Type V isotherms

Isotherms of Type 111 and Type V, which are the subject of Chapter 5, seem to be characteristic of systems where the adsorbent-adsorbate interaction is unusually weak, and are much less common than those of the other three types. Type III isotherms are indicative of a non-porous solid, and some halting steps have been taken towards their use for the estimation of specific surface but Type V isotherms, which betoken the presence of porosity, offer little if any scope at present for the evaluation of either surface area or pore size distribution. 

Type III and Type V Isotherms The Special Behaviour of Water... 

Both Type III and Type V isotherms are characterized by convexity towards the relative pressure axis, commencing at the origin. In Ty )e III isotherms the convexity persists throughout their course (Fig. 5.1(a), whereas in Type V isotherms there is a point of inflection at fairly high relative pressure, often 0-5 or even higher, so that the isotherm bends over and reaches a plateau DE in the multilayer region of the isotherm (cf. Fig. 5.1 (b)) sometimes there is a final upward sweep near saturation pressure (see DE in Fig. 5.1(b)) attributable to adsorption in coarse mesopores and macropores. 

Types III and V isotherms arc characteristic of weak gas-solid interactions, the Type III isotherm being given by a nonporous or macroporous solid and the Type V isotherm by a mesoporous or microporous solid. 

Type III end Type V isotherms speciel behaviour of weter... 

Type III (and Type V) isotherms may originate through the adsorption of either nonpolar or polar molecules, always provided that the adsorbent-adsorbate force is relatively weak. 

Type III and Type V isotherms special behaviour of water... 

One must conclude therefore that the BET procedure for evaluation of monolayer capacity is not applicable to a Type III (nor by implication, to a Type V) isotherm. 

Type V isotherms of water on carbon display a considerable variety of detail, as may be gathered from the representative examples collected in Fig. 5.14. Hysteresis is invariably present, but in some cases there are well defined loops (Fig. 5.14(b). (t ), (capillary-condensed water. Extreme low-pressure hysteresis, as in Fig. 5.14(c) is very probably due to penetration effects of the kind discussed in Chapter 4. 

The advantage of equation 17.14 is that it may be fitted to all known shapes of adsorption isotherm. In 1938, a classification of isotherms was proposed which consisted of the five shapes shown in Figure 17.5 which is taken from the work of Brunauer et alSu Only gas-solid systems provide examples of all the shapes, and not all occur frequently. It is not possible to predict the shape of an isotherm for a given system, although it has been observed that some shapes are often associated with a particular adsorbent or adsorbate properties. Charcoal, with pores just a few molecules in diameter, almost always gives a Type I isotherm. A non-porous solid is likely to give a Type II isotherm. If the cohesive forces between adsorbate molecules are greater than the adhesive forces between adsorbate and adsorbent, a Type V isotherm is likely to be obtained for a porous adsorbent and a Type III isotherm for a non-porous one. 

The regions of Type IV and Type V isotherms which are concave to the gas-concentration axis at high concentrations correspond to the bulk condensation of adsorbate in the pores of the adsorbent. An equation relating volume condensed to partial pressure of adsorbate and pore size, may be found by assuming transfer of adsorbate to occur in three stages ... 

The formation of silicon-flvxyride bonds on the surface of silica after treatment with hydrogen fluoride was never proven directly. However, there is a pronounced change in the adsorption and wetting properties. The silica becomes hydrophobic as was mentioned in a patent to Kimberlin (279a). Neimark and collaborators (279b) found a type V isotherm in the methanol adsorption on silica gel which had been treated with a solution of SiF in absolute alcohol. Wilska (280) obtained a water-repellent silica when solutions of HaSiPg were precipitated with ammonia. The Si—F bond is hydrolyzed only slowly. A considerable fluorine content of 7-10% F was reported in an older patent (281) for a silica that had been prepared by hydrolysis of SiF. ... 

Type V isotherms result from small adsorbate-adsorbent interaction potentials similar to the type III isotherms. However, type V isotherms are also associated with pores in the same range as those of the type IV isotherms. 

The types IV and type V isotherms resemble the type II and type III isotherms, respectively, at the bottom or low-pressure end. However, at the high-pressure end they turn toward the P/Pq = 1 line and as indicated... 

Type V isotherms are related to the type III isotherms and are also characteristic of weak adsorbate-adsorbent interactions (i.e., water vapor adsorption on charcoal). 

It is not correct, however, to regard activated carbon as hydrophobic. The equilibrium sorption of water vapor on an anthracite-derived activated carbon is compared with that of other sorbents in Fig. 2. The sorption of water vapor on activated carbon follows a Type V isotherm (according to... 

The Type V isotherm is uncommon it is related to the Type III isotherm in that the adsorbent-adsorbate interaction is weak, but is obtained with certain porous adsorbents. 

The Type V isotherm is initially convex to the p/p° axis and also levels off at high relative pressures. As in the case of the Type III isotherm, this is indicative of weak adsorbent-adsorbate interactions. A Type V isotherm exhibits a hysteresis loop which is associated with the mechanism of pore filling and emptying. Such isotherms are relatively rare. 

Truly reversible Type V isotherms are quite rare. It is significant that the example reported by Dubinin (1980) was obtained on a low bum-off (5.7%) carbon, which was certainly ultramicroporous. It was pointed out that with an activated carbon obtained by 20% bum-off, the hysteresis extended over virtually the whole range of pore filling - the water isothenn then having a very similar appearance to that of the Carbosieve isotherm in Figure 9.26. 

The appearance of the initial section of a Type V isotherm is very similar to that of a Type III isotherm for a similar gas-solid system (e.g. water/carbon). In this case, however, the sharp increase in adsorption at higher p/pa is dependent on the pore size. For example, the ulbamicropores in a molecular sieve carbon are filled with water at a much lower p(p° than are the wider pores in a supermicroporous carbon. 

Also, the variation in the C parameter along the isotherm serve to account for the different shapes of the isotherms. From a mathematical point of view, the C constant of the BET equation is intimately related to the shape of the isotherm. A detailed analysis of this can be found in the book by Grengg and Sing [2], according to which when the C constant is lower than 2 the BET equation affords a convex curve, with the shape of the Type III isotherm. However, when the C constant is above 2 the curve acquires the shape of the Type V isotherm. What is absolutely clear is that the most important consequence of C changing with the surface coverage is that this circumvents one of the most important criticisms that have been made about the BET model. Now, the adsorption heat in the first layer changes with the amount adsorbed, as happens in real systems. 

R. M. Barrer (Imperial College, London) Type V isotherms similar to those shown in your Figure 2 have also been found by us (Barrer and Kanellopoulos, /. Chem. Soc.) when equimolecular mixtures of NH3 and HCl are sorbed between 200° and 300°C in zeolites. The sorptions of NH3 alone and of HCl alone in the same temperature ranges are relatively small, but together they interact strongly and are copiously sorbed. Thus, interaction may be strong between unlike molecules and is responsible for the peculiar isotherm shape observed in mixed sorption. 

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