Physical adsorption isotherms at extremely low pressures

Assuming a homogeneous model of adsorption and employing the results of the theory of imperfect gases. Henry's Law constant has been calculated quantum mechanically and classically for several potentials of interaction. The quantum mechanical treatment yields theoretical adsorption isotherms for the various interaction potentials which are very different from each other, while the isotherms calculated classically all fall within a relatively narrow range. These results are compared with the experimental data of Hobson for He on Pyrex glass at 4.2°K and with the data of Hobson and Armstrong for N₂ on Pyrex glass in the temperature range 63.3°K to 90.2°K. In all cases the agreement with experiment is poor. The present calculations based on the homogeneous model suggest that for the Morse potential, the simple harmonic oscillator approximation and the straight line approximation pressures outside of present research techniques in ultra high vacuum physics will have to be attained before adsorption isotherms are described by Henry's Law.