Experiments on the Capillary Condensation/Evaporation Hysteresis of Pure Fluids and Binary Mixtures in Cylindrical Nanopores

A new experimental method, i.e., isochoric cooling/heating cycle measurement using differential scanning calorimetry (DSC), is applied to investigate the hysteretic phenomenon of pure components and mixtures confined in two different types of cylindrical nanopores (SBA-15 and KIT-6). The capillary condensation/evaporation hysteresis end point T-he for a specific system is determined by performing a series of measurements with different densities by increasing initial temperature and pressure. We confirm that for a pure component, T-he is lower when confined in smaller nanopores, whereas the T-he values for CH4/CO2 and CH4/C2H6 mixtures at the specific composition investigated are higher than those of pure CO2 and C2H6, respectively, when confined in the same nanopores. To the best of our knowledge, this is the first time that T-he of fluid mixture is experimentally studied, which further demonstrates that binary mixtures behave similarly to pure fluids when confined in nanoporous media. At low temperatures, where hysteresis is observed, the shape as well as the duration of the exothermic peak (due to capillary condensation) and endothermic peak (due to capillary evaporation) indicate that capillary condensation, rather than capillary evaporation, takes place at/near thermodynamic equilibrium. Within the accuracy of the measurements, we find that the total heat of capillary condensation is identical to that of capillary evaporation for all systems examined, regardless of the presence of hysteresis. By comparing the heat per unit volume among different systems, we also find that the heat per unit volume of the phase transition of confined pure fluid is lower than that of bulk, which further demonstrates that at the same temperature the molar density of confined pure liquid is less than that of bulk pure liquid at saturation.