Isotope effect on physical adsorption

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Abstract
The gas-chromatographic technique is useful for the study of isotope effects on adsorption at zero coverage. Experimental data for several isotopic pairs, adsorbed on various surfaces at various temperatures, are discussed in terms of the theory of isotope effects in condensed systems. The variation of the logarithm of the gas-chromatographic isotopic separation factor is approximately described by a relationship of the type B/T–A/T2+C. The reverse isotope effect can be quantitatively explained by differences in adsorption potentials, produced by differences in polarizability, without invoking any decrease in the molecular stretching frequencies on adsorption. The B term, responsible for the cross-over phenomenon, is related to differences in polarizability and good agreement is found for isotopic pairs which definitely exhibit non-specific interactions with the adsorbing media. Deviations of up to 50 %, which have been found for isotopic pairs containing double bonds, are discussed briefly in terms of a slight specific interaction with the adsorbing media and differences in equilibrium distances with respect to the adsorbing surface.

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