Classification of Composite Isotherms

FIGURE 3.8 Composite and individual isotherms for binary systems (solid line, composite dashed line, individual for components). (After Ostwald, W. and de Izaguirre, R., Kolloid Z., 30, 279, 1922. With permission.) 

FIGURE 3.9 Types of composite isotherm (a) U-shaped, (b) S-shaped, (c) hnear. (After Kipling, J.J., in Adsorption from Solutions of Non-Electrolytes, Acad. Press, London, 1965. With permission.) 

A two-branch S-shaped isotherm in which the two branches usually have different sizes 

U-shaped or S-shaped isotherms are more common. Although U-shaped isotherms correspond to Type (a) of the Ostwald-de Izaguirre, S-shaped isotherms may replace almost all of the remaining types. Linear isotherm (Type c) is generally obtained for adsorption by molecular sieves when only one component of the solution can enter the pores of the adsorbent while the entry of the second component is inhibited. 

FIGURE 3.10 Classification of composite isotherms. (After Nagy, L.G and Schay, G., Mag. Kem. Foly, 66, 31, 1960. With permission.) 

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The adsorption from binary solutions on sohd adsorbents in general and on activated carbons in particular is discussed in Chapter 3. The nature and types of adsorption and adsorption isotherms from dilute solutions and from completely miscible binary solutions are described. The composite isotherm equation is derived. The shapes and classification of composite isotherms and the influence of adsorbate-adsorbent interactions, the heterogeneity of the carbon surface, and the size and orientation of the adsorbed molecules on the shapes are examined. The thickness of the adsorbed layer and the determination of individual adsorption isotherms from a composite isotherm are also described. 

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