The thermal properties and heats of adsorption of films on vitreous silica

Abstract

As a contribution to the few existing data of sufficient accuracy for the calculation of heats of adsorption and other thermal properties of films on adsorbents simpler than charcoal, a series of measurements has been made of the adsorption of the vapours of benzene, acetone, and methyl alcohol on vitreous silica at temperatures between 70 and 25° C. Previous experience with this adsorbent (Palmer and Clark 1935; Palmer 1937) showed that it was well suited for the purpose, particularly in respect to the very rapid attainment of equilibrium. Complete isothermals at 25° for these vapours up to saturation pressures have already been investigated and recorded (Palmer 1937). The use of vapours rather than gases not only admits of experimental simplicity, but has the further advantage that the free energy of adsorption (or adsorption potential) is immediately calculable from the relative pressure at equilibrium. It is hoped that the selection of substances is representative, in that they are respectively non-polar but polarizable, polar but non-associated, polar and associated. These molecular distinctions appear prominently in the properties of the adsorption films. Experimental The preparation of the silica adsorbent, manipulation, and methods of measurement were in principle the same as those previously described, with the following modifications. The volumenometric portion of the apparatus, including the manometer bulbs b 1 and b 2 (fig. 1; cf. Palmer 1937, fig. 1) was housed in an air thermostat T 1 regulated to 42·5°, and the horizontal adsorption tube A immersed in a water thermostat T 2 that could be set at temperatures ranging from 70 to 20°. Connecting tubes were kept at 43° by means of electrically heated wrapping wire. Since there was a maximum temperature difference of only about 20° along the tube t (of 1 mm. bore) and a pressure of vapour never less than about 1 mm., no correction for thermal pressure difference was necessary (see also below, p. 194). The powdered adsorbent weighed about 15 g. and exposed about 7 sq. m. of surface; its exact specific surface was determined by the rate of solution in hydrofluoric acid, as previously described (Palmer and Clark 1935).