Porous structure of gels

2 SIZE EXCLUSION CHROMATOGRAPHY FOR POROUS STRUCTURE ANALYSIS 2.1 Porous Structure of Gels 

A gel is an intermediate state between solid and liquid, composed of a three-dimensional network of a polymeric substance and the imbibed liquid. The scientific and practical importance of gels arises from their peculiar 

Much useful information on gel structures has been obtained by using conventional physicochemical techniques such as mercury intrusion, gas 

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Fig. 7,2. Influence of colloidal sol stability on the porous structure of gel layers (a) stable sol with non-aggregated particles (b) partially aggregated sol with weakly charged particles.

Porath [67] and Halasz et al. [68] suggested the use of gel chromatography for the characterization of the porous structure of gels (inverse gel chromatography). GPC... 

Chul, M Phillips, R McCarthy, M, Measurement of the Porous Microstructure of Hydrogels by Nuclear Magnetic Resonance, Journal of Colloid and Interface Science 174, 336, 1995. Cohen, Y Ramon, O Kopeknan, IJ Mizrahi, S, Characterization of Inhomogeneous Polyacrylamide Hydrogels, Journal of Polymer Science Part B Polymer Physics 30, 1055, 1992. Cohen Addad, JP, NMR and Statistical Structures of Gels. In The Physical Properties of Polymeric Gels Cohen Addad, JP, ed. Wiley Chichester, UK, 1996 39. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Tamon H, Ishizaka H, Mikami M, Okazaki M. Porous structure of organic and carbone aerogels synthesized by sol-gel polycondensation of resorcinol with formaldehyde. Carbon 1997 35 791-6... 

Non-silica-based RP-HPLC stationary phases have also been developed and their separation capacity has been compared with those of silica-based ones. The porous structure of crosslinked polymer gels may be responsible for the markedly different selectivity and retention characteristics. Up till now, the mode of separation on polymer stationary phases is not entirely understood at the molecular level. It has been established that the size-exclusion effect may influence the retention of analyses on polymer gels. 

To elucidate the influence of the preparation procedure (dynamic or static) two series of gluocose based adsorbents were pre-deposited on the surface of silica gel Si-100. The proportions of glucose to silica gel were the same for the static (S) and dynamic (D) series of carbosils. Additionally, the influence of water (formed during glucose decomposition) on the porous structure of silica was analyzed. 

A zeolite membrane is a membrane in which the transfer is controlled by the porous structure of the zeolite. Compared to sol-gel membranes, zeolite membranes can present some advantages for CMR applications. The most useful feature is that the pores of zeolites arc in the ultramicroporous range and have a very narrow size distribution (in this case pores are linked to the structure and not to the texture as in sol gel membranes). These characteristics of zeolite... 

Freeze drying is effective to retain porous structure of RF hydrogels. It is possible to prepare mesoporous RF drygels and carbon gels over wide ranges of R/C and R/W. 

Specific surface. The porous structure of silica gel may vary on a wide scale. Porous structure defines the size of surface area. If the pores are fine, the surface area is larger the resolution of small molecular size components is generally better. A larger size of molecules may require a wider pore size for their separation. Usually, the mean pore size may vary from 150 to 40 A with specific surfaces between 300 m /g to 600 m-/g. In some cases, the specific surfaces of TLC silica may be as low as 10 m /g, or as high as 100 m-/g. 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

V.V. Terskikh, I.L. Mudrakovskii and V.M. Mastikhin, Xe NMR Magnetic resonance studies of the porous structure of silica gels, /. Chem. Soc. Faraday Trans., 89 (23) (1993) 4239. 

Also, third-generation biosensors for superoxide anion (O ) have been developed based on superoxide dismutase (SOD) immobilised by thin silica-PVA sol-gel film on a gold electrode surface [633]. The preparation of SOD electrode is easy and simple. The uniform porous structure of the silica-PVA sol-gel matrix results in a fast response rate of immobilised SOD and is very efficient for stabilising the enzyme activity. 

As indicated at the beginning, the presented discussion of CPG is selective. It focuses on the many properties distinguishing CPGs from silica gels. These differences are found in the porous structures of these materials and - mainly - in their surface properties. Both are built of Si02 and have surface covered with hydroxyl groups, but boron residue remaining... 

The relatively high carbon content of Adsorbent X (2.46 %) does not cause any significant changes in the porous structure of the modified initial silica gel. Adsorbent H obtained through the pyrolysis of n-heptanol contains, in fact, the same amount (2.3 %) of carbon as Adsorbent X, but, in spite of this, as shown by Table 7, the adsorbents clearly differ in their surface characteristics. The n-heptanol carbonization products block more effectively the narrow pores of the modified silica than the carbon produced in the pyrolysis of dichloromethane. This is confirmed by the differences in the specific surface area of both adsorbents (Table 7). 

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