Motional State of Sorbed Nitrogen in Mordenite

Abstract

Sorption isosteres of nitrogen in mordenite have been accurately measured by a gravimetric method over a temperature range – 50 to 25°C. Sorption isotherms at 25, 50 and 75°C have also been measured by the use of a high pressure adsorption apparatus. Analysis of the experimental results by statistical mechanics, allows the thermal entropy of the sorbed nitrogen to be obtained as a function of temperature. It is well established, from entropy values, that the sorbed nitrogen is in a librational state at temperatures below – 20°C and freely rotating above 20°C. The transition from libration to free rotation occurs over a narrower temperature range than for the internal rotation of a molecule.

The equation of state for sorbed nitrogen is derived, and values for its virial coefficients are compared with those for gaseous nitrogen and discussed qualitatively in the light of the molecular theory of fluids.