The thermodynamic behavior of fluids confined in nanopores differs from that of bulk fluid by the fluid-pore wall interaction. Inspired by the heterogeneous distribution and layering transitions from fluid-wall interactions, we have developed a pore-size-dependent equation of state (EOS) extended from the Peng-Robinson equation of state (PR EOS) by describing the confined fluid as distinct populations of surface adsorbed and core fluid. The surface-adsorbed fluid physically corresponds to two surface-adsorbed layers and is therefore considered to interact with the pore wall through a dual-square-well (DSW) potential function. The presented EOS requires only one additional parameter of confinement energy (epsilon(sf)) beyond PR EOS, and it needs to be determined from experimental data. Using adsorption and differential scanning calorimetry experiments, we determine the confinement parameter epsilon(sf) for nitrogen, carbon dioxide, n-alkanes of methane to n-tetradecane, with respect to surfaces of native silica and silylated silica. The resulting database of confinement parameters is presented. This EOS models adsorption isotherms, phase transition temperature, and phase transition-pore size relations, in agreement with experimental data. The EOS is shown to be a unified tool for modeling the fluid phase behavior in nanopores. (C) 2019 Elsevier B.V. All rights reserved.