The effects of mechanical and thermal perturbations on states within the hysteresis of sorption isotherms of mesoporous materials

We investigate thermodynamic states inside the hysteresis in the adsorption–desorption isotherms of mesoporous materials by perturbing the adsorbate at a point inside the loop. Scanning curves were run to points inside the hysteresis loop and then the system was perturbed by both mechanical and thermal means. Mechanical perturbations were performed by placing a speaker inside the cryogenic bath to expose the system to vibrations. Thermal disturbances were performed by changing the temperature bath ±10 K and back or by removing the bath 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, thermal shocks do affect the system and the states inside the hysteresis loop are found to be unstable with respect to this sort of perturbation. 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.