I ~ O N A LGRAHAM I)
2792
Film Pressures. Film pressures, or changes in the surface free energy with adsorption, were calculated from the adsorption data for a coverage of one statistical monolayer by graphic integration of thc Gibbs equation as has been described.’ The values obtained for nitrogen, argon, and ethane were 12, 13, and 14 ergs/cm.2, respectively. I n contrast, the film pressure of a statistical monolayer of carbon tetrafluoride was only 9 ergs/cm.2, a value consistent with its lower free energy of adsorption.
Conclusions Polypropylene adsorbs the nonpolar gases, nitrogen, argon, carbon tetrafluoride, and ethane, with small to moderate heats of adsorption, the low coverage results indicating extensive surface heterogcneity. Nitrogen and argon are adsorbed as mobile two-dimensional gascs, argon retaining some “supermobility.” The polyatoniic gascs, carbon tetrafluoride and ethane, were adsorbcd with sornewhat restricted mobility, but the film prcssure of carbon tetrafluoride was lower than those of the other adsorbatcs, reflecting its lower frec energy of adsorption.
Discussion J. H. DE BOER(The Ilague, Netherlantls). I n your introductory remarks you refer to the independent) proof that 16.2 A . 2 is the right value to use for the molec.ular surface area of nitrogen, based on studies of the adsorption of iodine on carbon. It may be interesting to note that our recent work on the adsorption of lauric: acid on aluniina shows that one lauric wid nio1t:cule is adsorbed for every four i2yygen atoms of the surface. This eBec*t for a lauric acid molecule in this parleads to a value of 26.9 ticular type of adsorption and confirms the value of 16.2 ii.z for nit,rogen. P. K . I S A A ~ (W. S R. Crace Company, Clarksville). W’oultf a higher energy surface than polypropylene have less tendency to give rise to polyniolecular layers? You seen1 to iriil)ly that polymolecular layers are peculiar to low energy surfacw below the saturation point. I). GRAHAM. Physical adsorption at :my (werage is favored by a high adsorbent surface energy. On a high energy surfaw, the first monolayer may he cwnpleted at a low relative pressurc with rnultilayer deposit,ion becoming appreciable as the vapor pressure of the adsorptive gas is itpproached. On a low energy solid adsorbent the first statistic:tl nionolayer retains wnsiderable mobility and requires a higher relat,ive pressure f o r i t s completion, tending to obscure the usual sigmoid intlct+,iori in the isotherrn, i.e., there is no well defined “point I3”-see Pig. 1-4. (7) W . D. Harkins, “The Physiral Chemistry of Surface Films,” Reinhold Publishing Corp., New York, N. Y . , 1952, p. 211.
