Isochoric measurement of the evaporation point of pure fluids in bulk and nanoporous media using differential scanning calorimetry

As a continuation of recent series of work, a new approach applying an isochoric heating process using differential scanning calorimetry (DSC) is introduced to measure the evaporation point of pure fluids in both bulk phase and nanoporous media, as opposed to the previous approach of isochoric cooling to measure the condensation point [X. Qiu et al., Phys. Chem. Chem. Phys., 2018, 20, 26241-26248; X. Qiu et al., Phys. Chem. Chem. Phys., 2019, 21, 224-231]. Though these two approaches must arrive at the same phase-transition point for a specified density of bulk pure fluids, it is not necessarily true for confined fluids due to hysteresis in a temperature range sufficiently far below the bulk critical point. The isochoric heating process allows one to accurately measure the phase transition of non-volatile fluids that exist in liquid phase at relatively high temperatures. As the new approach operates without an inert gas, which substantially dissolves in the test sample at high pressures if the standard isobaric measurement ASTM E1782 is used, application to the high-pressure range is enabled with higher accuracy. This method can also be extended to confined systems, where the evaporation points of both bulk and confined fluids are successively measured in a single run of experiment. The results reveal that capillary evaporation, i.e., evaporation of fluids confined in nanoporous media, occurs at a higher temperature (isobarically), or at a lower pressure (isothermally), than that in bulk only after the liquid in bulk space is completely evaporated. The method introduced in this work paves a new way to study the condensation/evaporation hysteresis of confined fluids as well as the evaporation point of confined fluid mixtures.