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 poly morphs of silica (alpha-quartz and alpha-cristobalite) has been studie d by measuring the heat of reversible adsorption of water,ammonia, met hanol and tert-butyl alcohol as a function of coverage and through the comparison with ab initio results on cluster models. The adsorption o f tert-butyl alcohol is rather insensitive to both crystallinity and d egree of dehydration, being largely due to nonspecific dispersive inte ractions. Water, ammonia, and methanol reveal structural heterogeneity , though to a different extent; a peculiar induced heterogeneity trans mitted through H-bonding of interacting silanols is evidenced by the a dsorption of ammonia. The most dehydrated sample studied is the high-t emperature treated Aerosil: the heat of adsorption of water reveals so me heterogeneity because of the different role of isolated and geminal silanols; ammonia shows heterogeneity also due to the species origina ted by its dissociation on strained bridges, during its previous conta ct with the surface. On mildly dehydrated surfaces, patches of silanol s 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 throu gh 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 interac ting silanols fully confirm such a picture.