In this work, the adsorption isotherms of carbon dioxide on silica have been measured with a gravimetric method at nine different temperature values (from 312 up to 466 K) and at pressure levels up to 366 bar. The adopted technique allows for a direct evaluation of the density that is measured simultaneously to the adsorption load, thus avoiding possible systematic errors arising from the use of an equation of state to calculate density from pressure and temperature data. The enthalpy changes related to the adsorption process have been obtained from a thermodynamic analysis based only on excess properties.

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The adsorption of nitrous oxide (N2O) on silica gel has been studied using a gravimetric apparatus under near critical conditions. In gravimetric measurements, the determination of the excess amount adsorbed depends on a buoyancy correction factor which accounts for the volumes of the adsorbent and the solid parts in the cell. The accurate measurement of the volume of the sorbent and the metal parts in the measuring cell plays an important role in the precise measurement of the excess adsorbed amount especially at high densities. To this aim, a new protocol has been proposed which accounts for non-negligible helium adsorption and thermal expansion of the sorbent and the solid parts of the balance. Helium isochores under moderate temperature and densities have been experimentally measured. The adsorption behavior of helium has been characterized using a nonlinear mobile adsorption model. The proposed protocol has then been applied for the measurement of the excess adsorbed amount of supercritical N2O on silica gel very close to the critical point. Clear experimental evidence of critical depletion, i.e., the decrease of the excess adsorption along an isochore with decreasing temperature, has been obtained. Unlike the previous observation, where critical depletion was seen only in the direct vicinity of the critical isochore, in the present study this is observed on isochores in the range of 0.74 ≤ ρr ≤ 1.15.

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