The Surface Energies of Amorphous Silica and Hydrous Amorphous Silica

The total surface energies (or, more strictly, surface enthalpies) of amorphous silica and hydrous amorphous silica were determined by measuring the heats of solution in a mixture of nitric acid and hydrofluoric acid of samples having differing specific surface areas and bound water contents, and by measuring the surface areas by the B.E.T. method, using nitrogen as adsorbate. The molecular area of nitrogen was taken to be 16.2 Å 2 at 77.3 °K. The surface energy of amorphous silica of zero water content (or the energy of the pure siloxane surface) at 23 °C. was found to be 259 ± 3 ergs/cm. 2 The heat of hydration by liquid water of the siloxane surface to silanol surface at 23 °C. was found to be 258.6 ± 13.0 cal./gm. of water. From these two values, with the added assumption that the molecular area of bound water was 25 Å 2 , the surface energy of hydrous amorphous silica with a completely hydrated surface (or the energy of pure silanol surface) at 23 °C. was calculated to be 129 ± 8 ergs/cm. 2 This value is only slightly greater than the surface energy of liquid water. Surface area determinations were also made by water vapor adsorption at 25 °C. The packing of physically adsorbed water appeared to be determined by the geometry of the surface. The cross-sectional area of the adsorbed water molecule was found to be 12.5 Å 2 . The density of amorphous anhydrous silica was 2.28 to 2.29 gm./cc. Silica particles having an average dimension of 37 Å were dehydrated at lower temperatures and sintered at lower temperatures than particles having an average dimension of 64 Å.