ANALYSIS OF PHYSICAL ADSORPTION ISOTHERMS ON HETEROGENEOUS SURFACES AT VERY LOW PRESSURES

Abstract

A method previously described for finding heterogeneous energy distributions ƒ(E) from physical adsorption isotherms is applied to experimental data for argon, nitrogen, and helium obtained in an ultrahigh vacuum apparatus in the pressure range [Formula: see text] Torr. The adsorbent was Pyrex and matching temperatures were 77.4, 90.2, and 4.2 °K for the three gases respectively. ƒ(E) was found to be asymmetric about a maximum energy whose location depended critically upon the magnitude of assumed adsorbate–adsorbate interaction (Q). However, the isotherms calculated at temperatures other than the matching temperature were insensitive to Q, and hence to the detailed form of the local isotherm. These calculated isotherms showed excellent agreement, not only with the data on Pyrex, but also with the data of several workers using various adsorbents and temperatures. The latter result suggests a similar adsorbing surface for all these adsorbents.