Surface heterogeneity on hydrophilic and hydrophobic silicas: Water and alcohols as probes for H-bonding and dispersion forces

The hydrophilic/hydrophobic behavior of two silica polymorphe (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 SiO₂ 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.