Adsorption phenomena on glass surfaces: IV. Surface acidity of silica and glasses

Abstract

Batch and flow microcalorimetry were used to study adsorption of aliphatic amines on silica gel, controlled pore glass (CPG), high alumina glass, and surface-derivatized controlled pore glass. The adsorption of basic aliphatic amines is indicative of the acidity of the surface. Silica gel and controlled pore glass show the same average heat of adsorption for butylamine, 35 ± 2 kJ mole −1, but the CPG, in contrast to the silica gel has nearly homoenergetic surface. High alumina glass (17% Al 2O 3) has a less acidic surface, and the relevant value for the heat of adsorption of butylamine is only 4.7 kJ mole −1. Surface derivatization with a long-chain aliphatic silane (octadecylsilane) produces a highly hydrophobic surface, when water molecules are used as probes. Interaction with butylamine remains strong, as in the original state of CPG. Derivatization with aminopropylsilane yields a basic surface and the interaction with butylamine is weak, showing a heat of adsorption of only 3–4 kJ mole −1. The paper indicates that surface acidity of silica and glasses can be understood only as an interactive process between the surface and the adsorbate.