Disclosing the True Critical Point of Fluids Confined in Nanopores

Two methods for determining the pore critical point (PCP), i.e., the critical point of confined fluid, are currently available in the literature. The better-known method is based on the analysis of data obtained from adsorption isotherms, while the newer method analyzes the heat of capillary condensation data obtained from differential scanning calorimetry (DSC). For the first time, these two methods are implemented on the same confined system for comparison to explore which method provides the true PCP. While the heat released at the PCP derived from adsorption isotherms is measurably nonzero, implying that a phase transition still occurs in the pores, no heat is released at the PCP derived from the DSC measurements, which indicates the true critical point of the first-order phase transition. With this true PCP, the Peng-Robinson cubic equation of state (EOS), without any modification, is shown to accurately represent the whole capillary-condensation curve of confined pure fluids with small and large molecules as well as a confined mixture.