Potential Theory of Adsorption from Dilute Solutions

1 Potential Theory of Adsorption from Dilute Solutions 

The potential theory introduced by Polanyi for adsorption of gases and vapors has been discussed in detail elsewhere in this book. For a single gas or vapor, the adsorption potential s has been defined as equal to the work done in bringing one mole of the adsorbate from infinity to a specified distance from the surface. Polanyi modified the theory for adsorption from solutions of sparingly soluble solutes. The sigifificance of this approach is that it is not essential to postulate a thickness for the adsorbed layer. This modified equation can be written as 

Urano et al. combined the generalizations of the Polanyi potential theory and the Freundlich isotherm equation to derive a relationship that these workers claimed could successfully predict the adsorption isotherms for several organic compounds from aqueous solutions on active carbons. Their adsorption equation is represented as 

FIGURE 3.7 Comparison of theoretical and experimental adsorption isotherms of benzene derivatives. = Phenol A = Benzaldehyde = Benzonitrile = m. toluic acid, x = m.amino benzoic acid. (After Urano, K., Kochi, Y., and Yamamoto, Y, J. Colloid Interface Set, 86, 43, 1982. With permission.) 

When two components of a solution are miscible with each other in all proportions, then there is no distinction between the solute and the solvent so that neither of the two components can be regarded as a solvent over the whole range of concentrations. In such a system the change in concentration of the solution as a result of adsorption will be the resultant of the adsorption of both the components of the solution. It has been mentioned earher in this chapter that what is actually measured experimentally is the change in concentration of the solution so that the adsorption isotherm is actually the isotherm of change in concentration. More recently, however, this isotherm of change in concentration has been named as the composite isotherm, indicating that it combines the results of true (or individual) isotherms for adsorption of the two components of the binary solution. 

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