The physical adsorption of hydrogen on polyethylene and teflon

Abstract

The physical adsorption of hydrogen gas on polyethylene and Teflon has been studied at temperatures between 14 and 20°K and pressures between 0 and 20 microns of Hg. From the adsorption isotherms the differential heats and entropies of adsorption have been determined. Surface areas were evaluated by applying the BET theory to adsorption measurements with neon at 14°K. The maximum surface coverage studied was about one monolayer or less. No significant difference between the adsorption of para and normal H 2 was found with these adsorbents. Para H 2 was used in most of the measurements. Polyethylene appears to be a slightly better adsorbent for H 2 than is Teflon. A difference of about four per cent in the corresponding heats of adsorption will account for the results. This is consistent with estimates of the attractive Van der Waals potential between a H 2 molecule a -CH 2- or -CF 2- group, calculated from the equation of Kirkwood and Müller. The differential isosteric heat of adsorption decreases with increasing surface coverage for both adsorbents. The range measured was from about 800 to 450 calories per mole. There is some temperature dependence, particularly for polyethylene. The differential entropy ( S ads) of adsorbed para H 2 was determined, although with considerable experimental uncertainty. At the surface coverages studied here, S ads appears to increase rapidly with coverage, a fact which may be related to the corresponding decrease in the differential heat of adsorption. S ads shows considerable temperature dependence for both adsorbents. In an Appendix, the method is described by which the large thermomolecular pressure differences present in these studies were evaluated. The method involved making measurements with connecting tubing of successively smaller diameter and extrapolating to zero tube diameter. The limiting equation P 1/P 2 = T 1/T 2 could then be applied.