Supercritical Fluid Adsorption to Weakly Attractive Solids: Universal Scaling Laws

Adsorption of light hydrocarbons (C1-C3) confined in the pores of silica aerogel (nominal density 0.2 g/cm3) was studied experimentally and by computer simulation. Five isotherms of ethane (25.0, 32.0, 32.3, 33.0, and 35.0 degrees C) were obtained over a temperature range including the critical temperature of the bulk fluid (T-c, = 32.18 degrees C) to similar to 220 bar by the vibrating tube densimetry (VTD) method, which yields unique information impossible to obtain by excess sorption measurements. Two isotherms (35 and 50 degrees C) of methane adsorption in SiO2 aerogel were measured far above its critical temperature, T-c = -82.59 degrees C, at 0-150 bar using a gravimetric method. The experimental pore fluid properties of methane, ethane, propane, and CO2 (obtained earlier by VTD for the same SiO2 aerogel sample) show behavior conforming to the principle of corresponding states over the entire covered reduced temperature ranges from T-r = 0.976 to T-r = 1.696. The magnitudes of the excess adsorption maxima and the reduced fluid densities where the maxima occur for C1-C3 hydrocarbons and CO2 are similar functions of the reduced temperature. This behavior indicates that, at least under the conditions covered in this work, the differences in molecular size and shape have only minor impacts on the solid fluid interactions underlying the adsorption behavior. These findings are supported by large-scale GCMC simulations of a lattice gas confined in the pores of a computer representation of the SiO2 aerogel framework, yielding excess adsorption isotherms in good agreement with experimental measurements, as well as the underlying microstructures of the adsorbed fluids.