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Abstract: A detailed molecular-dynamics (MD) simulation study of the interfacial tension of the n-heptane+perfluoro-n-hexane (C 6F14 + C 7H16) binary mixture is carried out at different temperatures and compositions for the vapour–liquid and liquid–liquid interfaces. The molecules are represented as chains of united-atom segments using previously reported force-field pure-component parameters with no further modification, while the unlike interaction energy between the alkyl and perfluoroalkyl segments is adjusted to describe the experimental data for the fluid phase equilibria. In the case of the vapour–liquid interface a horizontal inflection at a critical composition, sometimes referred to as aneotropy, is represented by our model as found in experimental work. Interfacial tensions for both the liquid-liquid and vapour-liquid interfaces are determined using the test-area method. A prerequisite of the determination of the interfacial tension by direct MD simulation involves studies of the fluid phase equilibria (vapour–liquid (VLE) and liquid–liquid equilibria (LLE)). A unique unlike energetic interaction parameter is adjusted to improve the prediction of the LLE and used in all the other simulations. The resulting description of the fluid phase behaviour is compared with that calculated using an equation of state (SAFT), and with experimental data where available; good agreement is observed. [Copyright &y& Elsevier]

An efficient and versatile method to calculate the components of the pressure tensor for hard-body fluids of generic shape from the perspective of molecular simulation is presented. After due consideration of all the possible repulsive contributions exerted by molecules upon their surroundings during an anisotropic system expansion, it is observed that such a volume change can, for non-spherical molecules, give rise to configurations where overlaps occur. This feature of anisotropic molecules has to be taken into account rigorously as it can lead to discrepancies in the calculation of tensorial contributions to the pressure. Using the condition of detailed balance as a basis, a perturbation method developed for spherical molecules has been extended so that it is applicable to non-spherical and non-convex molecules. From a series of 'ghost' anisotropic volume perturbations the residual contribution to the components of the pressure tensor may be accurately calculated. Comparisons are made with prior methods and, where relevant, results are evaluated against existing data. For inhomogeneous systems this method provides a particularly convenient route to the calculation of the interfacial tension (surface free energy) from molecular simulations. [ABSTRACT FROM AUTHOR]

In this contribution we provide a review and reformulation of perturbation theories (generalised van der Waals approaches) for the description of the fluid phase behaviour and orientational ordering transitions of thermotropic nematic liquid crystals. Free-energy functionals of the basic Onsager form are used as the platform for the development of a general formulation that reduces to the specific forms of the various theories that have found common use. A novel closed analytical description of the thermodynamic properties and degree of nematic order is then obtained by employing the Onsager trial function to represent the orientational distribution function in terms of a single parameter. The latter essentially constitutes an algebraic equation of state for the nematic phase appropriate for use in engineering applications. The description of ordering transitions with scaled-Onsager theories and suitable trial functions has already been illustrated by its application to systems of hard spherocylinders (HSCs), indicating that the approach provides an excellent representation of the orientational order of the hard-core (athermal) system [Mol. Phys. 106, 649 (2008)]. Here, the hard-body model is extended to account for attractive interactions (treated at the van der Waals level) of a general isotropic/anisotropic form (e.g., Lennard-Jonesium (LJ), square-well (SW), Maier-Saupe (MS), etc.). The adequacy of our generalised van der Waals-Onsager theory is exemplified by an analysis of the vapour-liquid, liquid-nematic, and vapour-nematic phase equilibria for hard spherocylinders with attractive square-well interactions (HSC-SW). The effect of the potential range and molecular aspect ratio on the vapour-liquid-nematic equilibria and orientational ordering transitions is examined to investigate the van der Waals limit (corresponding states) of the ordering phase behaviour. In the case of systems with an aspect ratio of ∼5 the corresponding-states limit is reached when the range is about 16 times the molecular diameter. For progressively longer molecules with an attractive range that follows their long dimension, the fluid-nematic equilibrium is enhanced to the point that the vapour-liquid boundary becomes metastable relative to fluid-nematic equilibria. In the case of molecules of very large aspect ratio (∼50), an additional region of nematic-nematic coexistence is exhibited by the system. [ABSTRACT FROM AUTHOR]

A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linear-chain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an "aromatic-like" nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature-composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction energy parameter is employed for each solvent to reproduce experimental solubility behavior; this is sufficient for the quantitative prediction of both upper and lower critical solution points and the transition to the characteristic "hourglass" phase behavior for these systems. [ABSTRACT FROM AUTHOR]