Infra-red spectroscopic study of the adsorption of amines on the A-type and B-type hydroxyls of an aerosil silica gel

The interaction of the homogeneous series of primary, secondary and tertiary amines with the free hydroxyls (A-type) and hydrogen-bonded hydroxyls (B-type) of an Aerosil silica is investigated by i.-r.-spectroscopy. The shift Δν of the 3750 cm–1 band (A-type OH) toward lower frequency on adsorption of amines is correlated with the ionization potential I(eV), indicating that charge-transfer contributes strongly to the hydrogen bonding interaction. To investigate the possibility that steric effects of substituent groups in the adsorbate molecule influence the hydrogen-bonding interaction, Δν was also related to the Taft factors σ* and Es, which reflect the polar and steric effects of the substituent groups. A linear (Δν, σ*) relation is found with strong deviation for the tertiary amines. This deviation can be explained either by a steric effect between adsorbate molecule and surface groups, or by the different molecular symmetry of tertiary amines which gives this series a separate but parallel behaviour to the two other series. The extent of interaction of the amines with the B-type hydroxyls is greater for ammonia and the primary amines, than for the secondary and tertiary. This different behaviour is explained by the different packing of the adsorbing molecule. The higher the packing, the lower the proportion of species reacting.