Significant Adsorbent Properties

The most common adsorption systems consist of silica gel or alumina adsorbents in association with an organic solvent system. The adsorbent can exert a considerable influence on the separation of compounds. Alumina and silica gel, for example, have significantly different properties and can result in quite different separations. Activation of the adsorbent also influences sample retention. The presence of water on the adsorbent decreases the adsorbent activity due to blockage of active sites. If large quantities of water are present, a partition system may be set up which may extensively change the retention times due to the different chromatographic principle involved. Table 2.1 compares results obtained for the separation of the insecticide carbaryl (Sevin) and its hydrolysis product 1-naphthol on alumina and silica gel. Comparisons between activation and deactivation are made. The results show that separation of the two components is reversed with the two adsorbents examined. In most cases, activation of the plates caused the/ f values to increase relative... 

The exchange of sodium by calcium has a significant effect on the adsorbent properties of zeolite A. An abrupt change in the adsorptive properties occurs when between three and five Na+ ions are replaced. Thus, Ar and N2 are now both able to enter the channels at low temperature, although the lack of agreement between the different values of up in Table 11.2 is still evident. 

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between free and bound water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the bound component, and others rely on its availability as a solvent. He prefers a definition in which bound water is that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with bulk water in the same system, and does not freeze at -40"C."... 

Very recently , we have carried out an intercomperison study of hquid-hquid extraction (LLE) and sorption on polyurethane foam (PUF) and Amberlite XAD-2 for the analysis of aliphatic, aromatic and chlorinated hydrocarbons dissolved in seawater. The application of these methods, sampling in parallel the same body of water, has shown significant differences in the recovery of higher molecular weight components in the complex mixtures of both aliphatic and aromatic hydrocarbons. These are attributed to selective associations of these hydrophobic species with macromolecular organic matter such as fulvic and humic acids and to the effects of the dissolved organic molecules on adsorbent properties. 

The adsorbent properties of activated carbons are essentially due to their surface area, universal adsorption effect, highly microporous structure, and a high degree of surface reactivity. The availability of favorable pore size makes the internal surface accessible and enhances the adsorption rate. The most widely used activated carbons have a specific surface area of 800 to 1500 mVg. This surface area is contained predominantly within micropores that have effective diameters smaller than 2 nm. In fact, a particle of active carbon consists of a network of pores that have been classified into micropores (diameters < 2 nm), mesoporous (diameter between 2 and 50 nm) and macropores (diam. > 50 nm). The macropores do not contribute significantly toward surface areas but act as conduits for the passage of the adsorbate into the interior mesopore and the micropore surface where most of the adsorption takes place. 

When the size of an adsorbate molecule is close to the size of the micropore, diffusion of the molecule becomes restricted and the rate of transport in the micropore may have a significant effect in the overall adsorption rate. This type of diffusion in the micropore is an activated process which depends heavily on adsorbate properties. 

The surface coverage (Equation 2) and hence the resulting adsorptive stripping response (Equation 1), differ from compound to compound based upon its polarity, size or orientation. It is this dependence of the response upon the adsorbate properties that provides useful information in the studies of biologically-significant compounds. 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Areas affected by significant atmospheric concentrations of contaminants exhibiting the foregoing physical and chemical properties should also be considered as potentially affected by contaminant rainout and settling to surface media. Contaminants dissolved in rainwater may percolate to ground water, run off or fall directly into surface waters, and adsorb to... 

We should mention here one of the important limitations of the singlet level theory, regardless of the closure applied. This approach may not be used when the interaction potential between a pair of fluid molecules depends on their location with respect to the surface. Several experiments and theoretical studies have pointed out the importance of surface-mediated [1,87] three-body forces between fluid particles for fluid properties at a solid surface. It is known that the depth of the van der Waals potential is significantly lower for a pair of particles located in the first adsorbed layer. In... 

Complete resolution was not achieved due to the carryover of interfering substances which frequently occurs when separating the components of biological samples. The column carried a reverse phase, but as the mobile phase contained low concentrations of lauryl sulfate, some would have adsorbed on the surface of the stationary phase and significantly modified its interacting properties. The retention mechanism is likely to have involved both ionic interactions with the adsorbed ion exchanger together with dispersive interactions with any exposed areas of the reverse phase. 

Molecular modeling showed that WO3 surface has a structure where p-xylene may adsorb well, explaining the high activity of WO3 in p-xylene conversion. Experimental data in this study show that Sb addition modifies significantly the activity and selectivity properties of WO3 as well as the catalyst structure. Fig. 3 shows the minimum and maximum p-xylene... 

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