Gibbs Ensemble Simulates For Vapor Liquid Equilibria Of Binary Mixture Fluids

Phase transitions of real and model complex fluids are of significant scientific and technological interest. From a modeling point of view, the phase behavior of a simplified system contain the essential interactions of interest. Computer simulations are a natural choice for this purpose, as they allow complete freedom in specification of the potential model. The focus of the present contribution is on a relatively simple and computationally inexpensive set of techniques known collectively under the name of "Gibbs ensemble" method. While the Gibbs ensemble does not necessarily provide accurate data and is not applicable to many important classes of systems, it is now commonly used for obtaining the phase behavior of fluids and mixtures. The main reason for this widespread use is probably the simplicity of the method. In particular, the method is easy to describe and program, and has an intuitive physical basis. It also places minimal demands on its user in terms of information on the approximate location of the phase diagram or other aspects of the behavior of a new system under study. The Gibbs ensemble method is the technique of choice for determining the phase diagrams of fluid mixtures. Gibbs ensemble simulations are performed in two microscopic regions within the bulk phases, away from the interface. Each region is simulated within standard periodic boundary conditions with images of itself. The thermodynamic requirements for phase coexistence are that each region should be in internal equilibrium, and that temperature, pressure and the chemical potentials of all components should be the same in the two regions. System temperature in Monte Carlo simulations is specified in advance. The remaining three conditions are respectively satisfied by performing three types of Monte Carlo "moves", displacements of particles within each region (to satisfy internal equilibrium), fluctuations in the volume of the two regions (to satisfy equality of pressures) and transfers of particles between regions (to satisfy equality of chemical potentials of all components).The Gibbs ensemble programs are used by the author to calculate the pure and binary mixture of nonlinear molecular and polar molecular.The constant-volume GEMC method is applied to calculate the vapor-liquid coexistence properties for the pure component carbon dioxide, propane, n-butane and n-pentane. The constant-pressure GEMC method is applied to simulate binary mixtures of carbon-dioxide/propane, propane/ n-butane, and n-butane/n-pentane systems. Lennard-Jones combined with Coulombic potential model was utilized to describe the interactions between united molecule for pure component carbon dioxide, propane, n-butane and n-pentane systems, while Buckingham exp-6 and Coulombic potential models were used for binary mixtures carbon-dioxide/propane, propane/ n-butane, and n-butane/n-pentane systems. Then the liquid- vapor phase coexistence curves that are plotted according to the simulation results respectively.Densities of both vapor phase and liquid phase, vapor pressures for pure components were simulated. Composition-pressure diagrams and densities for vapor phase and liquid phase of mixtures were plotted. Densities and compositions for both vapor phase and liquid phase from simulation show fair agreement with the experiment data over a wide range of pressures.