Critical behavior of pure confined fluids from an extension of the van der Waals equation of state

The critical behavior of pure fluids confined in porous solids is investigated using an extension of the van der Waals equation of state. The effects of pore size and of the interaction between fluid molecules and pore walls are evaluated. Fluid molecules were assumed spherical, interacting with each other and with the walls of cylindrical pores through distinct square-well potentials. It was found that our model may predict either one or two mechanically stable critical points for the confined fluid, depending on its specifications. When two critical points are predicted, one is attributed to a major contribution of molecule-molecule interactions and the other to a major contribution of molecule-wall interactions. The confined fluid critical point(s) presented a complex dependence on the pore size, due to the interplay of molecule-molecule and molecule-wall interactions. It is shown that the prediction of two critical points for confined fluids is useful to describe adsorption isotherms with two phase transitions.