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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

Reactive sites on α-cristobalite and α-silicon carbide outgassed over a wide range of temperature (423–1573 K) have been studied by adsorption microcalorimetry. By successive adsorption–desorption cycles the fraction of each adsorbate reversibly adsorbed at room temperature and the effect of preadsorption of one adsorptive on the adsorption of the other have been evaluated. Both molecules are dissociated on strained bridges and adsorbed via H-bonds on surface hydroxyls, but they do not react strictly with the same sites. H-bonding of H2O takes place on two silanols located at a defined distance, with an enthalpy in the range 50–100 kJ mol–1, and on isolated silanols, with an enthalpy lower than the latent enthalpy of liquefaction (44 kJ mol–1). The two processes discriminate hydrophilic and hydrophobic patches at the surface. Ammonia interacts with single silanols, either isolated or terminal in a cluster: in the latter case electronic effects due to adsorption involve all SiOHs in mutual interaction. Strained siloxane bridges dissociate both molecules with formation either of two new silanols or of one silanol and one silylammine group. This latter surface functionality interacts with H2O, although with a lower enthalpy than SiOH, but does not interact with NH3. Silicon carbide exhibits similar behaviour to silicas, when outgassed up to 773 K, but after heating at 1073 K develops a peculiar reactivity, whereby large amounts of both H2O and NH3 are dissociated, in contrast to silica whose reactivity is progressively depressed upon thermal treatments. The above data are discussed in the context of systematic surface characterization of covalent solids, aiming to relate the surface properties of the particles to their lung toxicity when inhaled.