Phase equilibria of a square-well monomer-dimer mixture: Gibbs ensemble computer simulation and statistical associating fluid theory for potentials of variable range

The vapor-liquid equilibria of a monomer-dimer square-well mixture is examined. The square-well segments all have equal diameter, well depth, and range (lambda=1.5); the dimers are formed from two tangentially bonded monomers. The phase behavior in this system is thus governed by the difference in molecular length of the two components. Pressure-composition slices of the phase diagram are obtained from Gibbs ensemble simulation of the mixture for a wide range of temperatures, including both subcritical and supercritical states. A small negative deviation from ideality is observed. The simulation data are used to determine the vapor-liquid critical line of the mixture. Additionally, we extrapolate the mixture data to obtain an estimate of the pure component phase equilibria. The resulting values for the coexistence envelope are in good agreement with existing data, and new vapor pressures of the square-well dimer are reported. The vapor-liquid equilibria data are used to establish the adequacy of the statistical associating fluid theory for potentials of variable range. The theory is found to give an excellent representation of the phase behavior of both the pure components and of the mixture. [S1063-651X(98)12302-4].