Classification of adsorption isotherms

Type II isotherms are typical of physical adsorption on nonporous solids. In contrast to type I, the adsorbate molecules in these cases also have relatively strong mutual interactions, which leads to the tendency for multilayer formation. The initial rapidly rising part of the isotherm corresponds to the equivalent type I adsorption. Point B on the curve is identified with complete mono-layer coverage. Multilayer formation then begins which may lead to surface condensation. Type II isotherms are sometimes encountered for microporous solids, in which case point B would correspond to both completion of mono-layer coverage and filling of the micropores by capillary condensation. The rest of the curve would then correspond to normal multilayer formation. 

Types III and V isotherms are relatively rare and correspond to systems in which the interaction between adsorbate molecules is stronger than that between adsorbate and adsorbent. In these cases, the uptake of gas molecules is initially slow until surface coverage is sufficient so that interactions between adsorbed and free molecules begins to dominate the process. One might say that the processes are autocatalytic in terms of the adsorption process. 

FIGURE 9.7. Adsorption isotherms are generally divided into five main types depending on the degree of adsorption (monolayer or multilayer), the mechanism of adsorption (physical or chemisorption), the nature of the adsorbent surface (porous or nonporous), and the relative strengths of adsorbate-adsorbent interactions. 

Type IV isotherms are obviously similar to type II and usually correspond to systems involving capillary condensation in porous solids. In this case, however, once the pores have become filled, further adsorption to form multilayers does not occur and the terminating plateau region results. This would indicate a relatively weak interaction between the adsorbate molecules. While more complex isotherm classifications are available, they generally represent combinations and extensions of the five basic types described above. 

The graphic representation between amount adsorbed x and the pressure p at any constant temperatures is referred to as an adsorption isotherm. When the adsorbate 

Relative Pressure Range Obtained from the Linear Regions of the BET Plots Using the Classical and Alternative Equations 

BET Parameters Obtained from the Classical and Alternative Equations 

FIGURE 2.11 Relation between the degree of bum-off and the C parameter calculated from the traditional and alternative equations. (Source Parra, J.B., de Sousa, J.C., Bansal, R.C., Pis, J.J., and Pajares, J.J., Adsorption Sci. and TechnoL, 12, 51, 1995. With permission.) 

Depending on the physicochemical conditions, a great variety of adsorption isotherms are experimentally observed. Eight common examples are shown in Fig. 9.2 [7,365], 

The most simple type, A, is that of a linear increase. It is described by the Henry adsorption isotherm equation  

Type B is very common. It is concave with respect to the abscissa. Most surfaces are heterogeneous. There are adsorption sites, which have a high affinity, and regions, which have a low affinity. The high affinity sites are occupied first, which accounts for the steep increase at low pressure. Another reason is sometimes a lateral repulsion between adsorbed molecules. This type of adsorption isotherm is described by the Freundlich1 adsorption isotherm equation [366]  

Type C is called the Langmuir2 type because it can be described by the Langmuir adsorption isotherm equation  

6 is the relative coverage and KL is a constant, called the Langmuir constant . Tmon is the maximum amount adsorbed which, in the case of Langmuir adsorption, is a mono-layer. Type C adsorption isotherms are characterized by a saturation at high concentrations. A possible reason is that the surface is completely filled with adsorbed molecules. Langmuir adsorption is often observed for the adsorption from solution but only rarely for the adsorption of gases. This type of adsorption isotherm can also be observed for porous materials. Once all pores have been filled the isotherm saturates (see Section 9.4.3). 

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Brucine, 2 74 Brucite, 15 321, 323, 399 reserves of, 15 322 Brugnatelli, Luigi, 9 760 Brunauer classification of adsorption isotherms, 1 591... 

Figure 5.5 Brunauer s classification of adsorption isotherms (pn saturated vapour pressure)...

Figure 4.13. Classification of adsorption isotherms (adapted from Sposito, 1948a).

Interpretation and classification of adsorption isotherms 43 Type / isotherms 440... 

Figure 2.24. Phenomenological classification of adsorption isotherms from dilute solution. The units are arbitrary.

By analogy with the characterization methods based on gas adsorption and on the shape of the isotherms, a classification of adsorption isotherms from liquid solution can be thought to be useful. The difficulties in establishing such a classification were underlined [9].For dilute solutions Giles and Smith [48] proposed indeed 18 classes, Lyklema [15] simplified this down to 6, but we suggest retaining only 2 of them. Indeed, the shape of an adsorption isotherm from solution is the complex result of the balance between the solute—solute, solute—solvent, solute-surface, and surface—solvent interactions. Molecules do not only adsorb because they interact with the solid but also because the solvent may reject them. The surface is not itself a simple parameter because it is... 

Equilibrium Concentration of Solute in Solution Fig. 4.1. Classification of adsorption isotherm (after Giles et al., 1960). 

Figure Bl.26.3. The lUPAC classification of adsorption isotherms for gas-solid equilibria (Sing K S W, Everett D H, Haul R A W, Mosoul L, Pierotti R A, Rouguerol J and Siemieniewska T 1985 Pure. Appl. Chem. 57 603-19).

Even limiting the goal to a purely numerical exercise, the study of the behaviours of eq. (51) is manifestly a difficult task, because of the numerous parameters it contains. Such an analysis, carried out in a preliminary form in ref. [74], shows however that eq. (51) is able to account for the Brunauer classification of adsorption isotherms. 

Figure 5. How the C M equation accounts for Brunauer classification of adsorption isotherms.

Figure 4.8. The lUPAC classification of adsorption isotherm shapes (Sing etal., 1985)...

Adsorption measurements should preferably be supplemented by microcalorimetry, such as immersion and flow-microcalorimetry (see Figure 4.8. The lUPAC classification of adsorption isotherm shapes see also Figure 8.4). These techniques give additional information about the nature of surfaces of the adsorbent and the mode or mechanism of adsorption. 

At this point, we will distinguish the techniques mentioned thus far, from those used for molecularly thin films on solids. Isotherms in this regime generally fall under the classification of adsorption isotherms and the measurement techniques are quite different from those described here. For a review of these techniques, we refer the interested reader to the text by Adamson (8). 

There is a more fundamental classification of adsorption isotherms according to Bmnauer, which divides the adsorption isotherms into five types. Type 1 is the monolayer chemical Langmuir-type adsorption and type II is the multilayer physical BET adsorption isotherm. Type III is for - rather rarely occurring - weak adsorptions and types IV and V are for capillary condensation. There are, however, some adsorption isotherms that do not fit into Bmnauer s classification. 

There are six representative adsorption isotherms that reflect the relationship between porous structure and sorption type. This lUPAC classification of adsorption isotherms is shown in Fig. 3 [41]. These adsorption isotherms are characteristic of adsorbents that are microporous (type 1), nonporous and macroporous (types 11, 111, and VI), and mesoporous (types IV and V). The differences between types II and in and between types fV and V arise from the relative strength of fluid-solid and fluid-fluid attractive interactions. When the fluid-solid attractive interaction is stronger than that of fluid-fluid, the adsorption isotherm will be of types II and IV, and the opposite simation leads to types III and V. The type VI isotherm represents adsorption on nonporous or macroporous solid surfaces where stepwise multiplayer adsorption occurs. 

Fig. 3 lUPAC classification of adsorption isotherms. Copyright Wiley... 

Figure 7 Classification of adsorption isotherms according to Giles et...

Early classifications of adsorption isotherms at solid-vapor interfaces were foimd to lit one of five basic shapes (Figure 10.4) at temperatures below the critical temperature (rj of the adsorbate. Type I isotherms were originally interpreted to... 

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