Adsorption particles, porous

Because of the similarity with the SGC model the modified Donnan model will be called the Stern-Donnan (SD) model. The SD model is newly derived and intended to be of use for a description of ion adsorption in porous or permeable particles without having to specify the permeability or porosity precisely. The SD model with small islands... 

Dextranase has been immobilized by adsorption onto porous titanium(iv) oxide particles coated with diazotized 1,3-diaminobenzene. Investigations of the effects of changes in dextran concentration, pH, temperature, and flow rate upon a continuously operated column of the immobilized dextranase permitted assessment of the kinetic aspects of the system via Lineweaver-Burk plots. 

The Figure 17.6 shows spectra of L-BPA after immobilization on FeC microparticles at different weight ratios composite/L-BPA. Apparently immobilization occurs by physical adsorption into porous of composite. The highest absorption capacity of L-BPA for this composite 78.0 mg/g was detected at weight ratio composite/L-BPA equal 5. The maximal adsorption capacity of L-BPA 160.0 mg/g was reached for dextran-modified iron-particles. 

FIGURE 11.12 Physical characterization and diagram of mesoporous CDC and its impact on cytokine removal, (a) Scanning electron micrograph of the synthesized mesoporous CDC. (b) Superior performanee of mesoporous CDC (800°C) as compared to other materials with respect to the ability to remove cytokines from human Wood plasma concentration of IL-6 in the plasma solution initially and after 5, 30, and 60 min of adsorption. (c,d) Schematics of protein adsorption by porous carbons (c) surface adsorption in microporous carbon. Small pores do not allow proteins to be adsorbed in the bulk of carbon particles (d) adsorption in the bulk of mesoporous CDC. Large mesopores are capable to accommodate a larger fraction of the proteins. (Adapted from Yushin, G. et al. Biomaterials 27, 5755-5762, 2006.)... 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

It would be difficult to over-estimate the extent to which the BET method has contributed to the development of those branches of physical chemistry such as heterogeneous catalysis, adsorption or particle size estimation, which involve finely divided or porous solids in all of these fields the BET surface area is a household phrase. But it is perhaps the very breadth of its scope which has led to a somewhat uncritical application of the method as a kind of infallible yardstick, and to a lack of appreciation of the nature of its basic assumptions or of the circumstances under which it may, or may not, be expected to yield a reliable result. This is particularly true of those solids which contain very fine pores and give rise to Langmuir-type isotherms, for the BET procedure may then give quite erroneous values for the surface area. If the pores are rather larger—tens to hundreds of Angstroms in width—the pore size distribution may be calculated from the adsorption isotherm of a vapour with the aid of the Kelvin equation, and within recent years a number of detailed procedures for carrying out the calculation have been put forward but all too often the limitations on the validity of the results, and the difficulty of interpretation in terms of the actual solid, tend to be insufficiently stressed or even entirely overlooked. And in the time-honoured method for the estimation of surface area from measurements of adsorption from solution, the complications introduced by... 

In liquid-solid adsorption chromatography (LSC) the column packing also serves as the stationary phase. In Tswett s original work the stationary phase was finely divided CaCOa, but modern columns employ porous 3-10-)J,m particles of silica or alumina. Since the stationary phase is polar, the mobile phase is usually a nonpolar or moderately polar solvent. Typical mobile phases include hexane, isooctane, and methylene chloride. The usual order of elution, from shorter to longer retention times, is... 

Figure 16-9 depicts porous adsorbent particles in an adsorption bed with sufficient generality to illustrate the nature and location of individual transport and dispersion mechanisms. Each mechanism involves a different driving force and, in general, gives rise to a different form of mathematical result. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Figure 1.5 shows the cumulative pore volume curve for 5-/rm monosized porous PS-DVB particles with 50, 60, and 70% porosity. The curves were drawn by overlapping the measurements from nitrogen adsorption-desorption and mercury intrusion. A scanning electron micrograph of 5-/rm monosized particles with 50% porosity is shown in Fig. 1.6 (87). 

Figure 1c shows the spectrum of aerosil that has been slurried in water and then dried at 100 C. This treatment initiates gel formation, so that the sample is no longer a chain of silica particles held together by electrostatic forces, but a porous network held together by siloxane linkages. The most obvious features in this spectrum are an increase in the water adsorption features at 3400 cm"" and 1632 cm". In addition, a band at 976 cm" is evident, that was much less obvious in the spectra of the other two silica samples. This feature is due to siloxane bridges formed during gel formation (10,12). 

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