Monolayer-multilayer path

The physisorption isotherm on a mesoporous or macroporous adsorbent follows the same monolayer-multilayer path as on the corresponding non-porous surface until the secondary process of capillary condensation occurs. In the case of a macro-porous solid, the deviation from the standard monolayer-multilayer isotherm does not take place until very high relative pressures are attained (with nitrogen adsorption at 77 K, this would be at p)p° > 0.99). 

Characteristic features of the Type IV isotherm are its hysteresis loop, which is associated with capillary condensation taking place in mesopores, and the limiting uptake over a range of high p/p°. The initial part of the Type IV isotherm is attributed to monolayer-multilayer adsorption since it follows the same path as the corresponding part of a Type II isotherm obtained with the given adsorptive on the same surface area of the adsorbent in a nonporous form. Type IV isotherms are given by many mesoporous industrial adsorbents. 

The standard data are determined experimentally on a series of carefully selected reference materials, which give unrestricted monolayer-multilayer adsorption. If the appropriate standard data are selected for the particular gas-solid system, it is therefore possible to identify the influence of porosity (micropore filling or capillary condensation) on the path of the isotherm. In this manner it is possible to check the validity of the BET-area and in favourable cases to evaluate the total surface area, the internal area and the external area ... 

For. a number of reasons, nitrogen (at 77 K) is generally considered to be the most suitable adsorptive for standard surface area determination and for this purpose it is usually assumed that the BET monolayer is close-packed (with the molecular area taken as 0.162 nm2). One advantage of nitrogen is that the path of its multilayer isotherm is not very sensitive to differences in adsorbent structure. A useful check on the validity of nm is that the value of C(BET) should be neither too low nor too high if C(BET) < 50, Point B is not sufficiently sharp if C(BET) > 200, there is either a significant micropore filling contribution or localized adsorption on specific sites. 

For the moment no transitional states between these extremes will be considered. For these two limiting modes of adsorption the establishment of equilibrium proceeds via different paths in localized adsorption, adsorbed molecules can move to other sites only by desorption and re-attachment, whereas for mobile adsorption the path of tangential transport is also open. Typical representatives of localized isotherms are those of Langmuir. Frumkln-Fowler-Guggen-helm (monolayer) and BET (multilayer). For mobile adsorbates examples are the Volmer and the Hill-De Boer (or two-dimensional Van der Waals) equations. 

Both neutral and charge-transfer photochemistry is involved in the photodissociation at 248 nm of methyl iodide as a monolayer (1 ML) or up to 10 ML on a silverfl 11) surface. Such reactivity involves the fission of the C—I bond and the formation of methyl radicals and iodide adsorbed at the surface12,13a. Dissociation also occurs during X-ray photoelectron spectroscopy136. Multilayers of methyl iodide on silverfl 11) surfaces undergo C—I bond fission on irradiation at 248 nm. Several products such as ethane, ethyl iodide and iodoform are formed but the principal reaction path, which is somewhat time-dependent, yields methane and methylene di-iodide14. 

Path (a) corresponds to the nucleation and completion of a full monolayer of oxide before growth of a multilayer film of oxide to reach a steady state passive film. Path (b) corresponds to the growth of a passive layer of hydroxide with subsequent nucleation and growth of a layer of oxide in the inner part. 

The Type IVa isotherm is given by a well-defined mesoporous solid having a fairly narrow distribution of pore size. In the monolayer and initial multilayer region (i.e. up to p/p 0.4) this isotherm follows the same path as the corresponding Type Ila isotherm determined on the equivalent area of a non-porous surface of similar structure. In this case capillary condensation is responsible for the upward deviation of the isotherm in the multilayer region. Such isotherms are obtained only with those mesoporous solids having uniform pore structures. 

The experiment of immersion is sometimes performed under completely different conditions. The solid may be first put in equilibrium with the vapour of the immersional liquid at a given equilibrium pressure p (Paths 2 or 3 in Fig. 6.5). The adsorbed gas may be at submonolayer, monolayer or multilayer coverage, depending chiefiy on the value of p, but also on the nature of liquid and solid. When the solid is subsequently immersed in the liquid, the measured enthalpy change, called the enthalpy of immersional wetting, AwH, will be different from AimmH. The various stages of the immersion-adsorption-wetting cycle are shown in Fig. 6.5. 

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