Adsorption Hysteresis: An Inside Look

Physical adsorption is widely used to characterize porous materials. Mesoporous materials—materials with pores 2–50 nm wide—exhibit hysteresis in the adsorption isotherm, a well-documented but poorly understood phenomenon that is still a topic of active study decades after its discovery. Hysteresis itself is further complicated by the different forms of scanning behavior (Tompsett, et al., 2005) shown by different materials. In this group of studies, we investigate thermodynamic states inside the hysteresis in the isotherms of mesoporous materials by perturbing the adsorbate at a point along a scan, inside the loop. The fluid at these points is perturbed by both mechanical and thermal means to assess the resilience of the filled pores to these types of perturbations. Mechanical perturbations were performed by placing a piezoelectric speaker inside the cryogenic bath to expose the system to vibrations in the form of sound waves of various frequencies. Thermal perturbations were performed by changing the temperature bath ±10 K and back or by removing the bath and exposing the system to room temperature for 30–60 s. We find that vibrations up to 500 kHz have no substantial effect on the quantity adsorbed for states that lie within the loop. On the other hand, the states inside the hysteresis loop are found to be unstable with respect to thermal perturbations. The final state after the sample is returned to its original temperature bath depends on whether the temperature had been raised or lowered during the shock. For increases in temperature, the adsorbed state shifts onto the adsorption branch of the hysteresis; for decreases in temperature, the state shifts onto the desorption branch.

Reference:
Tompsett, G. A., L. Krogh, D. W. Griffin, and W. C. Conner. “Hysteresis and Scanning Behavior of Mesoporous Molecular Sieves.” Langmuir, 21, 8214 (2005).