STRUCTURAL AND INDUCED HETEROGENEITY AT THE SURFACE OF SOME SIO2 POLYMORPHS FROM THE ENTHALPY OF ADSORPTION OF VARIOUS MOLECULES

Surface heterogeneity of both amorphous (Aerosil) and crystalline polymorphs of silica (alpha-quartz and alpha-cristobalite) has been studied by measuring the heat of reversible adsorption of water,ammonia, methanol and tert-butyl alcohol as a function of coverage and through the comparison with ab initio results on cluster models. The adsorption of tert-butyl alcohol is rather insensitive to both crystallinity and degree of dehydration, being largely due to nonspecific dispersive interactions. Water, ammonia, and methanol reveal structural heterogeneity, though to a different extent; a peculiar induced heterogeneity transmitted through H-bonding of interacting silanols is evidenced by the adsorption of ammonia. The most dehydrated sample studied is the high-temperature treated Aerosil: the heat of adsorption of water reveals some heterogeneity because of the different role of isolated and geminal silanols; ammonia shows heterogeneity also due to the species originated by its dissociation on strained bridges, during its previous contact with the surface. On mildly dehydrated surfaces, patches of silanols are present: heat of adsorption of water distinguishes between such hydrophilic patches and hydrophobic ones. Extensively hydrated samples exhibit large patches of silanols, interacting with one another through H-bonds: adsorption of ammonia reveals that the H-bonding strength of the terminal hydroxyl increases with the size of the patch. Rupture of H-bonds among adjacent SiOH groups occurs with increasing ammonia coverage, revealed by a linear decrease of the heat of adsorption over a very wide range of coverage: accordingly, the experimental isotherm follows the Temkin model. Ab initio calculations on chains of interacting silanols fully confirm such a picture.