Adsorption hysteresis loop scan

To scan from the desorption to the adsorption branch, pure adsorbate is first adsorbed, then the adsorbate concentration is reduced to a value giving a relative pressure corresponding to the start of the scan on the desorption isotherm. When equilibrium is established, as indicated by a constant base line, the adsorbate concentration is increased to give a relative pressure between the desorption and adsorption isotherms. After equilibrium is again established, the coolant is removed and the resulting signal is calibrated to yield a data point between the desorption and adsorption isotherms. This procedure repeated, each time using a different final relative pressure, will yield a hysteresis loop scan from the desorption to the adsorption isotherm. 

Figure 15.13 Hysteresis loop scan for same sample as Figure 15.12. Desorption to adsorption, + adsorption to desorption, O.

Attempts to measure points on the desorption isotherm without first adsorbing near saturation will result in scanning the hysteresis loop from the adsorption to the desorption curve (see Fig. 15.13 and Section 15.6). 

One can conclude that stepped isotherms and low pressure hysteresis loops may only be observed on monodispersed and large crystals of network types of molecular sieves. The hysteresis loop was examined in detail by scanning the region between the adsorption and the desorption branch of the hysteresis. The points betweoi the two branches were stable for many hours, thus, the low pressure hysteresis is controlled by equilibrium properties and not by kinetical effects. Previous studies provided clear evidence that the stepped isotherms of argon and nitrogen on MFI-crystals can be rationally explained by localised adsorptive molecules at the channel walls and intersections [10]. 

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