Computer Simulations Of Vapor-liquid Phase Equilibria And Critical Asymmetry For Square-well Fluids

The vapor-liquid phase equilibrium properties and critical asymmetry of fluids with Square-Well potential were investigated by grand-canonical ensemble Monte Carlo simulation based on complete scaling theory. With the help of histogram reweighting technique and Finite-Size Scaling theory, we obtained the critical temperatures and critical densities of Square-Well chain molecules with λ=1.5and2,4,8,16segments. The values of critical temperatures are lower than that presented before based on incompelete scaling theory. And the critical temperature for an infinitely long Square-Well chain was estimated.The critical properties of Square-Well fluids with λ=1.25,1.75,2.0were reexamined using the Q-parameter method. The vapor-liquid coexistence curve in near-critical region for Square-Well monomers with λ=1.25,1.75,2.0were obtained by a scaling algorithms for precise simulation of fluid coexistence close to criticality. The results implied that the system with a smaller value of λ showed stronger critical asymmetry. Analyzing the coexistence diameter by fitting to a scaling relation made us concluded that the radio of coefficients in terms|t|2β and|t|1-α in Square-Well fluid with1=1.25was the largest, which means the smaller of inaction range, the more important in|t|2β term to coexistence diameter. This fact is in agreement with the conclusion in other papers.The vapor-liquid phase equilibrium properties and critical asymmetry of Square-Well dimer and tetramer with λ=1.5were studied by the same scaling algorithms. It showed that the term of|t|2β contributed more to coexistence diameter for Square-Well dimer than that for Square-Well monomer. That means fluids with a bigger molecule size had a much more dominating effect to the singular of coexistence diameter. While analyzing the results for Square-Well tetramer, we found that the domination of|t|2β term to the coexistence diameter was smaller for Square-Well tetramer fluid than that for Square-Well dimer fluid, and|t|1-α term dominate the singular behavior of coexistence diameter for Square-Well tetramer. The reason may be that the Square-Well tetramer molecule existed not in the form of a linear chain but in the form of a curly chain. Therefore, except the interacting range and molecule size, the molecule configuration of chain fluids was expected to impact the critical asymmetry. We also performed hyper parallel tempering Monte Carlo simulations for Square-Well dimer to predict coexistence properties far from critical point. The coexistence vapor and liquid densities were located by equal-area criterion for the double-peak of density probability distribution. The results are in good agreement with previous ones. Finally, the complete vapor-liquid coexistence curve of Square-Well dimer was showed.