Elastic properties and freezing of argon confined in mesoporous glass

We study the elastic properties of argon confined in mesoporous Vycor glass with a mean pore diameter of 8 nm. For this purpose volumetric adsorption and desorption measurements are combined with simultaneous ultrasonic measurements. Therewith we obtain the effective shear modulus as a function of the pore filling reflecting both the spatial arrangement and the freezing process of confined argon. Below the freezing point of argon the adsorption process proceeds in three steps. (I) The first adsorbed layers of argon do not contribute to the measured shear modulus, i.e., they do not behave like a solid. (II) In an intermediate range of pore filling the shear modulus increases linearly with increasing amount of adsorbate, i.e., the process of freezing starts, probably with the formation of capillary sublimate. (III) At a certain filling fraction an abrupt rise of the shear modulus up to a plateau value is observed. This step indicates either a change in the spatial distribution of the capillary condensate or a change in its intrinsic properties. The lower the temperature, the smaller the characteristic filling fractions at which these transitions occur. During desorption a hysteresis of the shear modulus, of the attenuation, and hence of the state of the adsorbate is observed.