An analysis by adsorption of the surface structure of graphite

Abstract

The work to be described had as objectives: to measure accurately sets of isothermals of non-polar gases on an inert adsorbent as a function of temperature and of quantity adsorbed; then to employ these isotherms and heats to obtain a modified Langmuir isotherm capable of a general and quantitative application to surfaces of variable adsorption potential. Owing to a quantization of the energy levels of the interacting molecules, in the van der Waals potential energy hollow, the liquid hydrogens H 2 and D 2 (Urey and Teal 1935) or the hydrogens adsorbed on charcoal (Barrer and Rideal 1935) have different vapour pressures; The quanta are larger for H 2 than for D 2 and therefore it is easier to evaporate H 2 . A counteracting influence must be considered, however (Lennard-Jones and Devonshire 1936), which is the effect of the mass on the wave functions whose product determines the probability of evaporation. Calculation shows for a simple case a separation factor which diminishes with temperature at a rate less than the original zero-point energy theory requires, though the zero-point energy effect is the greater. One might expect similar considerations to apply to other adsorbed molecules held by van der Waals forces only, and by lowering the temperature to cause transitions to lower vibrational levels of the adsorbed molecules against the solid, making it more difficult to desorb them. No investigation of the magnitude of these effects has been made, and so it was regarded as important to look for effects of temperature on pure van der Waals heats of sorption.