N. G. Almarza

Role of the interaction range in the shaping of phase diagrams in simple fluids. The hard sphere Yukawa fluid as a case study

We have investigated the influence of the interaction range on the shape of the phase diagram in a system of particles that interact via a hard‐sphere potential plus an attractive Yukawa interaction. The gas–liquid phase diagram is evaluated both by Gibbs ensemble Monte Carlo simulation and reference hypernetted chain integral equation theory. An estimate of the liquid–solid transition line was computed using a recently proposed one‐phase entropic criterion with thermodynamic and structural input from the integral equation results. The overall agreement between theory and simulation is remarkable. Our results clearly indicate that the range of temperatures for which the liquid is stable shrinks as the interaction range decreases. At the same time, the gas–liquid phase diagram tends to flatten out, in particular on the liquid side. For sufficiently short interaction ranges the system has no stable liquid phase.

Phase stability of binary non‐additive hard‐sphere mixtures: A self‐consistent integral equation study

We have tested the capabilities of a new self‐consistent integral equation, closely connected with Verlet’s modified closure, for the study of fluid‐fluid phase separation in symmetric non‐additive hard‐sphere mixtures. New expressions to evaluate the chemical potential of mixtures are presented and play a key role in the construction of the phase diagram. The new integral equation, which implements consistency between virial and fluctuation theorem routes to the isothermal compressibility, together with chemical potential and virial pressure consistency via the Gibbs‐Duhem relation, yields a phase diagram which especially at high densities agrees remarkably well with the new semi‐Grand Ensemble Monte Carlo simulation data also presented in this work. Deviations close to the critical point can be understood as a consequence of the inability to enforce virial‐fluctuation consistency in the neighborhood of the spinodal decomposition curve.

Monte Carlo simulations of liquid n-butane

We have carried out Monte Carlo (MC) simulations for a model of liquid n-butane in three thermodynamic states. In each case, both the flexible model in the limit when bond-length and bond-angle forces become infinite and the constrained model were studied. The different trajectories followed by each model lead to different ensemble averages for the intramolecular properties, such as the conformational distribution and intramolecular energy. The changes observed in the external configurational properties, such as the intermolecular energy and pressure are less than the statistical error. In all the cases we have found a weak shift of the trans conformational population with respect to the ideal-gas distribution (i.e. less than 3%). In addition, we have explored the possibilities of a purely repulsive model as a reference system for this liquid. As in the case of simple atomic liquids, the standard hard-core theoretical approaches are good starting points for obtaining the contribution of the external degre...

Monte Carlo simulation of liquid n‐alkanes. I. Intramolecular structure and thermodynamics

The conformational properties of liquid n‐alkanes (ranging from n‐pentane to n‐decane) have been investigated using Monte Carlo computer simulation techniques. The method of simulation combines the ‘‘reptation’’ method with a scheme of preferential sampling, which leads to an improvement of the simulation efficiency. The change of internal properties and structure as an effect of the density is studied.

Low density equation of state of asymmetric hard sphere mixtures

The equation of state of highly asymmetric hard sphere mixtures at low and moderate densities is investigated using NpT Monte Carlo simulations. The discrepancies of the estimated fifth virial coefficient with the reported values of Saija et al. (Saija, F., Fiumara, G. and Giaquinta, P. V., 1996, Molec. Phys., 89, 1181) led us to compute such a coefficient. The new results are fully compatible with the compressibility factors estimated by Monte Carlo simulation and show excellent agreement with the predicted coefficients from the Boublik and Mansoori et al. equation of state.

Statistical mechanics of small chain molecular liquids. I. Conformational properties of modeled n‐butane

The density functional formalism can also be profitably applied to the statistical mechanics of molecular fluids with internal degrees of freedom. The change of the intrinsic chemical potential for different conformers of nonpolar molecules is given by a zeroth order perturbation approach. This approach is applied to study the isomerization of n‐butane in liquids, which models the neat liquid and CCl4 solutions. We find that a nonpolar solvent medium produces a shift in the conformational equilibria of model liquid n‐butane from that found in the gas phase. The theoretical predictions show a good agreement with recent Monte Carlo and molecular dynamics simulations results. Discrepancies with Jorgensen et al.’s data are explained in terms of the attractive intermolecular forces used in their MC codes.

Structure and dynamics of selenium chain melts: A molecular dynamics study

A molecular dynamics (MD) study of liquid selenium modeled by 16 linear chains of 40 monomers each is presented. The simulated thermodynamic state corresponds to the experimental density of 3570 Kgu2009m−3 at 873 K. The structural and force constant data of the chains were obtained from previous studies of neutron diffraction experiments, lattice dynamics, and first principles calculations. The computed structural properties show a good agreement with available neutron scattering data. The flexibility of the chains and the high temperature thermodynamic state of the liquid enabled the observation of fast torsional motions and different spatiotemporal dynamic ranges, which can be described by the Rouse model for dense polymer solutions. We identify the crossover from an atomic to an intermediate or ‘‘universal’’ chain regime, and subsequently to global chain behaviors. The dynamics of the system is discussed in terms of time and space‐dependent transport coefficients. The generated MD trajectory thus provides ...

Statistical mechanics of small chain molecular liquids. II. Structure and thermodynamic properties of modeled n‐butane liquid

The theory of molecular liquids based on the extended reference interaction site model (RISM) equation is used to describe the intermolecular structure of a small hydrocarbon chain liquid like n‐butane. The model treats the methyl and methylene groups as single interaction centers with pair interactions modeled by a Lennard‐Jones (12:6) function and located at the position of the carbon nuclei in the molecule. The theory assumes that the equilibrium internal states distribution of nonpolar flexible molecules are determined primarily by the short range, hard repulsive portions of the intermolecular potential (see preceding paper). The molecular structure factor has been calculated from the theory, and the results are in good agreement with recent x‐ray scattering experiments. From this structural information a perturbative scheme to obtain the thermodynamic properties is employed. The theoretical predictions compare favorably with recent simulation results, giving quantitative agreement with the equation s...

Crossover from Hydrodynamic to Atomic Diffusion in Liquid Selenium: A Molecular-Dynamics Study

A molecular-dynamics (MD) study on a model of liquid selenium is presented. The model assumes the existence of linear chains with the required structural and force constant data obtained from experimental data, lattice dynamics and first-principles calculations. The computed properties show an excellent agreement with reported structure factor and density-of-states data. The analysis of the incoherent scattering function evidences the presence of strong, space-dependent effects which are a marked signature of the crossover between hydrodynamic (centre of mass) and atomic diffusion.

The virial coefficients of hard hypersphere binary mixtures

The third, fourth and fifth virial coefficients of hard hypersphere binary mixtures with dimensionality d = 4, 5 have been calculated for size ratios R ≥ 0.1, R = ≡ σ22/σ11, where σ ii is the diameter of component i. The composition independent partial virial coefficients have been evaluated by Monte Carlo integration of the corresponding Mayer modified star diagrams. The results are compared with the predictions of Santos, S., Yuste, S. B., and Lopez de Haro, M., 1999, Molec. Phys., 96, 1 of the equation of state of a multicomponent mixture of hard hyperspheres, and the good agreement gives strong support to the validity of that recipe.