New Conceptualization and Solution Approach for the Ideal Adsorbed Solution Theory (IAST)

Despite the widespread use of the ideal adsorbed solution theory (IAST) to predict competitive adsorption of hydrophobic organic compounds from aqueous solutions, the underlying principles of the model are not widely understood. A new conceptualization is presented that treats the adsorbed phase as gas-like with a total surface pressure equal to the sum of the partial surface pressures of the adsorbed species. In this conceptualization, the key assumption of the IAST is reformulated as indicating that the partial surface pressure exerted by an adsorbate depends only on its adsorption density and a surface activity coefficient that depends on the total surface pressure. The parallels of the model parameters and assumptions with those in gas/liquid equilibrium facilitate a more intuitive understanding of the model concepts. This understanding, in turn, clarifies the reasons underlying the predictions of the IAST for limiting conditions where one adsorbate dominates the surface phase. Finally, a new solution algorithm is introduced that allows the model equations to be solved using a simple spreadsheet analysis in which only two, independent parameters need be evaluated regardless of the number of adsorbates in the system or the characteristics of the single-solute isotherms.