On the use of the Dubinin-Radushkevich equation to distinguish between physical and chemical adsorption at the solid-water interface

Abstract

Defining the binding mode of ions and molecules at the solid-water interface is a key step to predict their reactivity. The Dubinin-Radushkevich (D-R) equation has been used in thousands of investigations to define if there is chemical or physical adsorption on solid surfaces from water. This paper evaluates the validity of the D-R equation to achieve this purpose. It is acknowledged that, either chemical or physical, the adsorption from solution is always an exchange of molecules or ions and thus enthalpy and free energy changes can take very small or nearly zero values. Therefore, characteristic energy values obtained from the D-R equation cannot be used as indicative of the adsorption mode. It is shown for the first time that the application of the equation always leads to “chemical” binding when molar units are used for the solute concentration in solution, and to “physical” binding when mg/L units are used.