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The hydrophilic/hydrophobic behavior of two silica polymorphs (a finely ground cristobalite and a pyrogenic amorphous Aerosil) was studied by means of adsorption microcalorimetry, as a function of nature and extension of the surface hydroxylated layer. Water and two alcohols (methanol and tert-butyl alcohol) were used as probes for polar and apolar sites, available for H-bonding and for dispersion forces interactions. It was found that in most cases small portions of the surface were irreversibly modified upon the earliest contact with the probe, owing to dissociative reactions occurring at 303 K. However, the main process involved during the interaction was a reversible molecular adsorption: on both hydrophilic and hydrophobic SiO 2 the process was by far more energetic for the two alcohols than for water, as indicated by the heat of adsorption values. The contribution of the dispersion forces to the overall energy of interaction on hydrophilic silica was 16 kJ/mol for methanol and 34 kJ/mol for tert-butyl alcohol. For hydrophobic silica the dispersive forces interaction was found to be a major component of the overall process, and the energetic described for hydrophilic silica was confirmed.

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 themore » size of the patch. Rupture of H-bonds among adjacent SiOH groups occurs with increasing ammonia coverage: accordingly, the experimental isotherm follows the Temkin model. Ab initio calculations on chains of interacting silanols fully confirm such a picture. 39 refs., 11 figs., 2 tabs.« less