Closed-loop phase equilibria of a symmetrical associating mixture of square-well molecules examined by Gibbs ensemble Monte Carlo simulation

A closed loop of liquid-liquid immiscibility for a simple model binary symmetrical mixture of square-well monomers with a single short-ranged interaction site has been recently observed using the Gibbs ensemble Monte Carlo technique @L. A. Davies, G. Jackson, and L. F. Rull, Phys. Rev. Lett. 82, 5285 ~1999!#. This model system has unfavorable mean-field interactions between unlike components which leads to phase separation at intermediate temperatures; the addition of a directional bonding site leads to association and miscibilty of the system at low temperatures. In this work we present a detailed study of the effect of a variation in pressure and of the strength of the bonding interaction on the phase equilibria of such a model system by Gibbs ensemble simulation. The phase diagram is dominated by regions of liquid-liquid immiscibility which are bounded at high temperatures by an upper critical solution temperature and by a lower critical solution temperature ~LCST! for specific values of the pressure and association strength. This closed-loop region is seen to increase in size as the pressure of the system is increased. For weak bonding interaction strengths the system does not possess a LCST and is seen to exhibit regions of two-phase vapor-liquid coexistence which are separated from the region of liquid-liquid immiscibility by a three-phase line. The phase equilibria of the same model system is also determined using the statistical associating fluid theory as adapted for potentials of variable range; the theory provides a good description of the closed-loop immiscibility and other features of the phase diagram

Effect of dipolar interactions on the phase behavior of the Gay–Berne liquid crystal model

A computer simulation study of the phase behavior of the dipolar Gay–Berne liquid crystal model is presented. The phase transitions are determined with isothermal–isobaric ~NPT! Monte Carlo simulations, utilizing the reaction field method. The electrostatic forces are found to have a considerable effect on the nature of the observed phases, but the density at which the isotropic fluid becomes unstable with respect to partially ordered phases is seen to be remarkably insensitive to the strength of the dipole. We pay particular attention to the structure of the mesophases, combining information from several singlet and pair distribution functions to build up an accurate picture of the molecular arrangement of the systems

Isothermal molecular-dynamics calculations

We have performed long-time runs of molecular-dynamics computer simulations of a two-dimensional Lennard-Jones system, without any scaling procedure. The thermodynamic properties show spontaneous fluctuations except when the system is far from the melting zon

Computer simulation of vapor-liquid equilibria of linear dipolar fluids: Departures from the principle of corresponding states

Liquid-vapor equilibrium of linear dipolar fluids has been determined by using the Gibbs ensemble simulation technique. Several elongations and values of the dipole moment were considered. Dipole moment increases the critical temperature and affects slightly the critical density and pressure. Compressibility factor at the critical point decreases as the dipole moment of the molecule increases. Dipole moment provokes deviations from the principle of corresponding states. It is shown that the temperature-density coexistence curve is broadened and that the slope of the vapor pressure curve increases with increasing dipole moment. We propose a new way of reducing the dipole moment so that the increase of the critical temperature becomes almost independent on the molecular elongation. We have also obtained the vapor-liquid equilibrium of models having both a dipole and a quadrupole moment. The obtained data were used to describe the behavior of some relatively complex fluids, namely, 1,1,1-trifluoroethane and 2,2,2-trifluoroethanol. Good agreement for coexistence densities and pressures was obtained. The results presented in this work for linear dipolar fluids along with previous work on linear quadrupolar fluids provide a very comprehensive view of the effect of polar forces on the vapor-liquid equilibrium of linear fluids

Molecular-dynamics ensembles: Fluctuations and correlations near the phase transitions

Computer simulations of liquid and solid systems very close to the melting-freezing transition zone have been performed for the microcanonical, canonical, and isothermal-isobaric molecular-dynamics ensembles. Temperature, pressure, and density fluctuations were studied over long evolution times, and graphical and analytical statistical-error methods were used to investigate correlations in the data. The Nosé-Hoover (NH) method combined with the Toxvaerd algorithm is proposed as a correct method of obtaining the true fluctuation and correlation of the thermodynamic variables in the system, because the temperature and/or pressure constraints in the NH method do not affect the dynamical evolution of the system, and because the fifth-order Toxvaerd algorithm gives very accurate behavior for the correlations, as has been shown in recent studies.Comisión Interministerial de Ciencia y Tecnología PB92-052

Liquid crystal behavior of the Kihara fluid

The liquid crystal phases of the Kihara fluid have been studied in computer simulations. The work focuses on the isotropic–nematic–smectic-A triple point region, especially relevant for the understanding of the properties and the design of real mesogens with specific phase diagrams. The Kihara interaction resembles more appropriately than other related models, the shape of elongated polymers and biomolecules, and a closer assertion is provided for the role of the configurational entropy and the dispersive interactions in the behavior of such molecules in dense phases or under macromolecular crowding conditions.Dirección Genaral de Investigación Científico y Técnica BQU2001-3615-C02Instituto de Salud Carlos III 01/1664Plan Andaluz de Investigación FQM-205, FQM-31

A novel orientation-dependent potential model for prolate mesogens

An intermolecular potential is introduced for the study of molecular mesogenic fluids. The model combines distinct features of the well-known Gay-Berne and Kihara potentials by incorporating dispersive interactions dependent on the relative pair orientation to a spherocylinder molecular core. Results of a Monte Carlo simulation study focused on the liquid crystal phases exhibited by the model fluid are presented. For the chosen potential parameters, molecular aspect ratio L*55 and temperatures T*52, 3, and 5, isotropic, nematic, smectic-A, and hexatic phases are found. The location of the phase boundaries as well as the equation of state of the fluid and further thermodynamical and structural parameters are discussed and contrasted to the Kihara fluid. In comparison to this latter fluid, the model induces the formation of ordered liquid crystalline phases at lower packing fractions and it favors, in particular, the appearance of layered hexatic ordering as a consequence of the greater attractive interaction assigned to the parallel side-to-side molecular pair configurations. The results contribute to the evaluation of the role of specific interaction energies in the mesogenic behavior of prolate molecular liquids in dense environments

Monte Carlo study of liquid crystal phases of hard and soft spherocylinders

We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: ~a! hard spherocylinders with an attractive square-well ~SWSC! and ~b! purely repulsive soft spherocylinders ~SRS!, in both cases for a length-to-breadth ratio L*55. Monte Carlo simulations in the isothermal–isobaric ensemble have been performed at a reduced temperature T*55 probing thermodynamic states within the isotropic ~I!, nematic ~N!, and smectic A ~Sm A! regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic–nematic transition of the hard spherocylinder ~HSC! fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic–smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I–N and N–Sm A transitions to higher pressures and densities. Moreover, the soft repulsive core of the SRS fluid induces a similar but quite more significant shift of both of these phase boundaries toward higher densities. This latter effect is apparently in contrast with very recent studies of the SRS fluid at lower temperatures, but this discrepancy can be traced back to the different effective size of the molecular repulsive core at different temperatures

Interplay between anchoring and wetting at a nematic-substrate interface

We use a generalized van der Waals molecular theory to study a model substrate-nematic interface in the regime of complete wetting and in a situation of competing interactions at the interface. The analysis shows that an anchoring transition between states with planar and homeotropic director configuration may play the role of a prewetting transition, and that reentrant anchoring must generally occur. As a result one expects complete wetting in a finite temperature range. The study provides a general context within which anchoring and wetting phenomena can be relate