Considerations about the Adsorption of Organic Molecules from the Gas Phase to Surfaces: Implications for Inverse Gas Chromatography and the Prediction of Adsorption Coefficients

Abstract

Whether surfaces are characterized from experimental gas-phase adsorption coefficients of probe molecules with known properties (as is done in inverse gas chromatography (IGC)) or whether adsorption coefficients are to be predicted from known properties of the adsorbate and the adsorbent (as is desirable in environmental chemistry) in both cases a correct quantitative description of the van der Waals and acid–base interactions of organic molecules at surfaces is necessary. The model used to date in IGC for the van der Waals interactions tacitly assumes that the interactions of a dilute gas with a surface can be treated like the interactions between two condensed phases. This only works if the contact area of the adsorbed molecule is treated as an adjusted parameter which makes up for the occurring discrepancies. In this paper an improved equation for the van der Waals interactions will be suggested which works without adjusted parameters. For describing acid–base interactions of dilute gases at surfaces Gutmann's donor and acceptor numbers have found wide use in the IGC literature. However, these parameters are related to the heats of the acid–base interactions rather than the free energies. Thus this approach has the disadvantage of putting the acid–base characterization of surfaces in IGC on a different basis than the van der Waals characterization, and it also does not allow the prediction of gas-phase adsorption coefficients. Here, a different approach will be discussed that was originally introduced by van Oss and coworkers for the acid–base interactions between condensed phases (1). This approach uses acceptor and donor parameters that are free energy related. Validation with experimental data from the literature shows a good performance of the equations introduced in this paper for IGC purposes as well as for the prediction of gas-phase adsorption coefficients.