IR STUDY OF THE ADSORPTION OF UNSATURATED-HYDROCARBONS ON HIGHLY OUTGASSED SILICA - SPECTROSCOPIC AND THERMODYNAMIC RESULTS

Fourier transform IR data are reported concerning the adsorption of et hyne, propyne, 1-butyne, 3-hexyne, benzene, and cyclohexene on Aerosil outgassed at 1073 K. With the three lightest acetylenic molecules, an ideal Langmuir-type interaction occurs. With the three six-carbon ato m species, physisorption competes with the formation of H-bonded compl exes, which are best studied at low coverage. From the intensity of th e nu(OH) mode of the free hydroxyls as a function of pressure, the equ ilibrium constant for H-bond formation is calculated in different ways , according to the role of physisorption. Several correlations are dra wn for Delta nu((OH)) and Delta G(o), based on the different basicitie s of the C-C triple bond as modulated by the alkyl substituents, and o n the different C-C bond order in the other molecules: An estimate for Delta H-o is made from Delta nu((OH)) so that a fairly complete therm odynamic characterization of the H-bonding process is achieved. An exp lanation is proposed for the complex shape of the nu((OH)), band in th e case of 3-hexyne. The interplay between sheer H-bonding formation an d dispersive contributions to the interaction is also discussed.