Specific adsorptive powers

Multilayers of sterols,3 chlorophyll,4 and proteins5 have also been built. Those of sterols show specific adsorptive powers for digitonin, according to the stereochemical configuration of the hydroxyl group those of chlorophyll showed no fluorescence, although the quantity deposited was sufficient to give marked fluorescence in solution. 

SPECIFIC ADSORPTIVE POWER OF DIFFERENT ACTIVATED CARBONS IN AQUEOUS SOLUTION ... 

At one time the process of oxidation seemed to be nothing but a preferential oxidation of hydrocarbons that had been deposited on the surface during the carbonization stage. According to this theory, the removal of the hydrocarbons was supposed to leave the carbon surface free to attract and adsorb other substances. There can be little doubt that some such process does play a part, perhaps an important part, and explains much of the general improvement in adsorptive power however, it does not explain differences in specific adsorptive powers. Why, for example, does oxidation with steam at 800° C provide specific adsorptive powers unlike those produced by activation with air at 400° C Or, to use another illustration, why should the type of adsorptive power produced by activation with steam at 800° C depend upon the previous treatment of the carbon, e.g., on whether the carbonization was conducted in the presence or absence of certain salts 71... 

RELATION OF SURFACE AREA TO SPECIFIC ADSORPTIVE POWER... 

Such effects suggest an influence arising from the chemical properties of the catalyst. Specific catalytic effects depend on properties of the surface atoms, and these properties, in turn, are a function of the chemical nature of the catalytic substance. This is shown by the fact that the same type of active surface is not developed on copper as on platinum, and both differ from the active surface that can be formed on carbon. Properties of surface atoms that are important in catalysis appear to be linked to specific adsorptive powers, thus hydrogenating and dehydrogenating catalysts adsorb hydrogen, whereas oxidizing catalysts adsorb oxygen. 

The temperature at which certain other adsorptive powers develop often corresponds to a temperature favorable to the formation of one of the surface oxides. This, however, does not establish a direct connection between a specific adsorptive power and the surface oxide. Let us consider the adsorption of Malachite Green. For this, certain temperatures of activation are favorable but only when the carbon has been given suitable previous treatment. Moreover, adsorptive power for Malachite Green when once developed is not... 

Under the conditions of gelling and drying which were employed, the polymerization of the silica apparently leads to the formation of a silica gel surface with a configuration which is molded around the dye molecule originally present, so that the surface thereafter exhibits specific adsorptive power for a molecule of that type. This implies that the particles making up this type of gel structure have to be of molecular dimensions. 

To a large extent, these disadvantages are offset by the possibility of obtaining additional information that is difficult to obtain for mercury, such as electroklnetic potentials, colloid stability data and directly measured adsorptions. Even in cases where the evaluation and interpretation of i -potentlals from electrokinetics are under discussion, the technique is powerful because it informs us about the sign of the diffuse part of the double layer and hence helps to detect (super-)equivalency of specific adsorption. Quantitative information on specific adsorption can also be obtained by comparing p.z.c. s and l.e.p. s. Because of all of this, over the past decades systematic studies on a number of well-characterized disperse systems have led to a number of qualitatively new features, whereas the progress in the domain of mercury double layers has been rather quantitative. 

In addition it was found that molecules which only differ in one methyl group can be distinguished [48] and that there seemed to be a lower limit for a suitable template size in these systems since, e.g., dimethylaniline and diethylani-line exhibited the same adsorption power [37]. The most recent trend in imprinted silicas uses aluminium ions as dopants to introduce more specific interactions with the template. The silica gel is only surface-modified in the presence of e.g. phenanthrene by treatment with diazomethane. After removal of the template, the silica shows a stronger retention for all polycyclic aromatic compounds, but no particular affinity towards the template [52]. 

Pine charcoal. A fine easily scattered black powder, which has an apparent specific gravity of about 0.37 g/cc. The adsorption power of dye in water is quite small which may show that it is not so porous. 

Paulownia charcoal. A light powder, which is easily scattered and has an apparent specific gravity of 0.12 g/cc. This value is the lowest for all the kinds of charcoal at present available. The dye adsorption power in water lies between pine charcoal, and hemp coal which has the largest. One analysis showed that it contained. 9% moisture and 2.5% ash. The pH value of the wash water(5 grams of charcoal / 30cc of water) was 7 5 A mixture of the paulownia charcoal, sulphur and potassium nitrate burns as well as the pine charcoal, but it produces less fire dust than the pine charcoal. 

A black fine powder which has a somewhat hygroscopic feeling. The apparent specific gravity is 0.22g/cc. The dye adsorption power is the ... 

A black fine powder which has a somewhat hygroscopic feeling. The apparent specific gravity is 0.22g/cc. The dye adsorption power is the largest of all the kinds of plant coal. A mixture of hemp coal, sulphur and potassium nitrate burns to produce a violet flame and less fire dust than pine charcoal. It is used to obtain a large force of explosion as a component of black powder or in combination with potassium chlorate or perchlorate for the bursting charge of chrysanthemum shells. 

In asking suppliers for samples of carbon, it is well to specify requirements on any properties such as those listed in Table 7 3. In so doing, one should avoid imposing specifications that are needlessly restrictive. If a moderate amount of inorganic substances can be tolerated, it would be unwise to insist on a low inorganic content which would add to the cost and possibly exclude grades that furnish needed adsorptive powers. 

After making a decision on the type of carbon to be purchased, the buyer must reach an understanding with the supplier on measures to ensure that the quality of carbon in all deliveries will equal that of the trial lot. Specifications can readily be framed for physical and chemical properties such as ash, density, mesh size. Arrangements for measurement of adsorptive powers will be governed by traits and characteristics of each individual system. 

The question arises Is it possible to anticipate and be prepared for such situations Obviously we cannot know in advance the identity of the processes and products that will appear in the future, nor of the specific properties they will require of carbon. But we do know that many forms of usefulness have their roots in adsorptive powers that can be detected and measured by appropriate synthetic tests. Consequently, experiences such as that with streptomycin suggest a need to invent new tests that will measure properties not revealed by existing tests. Such new tests could unfold previously unrecognized properties of activated carbon. 

We are far from having a clear picture of just what occurs during activation aside from the fact that the surface area is increased and a very porous structure is developed. This lack of knowledge is not surprising in view of the various methods by which an activated carbon can be produced. One is often perplexed to find a step described as necessary in one process and omitted from another. In some cases, this is because the value of a particular stage may depend upon the previous history as well as on the treatment that follows. Then, too, the value of certain steps is the fact that they confer certain specific powers that may be needed in one application and not in another. It is to be remembered that each method of activation leaves a characteristic imprint on the adsorptive powers.71 Therefore, it is not uncommon to combine several different processes to provide a diversity of properties. 

Although the mechanisms that have been mentioned indicate how noncarbon ingredients could enhance adsorptive power in general, they fail to explain specific effects. Why, for example, should a carbon prepared with calcium chloride be more adsorptive for caramel, whereas a carbon prepared with zinc chloride is more effective for iodine 71 A convenient interpretation is that each... 

It is at least possible that many of these strongly held foreign elements provide adsorptive bonds. Support for this view is found in the fact that specific adsorptive and catalytic powers have been traced to the presence of noncarbon atoms—oxygen, iron, and nitrogen. The influence of noncarbon atoms may extend to adjacent carbon atoms in a manner analogous to the way in which a polar group introduced into an organic compound affects the chemical properties of remote atoms. 

One alternative hypothesis proposed to explain the data in Table 9 2 is Phenol-adsorptive spaces are created on the final portions of new surface but the gain is offset by a simultaneous mutation of phenol spaces created initially. This hypothesis would regard the erosion of a carbon during activation as an etching process in which simple patterns are etched initially on the surface, but as the activation continues, these initial patterns are modulated into other patterns to provide progressive changes in adsorptive power. That specific affinities can reside in separate areas is indicated in other studies, one of which is the way certain adsorptive powers can... 

Many reactions are catalyzed by activated carbon.36,37 The catalytic power of an activated carbon is often quite specific. The specific properties depend on the method of manufacture. Two carbons, each made by a different process, may be similar in adsorptive properties and yet be dissimilar in specific catalytic power. One of them may be more effective for certain reactions whereas the other carbon may prove more appropriate for other reactions. 

Considerable specificity is shown in the adsorption characteristics of colloids from different sources. Thus carbon R removed more colloids from a cane sugar syrup, whereas, carbon T was more effective for a beet sugar syrup. The adsorptive power of a carbon for colloids is usually increased by pulverizing it to a small particle size. 

It is to be emphasized that the data given for Carbon W in Table 14 5 and the deductions as to suitable control tests apply only to that particular grade of carbon. Another grade, particularly one made by a different process, would furnish other assortments of adsorptive power. A corollary is that specifications for the purchase of a certain grade of carbon should not be used automatically for the purchase of a different grade. Furthermore, if and when a switch to another carbon is being considered the decision should rest on a comparative performance with the solution to be purified. Carbons must never be substituted for one another on the basis of synthetic test solutions in an industrial application. 

---