B. Garzón

Virial coefficients and equation of state of hard alkane models

The virial coefficients up to the fifth of hard conformers of alkanes have been evaluated numerically. The conformers were selected by using the rotational isomeric state approximation. The effect of branching and length of the chain on the anisotropy of alkanes is discussed and a relation between the nonsphericity parameter α and the acentric factor ω for this kind of molecule is proposed. We modified the equation of state for tangent hard spheres first proposed by Wertheim [M. S. Wertheim, J. Chem. Phys. 87, 7323, (1987)] to allow for overlapping of the constituting hard spheres. This modified version predicts with high accuracy the virial coefficient of the hard conformers of alkanes. Moreover comparison of this equation of state with simulation results of repulsive models of n‐alkanes reveals very good agreement. The effect of condensed phase effect on the conformational equilibria of n‐alkanes up to n‐octane is analyzed.

Reaction field simulations of the vapor-liquid equilibria of dipolar fluids: Does the reaction field dielectric constant affect the coexistence properties?

Abstract Vapor-liquid equilibria of two Stockmayer fluids have been obtained by computer simulations in the Gibbs ensemble. The long-range dipolar interactions have been considered within the reaction field geometry. Two different criteria for the reaction field dielectric constant, (ϵ rf , have been applied. It is shown that ϵ rf does not significantly affect either the coexistence properties or the structure of coexisting phases for the simulated systems.

Computer simulation of vapor–liquid equilibria of linear quadrupolar fluids. Departures from the principle of corresponding states

Vapor–liquid equilibria of different quadrupolar linear Kihara fluids have been studied, by using the Gibbs ensemble Monte Carlo technique. Coexistence curves for fluids with elongations L*=L/σ=0.3, 0.6, and 0.8 and different quadrupoles are given. We analyze the effect of quadrupole moment on critical properties. Quadrupole moment increases the critical temperature, pressure, and density. The magnitude of the increase depends on both anisotropy and quadrupole moment. A new way of reducing the quadrupole is proposed, so that the variation of critical properties due to the quadrupole follows a universal behavior. Quadrupole provokes deviations from the principle of corresponding states. A broadening of the coexistence curve is observed due to the quadrupole. The quadrupole moment increases the slope of the vapor pressure curve vs temperature inverse. Simulation data are used to describe vapor–liquid equilibria of carbon dioxide. Good agreement between simulation and experiment is achieved.

Linear hard sphere models Virial coefficients and equation of state

Virial coefficients of tangent hard spheres in a linear configuration have been determined numerically. Trends of the virial coefficients with the molecular anisotropy are similar to those of other linear models, such as hard spherocylinders or hard ellipsoids. Theoretical predictions of virial coefficients from different equations of state of hard body fluids are compared with the numerical results. None of them provides a completely satisfactory description of the lower virial coefficients when the anisotropy of the molecule is large. We propose a new method to build up an equation of state of hard linear models (prolate or oblate) from the knowledge of the first five virial coefficients. The equation of state obtained in this way provides a very good description of the equation of state of hard linear fluids at low, medium and high anisotropies.

Vapour–liquid equilibrium of fluids composed by oblate molecules

Gibbs ensemble Monte Carlo simulations are performed to obtain the vapour–liquid equilibrium of oblate-like fluids interacting through the Kihara intermolecular potential. Results confirm the validity of a perturbation theory for Kihara fluids, whose accuracy for prolate fluids was tested some years ago. As in the case of hard ellipsoids, the symmetry of the phase diagram of oblate and prolate models is analysed. An interesting relation of Boyle temperature and critical parameters with molecular volume is found for the considered models. As a particular application, this relation allows the prediction of some thermodynamic properties of a new promising biofuel 2,5dimethylfuran.

Liquid–vapour equilibrium of multipolar square-well fluids.Gibbs ensemble simulations and equation of state

Simulation results for a system comprising a square well plus either a point dipole or a point quadrupole are presented. The properties obtained are the vapour–liquid equilibrium densities and the critical properties. Critical densities are not very sensitive to the values of dipole or quadrupole, while critical temperatures increase significantly when the multipole strength rises. A comparison with a perturbation theory for multipolar square-well systems is presented. Overall agreement between simulated and theoretical values is good when comparison is restricted to quadrupoles or dipoles corresponding to the most relevant real polar substances but is only moderate for the largest multipolar strengths considered.

Monte Carlo simulations of dipolar and quadrupolar linear Kihara fluids. A test of thermodynamic perturbation theory

Several simulations of dipolar and quadrupolar linear Kihara fluids using the Monte Carlo method in the canonical ensemble have been performed. Pressure and internal energy have been directly determined from simulations and Helmholtz free energy using thermodynamic integration. Simulations were carried out for fluids of fixed elongation at two different densities and several values of temperature and dipolar or quadrupolar moment for each density. Results are compared with the perturbation theory developed by Boublik for this same type of fluid and good agreement between simulated and theoretical values was obtained especially for quadrupole fluids. Simulations are also used to obtain the liquid structure giving the first few coefficients of the expansion of pair correlation functions in terms of spherical harmonics. Estimations of the triple point temperature to critical temperature ratio are given for some dipole and quadrupole linear fluids. The stability range of the liquid phase of these substances i...

Understanding the phase diagrams of quadrupolar molecules

Abstract Recent research on the effect of a quadrupole moment on the phase diagram of linear molecules is reviewed. In particular the effect of the quadrupole on the vapour-liquid and solid-liquid equilibria will be discussed. It is found that the quadrupole moment increases the critical temperature, pressure and density of the fluid over the model without a quadrupole and leads to deviations from the principle of corresponding states. The effect of the quadrupole on a molecular model with a spherical repulsive core is larger than on one with a nonspherical core. The presence of the quadrupole stabilizes solid structures which are not close packed. This leads to a shrinkage of the liquid range as measured by the ratio of the triple to critical point temperature and exhibited by systems like carbon dioxide and acetylene.

Dynamical properties and transport coefficients of Kihara linear fluids

Transport properties of spherical and linear molecules modeled by the Kihara potential are studied by molecular dynamics simulations. Diffusion coefficients, shear viscosities, and thermal conductivities are calculated for a wide range of the fluid region and for several elongations. The corresponding individual and collective correlation functions are discussed along with angular velocity and reorientational correlation functions. Relaxation times and simple models relevant to orientational motion are also studied. The results obtained are discussed in a corresponding states framework, using previous Gibbs ensemble Monte Carlo data for the liquid–vapor equilibria of the models. In this way, the role of elongation can be studied. It is found that in most of the liquid region, the diffusion coefficient is weakly dependent on elongation. On the other hand, both viscosity and thermal conductivity are found to decrease with elongation. The dependence of transport coefficients on density and temperature is als...

Nonequilibrium properties of linear polar Kihara fluids from molecular dynamics. Results for models and for liquid acetonitrile

Molecular dynamics simulations for polar Kihara fluids are reported for linear models of different lengths at several dipole and quadrupole values and at three different thermodynamic states. Two of these states are close to the vapor–liquid equilibrium curve and the third one is at the same density as the first and at the same temperature as the second. Self-correlation functions and translational and orientational times are calculated and analyzed. Transport properties, diffusion, thermal conductivity, and shear viscosity are also reported and discussed in terms of multipolar forces. Correlation terms are used to calculate band broadening in different kinds of molecular spectra. Finally, it is shown how it is possible to discriminate between two models of acetonitrile that fit equilibrium properties equally well by using dynamic properties.