(177ai) Studies of Sorption Equilibria with Unique Stability of Temperature and Relative Pressure

As an example we studied sorption phenomena with CO₂ at 195 K. Such Sorption studies with CO₂ are still in the spot light of current research projects. Not only because of climate related investigations but also to characterize nano-sized and porous materials.

Classically, CO₂ at 273 K is used to study very narrow micropores. Despite the fact that the use of CO₂ at 195 K is also common practice, there are actually several inconsistencies, such as temperature control and the use of adsorbate parameters. The temperature of 195 K is still set in many laboratories using dry ice and isopropanol. However, it is known that a temperature gradient forms over time, which has a negative influence on the isotherm measurement, as the saturation pressure p₀ of CO₂ also changes with temperature. The biggest discrepancy, however, can be found in the actual value of p₀. Some authors assume the sublimation pressure of solid CO₂ for measurements below the triple point of 216.59 K (-56.56 °C).^[1] Others used the vapor pressure of the undercooled liquid^[2] But which one is correct? To answer this question we used a mesoporous material, namely MCM-41 to investigate the adsorption of CO₂ at different temperatures. From these experiments we could extract the heat of adsorption of CO₂ on MCM-41 which is lower than the sublimation energy but higher than heat of condensation. We therefore can prove a liquid-like state of CO₂ within the pores of CO₂.

[1] S. Yang et.al, Nat. Mater 11, 710 (2012).

[2] P. Wu et. al. Nat. Commun 10, 4362 (2019).