Binary fluid mixtures confined in nanoporous media: Experimental evidence of no phase coexistence

The phase behavior of two binary fluid mixtures (CH4/C2H6, CH4/CO2) confined in well-defined, ordered nanoporous media (SBA-15) has been studied using Differential Scanning Calorimetry (DSC) with isochoric measurement procedures. For the first time, it is experimentally demonstrated that binary fluid mixtures confined in nanopores, at a specified bulk composition, behave similarly to confined pure fluids. As in confined pure fluids, the result reveals that capillary evaporation could only occur after all the bulk liquid completely evaporates in an isochoric heating process, while capillary condensation completely ends before the occurrence of bulk condensation in an isochoric cooling process. Both capillary condensation and capillary evaporation can therefore only occur when the bulk mixture outside the nanopores is in the vapor phase, and the phase transition is complete upon crossing the capillary-condensation or evaporation curve. The capillary condensation and evaporation curves at a specified bulk composition are found to coincide with each other except at low-temperature range where hysteresis occurs, which is also well known in the case of confined pure fluids. Therefore, this work experimentally demonstrates that confined mixtures do not exhibit phase-coexistence region as the bulk mixtures do.

Automated summary

The study demonstrates that binary fluid mixtures behave similarly to confined pure fluids in nanopores, with capillary condensation and evaporation occurring only when the bulk mixture outside the nanopores is in the vapor phase. Hysteresis occurs at low temperatures, but other than that, the capillary condensation and evaporation curves coincide, indicating no phase-coexistence region for confined mixtures.