The vapour pressure of argon

The condensation of atmospheric gases on cold surfaces

Data for the deposition of argon, nitrogen and carbon dioxide on cold, metal surfaces are considered in detail. It is concluded that argon and nitrogen deposit when the incident gas pressure equals the sublimation pressure at the respective surface temperature, and growth therefore proceeds without any significant intermediate nucleation barrier. Carbon dioxide, however, requires considerable supersaturation of the gaseous phase and consequently bulk deposition is inhibited by a nucleation barrier. The results are analysed with a view to determining the critical nucleus size and the adsorption energy. The ‘classical’ method of analysis gives unsatisfactory and inconclusive results. In contrast, the ‘atomistic’ approach is found to give a good account of the critical deposition phenomenon. The onset of gross deposition is found to be due entirely to capture of single molecules by stable nuclei, rather than by the formation of critical nuclei as the ‘ classical ’ theory wrongly assumes. The number of molecules in the critical nucleus is found to be about nine and a value of 9.4 kJ mol -1 is obtained for the adsorption energy, suggesting that nucleation occurs on top of a strongly bound adsorbed layer of contaminants or carbon dioxide itself, rather than on bare metal.

Deviations from the principle of corresponding states

Certain intermolecular potential energies which are functions of the relative orientation of the molecules may be reduced to a form similar to that of simple spherical molecules by taking a statistical average over all orientations. Such an average energy is an explicit function of the temperature. Two such intermolecular potentials are used to calculate the difference in the equations of state of assemblies of elliptical molecules (part I), of dipolar molecules (part II), and an assembly of equivalent spherical molecules, which conforms to the principle of corresponding states. These calculations are compared with the observed deviations from this principle of the vapour pressure and rectilinear diameter of thirteen liquids.

The expansion of porous glass on the adsorption of non-polar gases

The design and construction of an absolute dilatometer of high sensitivity to measure the small expansions resulting from the adsorption of non-polar gases on to porous glass at liquid-air temperatures are described. The expansions measured with this instrument are the first recorded with a rigid adsorbent for the case in which the adsorption is, with certainty, physical in nature. Experiments in which the gases argon, nitrogen, oxygen, hydrogen and krypton were used as adsorbates are reported. The effect of organic matter adsorbed on to the glass from the atmosphere, of unknown composition, on the expansion characteristics is indicated. A technique for the removal of this organic matter in situ without affecting the area of the glass is described. The results have confirmed the correctness of an equation which has been derived by the author (Yates 1952) (∂ F /∂ V ) T = –3/2 K , where ∂ V is the volumetric expansion at constant temperature resulting from ∂ F , the surface free energy lowering, and where K is the bulk modulus of the adsorbent.