HEATS OF INTERACTION OF STRONG BRONSTED ACIDS WITH SILICA

Heats of immersion (DELTA-H(imm)) are reported for a well-characterized commercial silica into acetonitrile solutions of a series of strong protonic acids at 25-degrees-C. Although the acids are of widely varying strength and might be expected to protonate the surface hydroxyl groups of the silica, or at least donate strong hydrogen bonds to them, there is only a modest variation in DELTA-H(imm). Significantly, all of the DELTA-H(imm) values are less exothermic than the heat of immersion of the silica into neat acetonitrile, a poor hydrogen-bond donor but a moderately good acceptor. These surprising results are interpreted to mean that the various acids, both carboxylic and sulfonic, are actually behaving as hydrogen-bond acceptors from the surface of the silica on which cooperative hydrogen bonding between siloxy groups enhances the hydrogen-bonding donor ability of the surface sites. This interpretation is supported by FTIR examination of the surface, and by demonstration that the heat of immersion of the silica in dichloroacetic acid decreases steadily as the density of hydrogen-bonded silanols is reduced by high-temperature activation.