Classification adsorption isotherms

Figure 10.5. Adsorption isotherm classifications covering greater concentrations of adsorbate.

Fig. I.l The five types of adsorption isotherm, I to V, in the classification of Brunauer, Deming, Deming and Teller (BDDT), together with Type VI, the stepped isotherm.

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

C. H. Giles, T. H. MacEwan, S. N. Natchwa and D. Smith, Studies in Adsorption, Part XI A System of Classification of Solution Adsorption Isotherms and its Use in Diagnosis of Adsorption Mechanism and its Measurement of Specific Surface Area of Solids, J. Chem. Soc., p. 3973,1960. 

The adsorption capacities of the adsorbents are usually determined from modeling of the adsorption isotherms according to the Giles s classification [36] (Figure 15.1). 

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

Given the complexity of the pore structure in high-surface-area catalysts, six types of adsorption isotherms have been identified according to a classification advanced by IUPAC 145-481. Out of these six, only four are usually found in catalysis ... 

Figure 4.30 Adsorption isotherms according to IUPAC classification.

Figure 9.13 Basic types of adsorption isotherms by IUPAC classification [53],...

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. 

According to another system of classification of solution adsorption isotherms by Giles et al. [18] the main classes are S, L, H and C isotherms. These workers indicated that L curves occur in probably the majority of cases of adsorption from... 

Microporous substances with pore widths smaller than twenty Angstroms exhibit type 1 isotherms (lUPAC classification) in the absence ofmesopores, as illustrated in Figure 13.1 [4]. The steep uptake of N2 at very low relative pressures is due to the capillary condensation in micropores. Following the filling of micropores, adsorption isotherms become nearly fiat because further N2 uptake can now only occur at the external surface area, which is typically much lower relative to the total surface area. 

Based on their molecular properties as well as the properties of the solvent, each inorganic or organic contaminant exhibits an adsorption isotherm that corresponds to one of the isotherm classifications just described. Figure 5.1 illustrates these isotherms for different organic contaminants, adsorbed either from water or hexane solution on kaolinite, attapulgite, montmorillonite, and a red Mediterranean soil (Yaron et al. 1996). These isotherms may be used to deduce the adsorption mechanism. 

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

Many adsorption isotherms are borderline cases between two or more of the above types. In addition, there are some isotherms which do not fit into Brunauer s classification at all, the most notable being the stepwise isotherms, an example of which is given in Figure 5.6. Stepwise isotherms are usually associated with adsorption on to uniform solid surfaces, each step corresponding to the formation of a complete monomolecular adsorbed layer (see page 133). 

Giles, C. H, Smith, D., and Huitson, A. (1974). A general treatment and classification of the solute adsorption isotherms. I. Theoretical. J. Coll. Interface Sci. 47,755-765. 

The first systematic attempt to interpret adsorption isotherms for gas/solid equilibria was by Braunauer, Deming, Deming, and Teller (BDDT) in 1940.78 They classified the adsorption isotherms into five types, and the BDDT classification became the core of the modem International Union of Pure and Applied Chemistry... 

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

Adsorption isotherms are plots of the amount of gas adsorbed at equilibrium as a function of the partial pressure p/p°, at constant temperature. The quantity of gas adsorbed is mainly expressed as the mass of gas (usually g) or the volume of gas reduced to STP (standard temperature and pressure). The majority of isotherms which result from physical adsorption may conveniently be grouped into five classes — the five types I to V included in the classification originally proposed by Brunauer, Deming, Deming and Teller — sometimes referred to simply as the Brunauer classification [2]. The essential features of these types are indicated in Fig. 12.1. 

Adsorption/separation processes are based on adsorption isotherms (thermodynamics) and intracrystalline diffusivity (kinetics). Figure 16.1 illustrates various shapes of adsorption isotherms depending on the VOC nature, trichloroethylene (TCE) and tetrachloroethylene (PCE), and of the zeolite, MFI with Si/Al > 500 and FAU (Si/Al > 100) (14). The isotherms of VOCs adsorbed on FAU present a more or less S-shape which corresponds to type V of the IUPAC classification. In contrast, the isotherms of VOCs on MFI are more of type I, with the additional particularity of a step at 4 molecules per u.c. for PCE adsorption. The... 

Experimental adsorption isotherms recorded in the literature, measured on a wide variety of gas-solid systems, have a wide variety of forms. Nevertheless, the majority of these isotherms which result from physical adsorption may conveniently be grouped into six classes in the IUPAC classification (cf. Figure 1.7). The first five types (I to V) of the classification were originally proposed by S. Brunauer, L.S. Deming, W.S. Denting and E. Teller as the BDDT classification (1940), sometimes referred to as the Brunauer classification (1945). 

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

Figure 5.7 Classification of typical adsorption isotherms (I-V) showing vapor adsorption as a function of relative pressure p/po see discussion in text. (Adapted with permission. See Ref. [102], 1940 American Chemical Society.)...

FIGURE 9.11 Classification of excess adsorption isotherms for binary solutions with o° solubility. After Kipling [39]. Reproduced by permission from Quantitative Review by Ripling Royal Society of Chemistry, Cambridge, UK, 1951. 

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