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-co mposition 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 extrap olate the mixture data to obtain an estimate of the pure component pha se equilibria. The resulting values for the coexistence envelope are i n good agreement with existing data, and new vapor pressures of the sq uare-well dimer are reported. The vapor-liquid equilibria data are use d to establish the adequacy of the statistical associating fluid theor y for potentials of variable range. The theory is found to give an exc ellent representation of the phase behavior of both the pure component s and of the mixture. [S1063-651X(98)12302-4].