Sorption of Helium and Nitrogen on Vycor Porous Glass

Sorption isotherms of 4 He on Vycor porous glass have been measured at temperatures between 1.7 ° and 4.22 °K, and of nitrogen at 77.7 °K, using a specimen in which the flow of superfluid helium has already been investigated. In agreement with previous experiments with other substrates, the helium adsorption is found to be anomalous when treated by standard Brunauer-Emmett-Teller theory. The data may be analysed to show directly, independently of any adsorption model, that a part at least of the helium is abnormally dense, which may be represented by an underlayer adjacent to the substrate of thickness 4.8 Å and density 0.21 g cm -3 , equivalent to between one and two layers of solid density. These data are used to show that a modified theory, based on the assumption of a BET adsorption build-up on top of the basic first layer of approximately solid density, fits the results correctly up to 30% of saturation pressure. The theory is used to derive values of the heat of adsorption and density of the first `modified BET layer', and of the isosteric heats of adsorption at higher coverages, which are compared with corresponding quantities in ordinary BET theory. The general character of the isotherms is similar to type IV of Brunauer's classification, with hysteresis. In helium, the size of the hysteresis loops is strongly dependent on temperature in a fashion not expected from a simple capillary condensation model. Hysteresis appears to be unaffected by the occurrence of superfluidity, but unexpectedly disappears at 4.22 °K. From the desorption branch of the hysteresis loops, values of the pore radius r K may be derived by application of the Kelvin vapour pressure equation. The values found are different for nitrogen and for helium at different temperatures, but in all cases are much smaller than the radius r A = 35.4 Å calculated from nitrogen adsorption with BET theory. Taken in conjunction with previous nitrogen measurements on Vycor, these results indicate a clear trend towards increasing discrepancies ( r A- r K) with increasing pore radius.