Surface Energy Changes of Heat Treated TEOS Derived Silica Xerogels

Abstract

The surface energy of monolithic silica xerogels was examined by measuring the interaction of organic probes with xerogels heated at temperatures close to the gel-to-glass transition temperature. Values of the dispersive component of the surface energy, γ\(_S^D \), between 60 and 80 mJ ⋅ m-2 have been observed using n-alkanes for silica xerogels heated at 700, 800 and 900°C. At 1000°C, γ\(_S^D \) decreases to 8.37 mJ · m-2. Also the differential heat of adsorption, variation of standard free energy and entropy of adsorption decrease when the silica xerogel is heated at 1000°C, showing a lower interaction potential of the organic probes with the silica surface. For the silica xerogels heated between 700 and 900°C, the acid character varies in accordance with the variation of the chemical nature of the silica. Upon heating at 1000°C, both acid and base characters are very close in accordance with a neutral surface. Within the experimental conditions used in this work, the surface of the obtained monolithic silica xerogels behaves as a glass surface when the treating temperature is 1000°C.