliths

Expansion and contraction of silica gel monoliths show hysteresis upon heating and cooling. As long as the sample was cycled below approximately 500°C, there was no hysteresis and the monolith was thermally stable (6,23). 

Studies to reduce the time of analysis while maintaining the quality of resolution. To achieve this, short columns are used with stationary phases formed either from non-porous particles of small diameter (3pm or less), or of porous networks of silica gel (monolithic column). This allows faster flow rates, then more rapid separations (Figure 3.24). This technique is most frequently used in chemical industries, and areas as food processing, environment, pharmacy and biochemistry. 

Figure 1. Flow diagram of the sol-gel process for making silica gel monoliths composed of micrometer-sized particles.

The vibrational overtones and combinations of hydroxyl groups and their associated molecular water occurring in the spectra of various gel silica materials are summarized in Table 2 and discussed in References 3, 5, and 22. These peaks and bands found in the preparation of alkoxide-derived silica gel monoliths are identical to those described for silica gel powders (41). 

Tanaka s research group reported the analysis of a series of alkylbenzenes (C6H5-(CH2) H, n = 0-6) and PAHs (naphthalene, fluorine, phenanthrene, anthracene, pyrene, triphenylene, and benzo[a]pyrene) on tetramethoxysilane-based macroporous silica gel monoliths and achieved efficiencies up to 80000 plates per meter. 

Nakanishi, K. Minakuchi, H. Soga, N. Tanaka, N. Double pore silica gel monolith applied to liquid chromatography. J. Sol-Gel Sci. Technol. 1997, 8, 547 552. 

Adachi T., Sakka S. Preparation of monolithic silica gel and glass by the sol-gel method using N,N -dimethylformamide. J. Mater. Sci. 1987a 22 4407-410 Adachi T., Sakka S., Okada M. Preparation of the silica gel monolith by the sol-gel method using N,N -dimethylformamide and the vitrification ofthe gel. J. Ceram. Soc. Jpn. 1987b 95 970-975 Adachi T., Sakka S. Sintering of silica gel derived from the alkoxysilane solution containing N,N -dimethylformamide. 1988 100 250-253... 

Adachi T., Sakka S. Microstructural changes in sol-gel derived silica gel monolith with heating as revealed by the crack formation on immersion. J. Ceram. Soc. Jpn. 1989 97 203-207 Adachi T., Sakka S. Dependence ofthe elastic moduli of porous silica gel prepared by the sol-gel method on heat treatment. J. Mater. Sci. 1990 25 4732 737 Chen A., James P.F. Amorphous phase separation and crystallization in a lithium silicate glasses prepared by the sol-gel method. J. Non-Cryst. Solids 1988 100 353-358 Chen M., Lee W.E., James P.F. Preparation and characterization of alkoxide-derived celsian glass-ceramic. J. Non-Cryst. Solids 1991 130 322-325 Chen M., James P.F., Lee W.E. Densificaation and crystallization of celsian glass derived from alkoxide gel. J. Sol-Gel Sci. Tech. 1994a 2 233-237... 

The latest innovation is the introduction of ultra-thin silica layers. These layers are only 10 xm thick (compared to 200-250 pm in conventional plates) and are not based on granular adsorbents but consist of monolithic silica. Ultra-thin layer chromatography (UTLC) plates offer a unique combination of short migration distances, fast development times and extremely low solvent consumption. The absence of silica particles allows UTLC silica gel layers to be manufactured without any sort of binders, that are normally needed to stabilise silica particles at the glass support surface. UTLC plates will significantly reduce analysis time, solvent consumption and increase sensitivity in both qualitative and quantitative applications (Table 4.35). Miniaturised planar chromatography will rival other microanalytical techniques. 

Siouffi, A.M. (2003). Silica gel-based monoliths prepared by the sol-gel method facts and figures J. Chromatogr. A 1000, 801-818. 

A new desiccant formulation was prepared from a mixture of submicron-sized silica gel and molecular sieves to achieve the best combination of large water capacity, rapid moisture adsorption and easy regenerability. The formulation also tolerates the presence of VOCs and smokes. Commercial NaX and silica gel were crushed and mixed in the proportion of 1 2 and wash coated on a monolith for testing. The same test cell shown in Fig. 12.7-2 was used. The airflow to the saturator was adjusted to obtain the desired humidity in the feed air. Humidity sensors located at the flow cell inlet and outlet, were used to... 

Silica-based monolithic columns (Figure 9) are generally prepared using sol-gel technology. This involves the preparation of a sol solution and the gelation of the sol to form a network in a continuous liquid phase within the capillary. The precursors for the synthesis of these monoliths are normally metal alkoxides that react readily with water. The most widely used are alkoxysilanes such as tetramethoxysilane (TMOS) and TEOS. 

The sol-gel process for the preparation of silica-based monoliths involves heat treatment at lower temperatures for gelation and aging and the formation of mesopores by heating at high temperatures. 

FIGURE 1.4 Morphology and porosity of a typical monolithic rod, prepared by copolymerization of silane precursors, (a) SEM micrograph of the fractured surface of a monolithic silica gel rod. (b) Pore size distribution of a representative monolithic silica rod. (Reprinted from Guiochon, G., J. Chromatogr. A, 1168, 101, 2007. Copyright 2007, with permission from Elsevier.)... 

Although inorganic, monolithic columns attracted considerable attention in the last 10 years, the preparation of silica-based monoliths does not yet offer the broad chemical variety of precursors and porogens for specific adjustment of separation compared with their organic counterpart. The preparation of silica monoliths uses the classical sol-gel process of hydrolysis and polycondensation of organosilicium compounds. 

On-line hydrolysis of proteins catalyzed by trypsin or pepsin immobilized on monolithic silica beds was described by Kato et al. [86,195], whereas pepsin was encapsnlated into the silica-gel matrix (75 pm capillary column), without loss in enzymatic activity [195]. 

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