Hysteresis and irreversible phase transitions: xenon adsorbed on a mesoporous silica gel

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Abstract
Isotherms for xenon adsorbed on a mesoporous silica gel are determined at integer temperatures from 139 to 170 K. All isotherms are type IV and exhibit extensive hysteresis. The desorption branch of each isotherm shows an unusual step at temperatures from 139 to 165 K. At higher temperatures this step is not observed. Current theories of thin film–thick film (prewetting) transitions are applied to explain the phenomenon. The apparent contact angle of capillary liquid xenon on this adsorbent decreases from ca. 60° at 139 K to zero for T > 164 K. Adsorption progresses through the growth of an adsorbed film on the pore walls plus a capillary gas phase within the pore cavity. Capillary liquid forms by a second-order phase transition at high coverage. Hysteresis occurs when there is a range of relative pressures at which a capillary gas phase (adsorption) and a capillary liquid phase (desorption) can exist. Irreversible film thickness transitions also contribute to hysteresis.

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