Generalized critical shifts of confined fluids in nanopores with adsorptions

Confined fluids undergo substantial changes in terms of their physicochemical properties when the pore radius reduces to the nanometric scale and be comparable to the molecular size. Adsorptions, which are the corresponding changes in the concentration of a given fluid at the interface compared with the other contacting phase, strongly affect the vapour-liquid equilibrium (VLE), especially in nanopores. No published literature so far has been found to study the critical shifts of confined fluids with adsorptions in nanopores. In this study, first, the van der Waals (vdW) and Soave-Redlich-Kwong (SRK) equations of state (EOSs) are extended to calculate the VLE of confined fluids in nanopores. Second, a new empirical correlation is developed to calculate the fluid adsorption thickness in nanopores. Finally, two generalized analytical formulations, on a basis of the modified vdW and SRK EOSs, are initially proposed to calculate the shifts of critical temperatures and pressures by considering the fluid adsorptions in nanopores. All the extended EOSs, new adsorption correlation, and two generalized formulations have been validated to be accurate by comparing with the experimental or literature data. Overall, the critical properties of the confined fluids are found to shift more with adsorptions in nanopores. Moreover, the compressibility factors of the confined Lennard-Jones fluids are proven to be universal and independent of the pore radius effect.