judging the technical performance of materials like polymers Whether surfaces are characterized from experimental gasor fibers (4,5). In IGC, probe molecules of known properties phase adsorption coefficients of probe molecules with known propare injected into an isothermal gas chromatographic system erties (as is done in inverse gas chromatography (IGC)) or with the surface of interest as stationary phase. From the net whether adsorption coefficients are to be predicted from known retention volume, the adsorption coefficient (Henry's law properties of the adsorbate and the adsorbent (as is desirable in constant), defined as environmental chemistry) in both cases a correct quantitative description of the van der Waals and acid-base interactions of or-K ads Å adsorbed concentration per unit surface area/ ganic molecules at surfaces is necessary. The model used to date concentration in the gas phase, 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 can be determined. By adopting de Boer's definition of the interactions between two condensed phases. This only works if the adsorbed standard state (6, p. 112), the adsorption coefficontact area of the adsorbed molecule is treated as an adjusted cient K ads (m) can be related to the excess free energy of parameter which makes up for the occurring discrepancies. In this adsorption DG e ads by the following equation ( 7):
paper an improved equation for the van der Waals interactions will be suggested which works without adjusted parameters. For DG e ads Å 0RT ln(2.99 1 10 8 (m 01 ) K ads ).
[1]
describing acid-base interactions of dilute gases at surfaces Gutmann's donor and acceptor numbers have found wide use in the
In the case of a dilute gas interacting with a surface, as IGC literature. However, these parameters are related to the heats is a typical situation in IGC, one can neglect interactions of of the acid-base interactions rather than the free energies. Thus molecules in the gas phase (assumption of an ideal gas) as this approach has the disadvantage of putting the acid-base charwell as interactions between neighboring adsorbed moleacterization of surfaces in IGC on a different basis than the van der cules (adsorption in the zero coverage region). Conse-Waals characterization, and it also does not allow the prediction of quently, the excess free energy of adsorption DG e ads simply gas-phase adsorption coefficients. Here, a different approach will represents the interactions between the adsorbate and the be discussed that was originally introduced by van Oss and cosurface. DG e ads can be broken down into two principally workers for the acid-base interactions between condensed phases different kinds of interactions: van der Waals interactions
(1). This approach uses acceptor and donor parameters that are free energy related. Validation with experimental data from the and Lewis acid-base interactions (with hydrogen-bonds as literature shows a good performance of the equations introduced a subclass) (1, 8, 9). 2 in this paper for IGC purposes as well as for the prediction of gas-phase adsorption coefficients. ᭧ 1997 Academic Press
2 Note that the van der Waals interactions can be treated without differ-Key Words: inverse gas chromatography; acid-base interacentiating between the different contributions that arise from dispersive, tions; van der Waals interactions; adsorption; surface free energy. dipole-dipole, and dipole-induced dipole interactions (1). In fact, the term g d which had been introduced by Fowkes (8) in a much cited paper in 1963, and which was interpreted as the dispersive component of the surface free energy, was shown by Chaudhury and coworkers (1) to include all the interpretation that divides intermolecular interactions into a dispersive of solids and liquids (2, 3,4). These data are helpful for component on the one hand and into a specific component including acidbase and dipole interactions on the other is still wide spread.