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We have performed molecular dynamics simulations of dense liquids using nonconformal and Gaussian potential models. We investigate the effect of the softness of the potential on the pressure tensor of liquids and dense fluids when the simulations are carried out using parallelepiped cells. The combination of periodic boundary conditions and small cross sectional areas induces an anisotropy in the diagonal components of the pressure tensor. This anisotropy results in an artificial stress in the system that has to be taken into account in simulations of explicit interfaces, where the artificial stress introduces errors in the computation of the surface tension. At high liquid densities the stress anisotropy exhibits an oscillatory dependence with the cross sectional area of the simulation box. We find that the softness of the potential has a dramatic effect on the amplitude of the oscillations, which can be significantly reduced in soft potentials, such as those used in the modeling of hydrocarbon liquids or polymers. [ABSTRACT FROM AUTHOR]

The simulation results of surface tension at the liquid-vapor interface are presented for fluids interacting with Lennard Jones and square-well potentials. From the simulation of liquids we have reported [M. González-Melchor et al., J. Chem. Phys. 122, 4503 (2005)] that the components of pressure tensor in parallelepiped boxes are not the same when periodic boundary conditions and small transversal areas are used. This fact creates an artificial oscillatory stress anisotropy in the system with even negative values. By doing direct simulations of interfaces we show in this work that surface tension has also an oscillatory decay at small surface areas; this behavior is opposite to the monotonic decay reported previously for the Lennard Jones fluid. It is shown that for small surface areas, the surface tension of the square-well potential artificially takes negative values and even increases with temperature. The calculated surface tension using a direct simulation of interfaces might have two contributions: one from finite-size effects of interfacial areas due to box geometry and another from the interface. Thus, it is difficult to evaluate the true surface tension of an interface when small surface areas are used. Care has to be taken to use the direct simulation method of interfaces to evaluate the predicted surface tension as a function of interfacial area from capillary-wave theory. The oscillations of surface tension decay faster at temperatures close to the critical point. It is also discussed that a surface area does not show any important effect on coexisting densities, making this method reliable to calculate bulk coexisting properties using small systems. [ABSTRACT FROM AUTHOR]

Finite size effects due to periodic boundary conditions are investigated using computer simulations in the canonical ensemble. We study liquids with densities corresponding to typical liquid coexistence densities, and temperatures between the triple and critical points. The components of the pressure tensor are computed in order to analyze the finite size effects arising from the size and geometry of the simulation box. Two different box geometries are considered: cubic and parallelepiped. As expected the pressure tensor is isotropic in cubic boxes, but it becomes anisotropic for small noncubic boxes. We argue this is the origin of the anomalous behavior observed recently in the computation of the surface tension of liquid-vapor interfaces. Otherwise, we find that the bulk pressure is sensitive to the box geometry when small simulation boxes are considered. These observations are general and independent of the model liquid considered. We report results for liquids interacting through short range forces, square well and Lennard-Jones, and also long range Coulombic interactions. The effect that small surface areas have on the surface tension is discussed, and some preliminary results at the liquid vapor-interface for the square well potential are given. [ABSTRACT FROM AUTHOR]

Properties of the liquid–vapor interface of square-well fluids with ranges of interaction λ=1.5, 2.0, and 3.0 are obtained by Monte Carlo simulations and from square-gradient theories that combine the Carnahan–Starling equation of state for hard spheres with the second and third virial coefficients. The predicted surface tensions show good agreement with the simulation results for λ=2 and for λ=3 in a temperature range reasonably close to the critical point, 0.8≤T/Tc≤0.95. As expected, the surface tension increases with the range of interaction and decreases monotonically with temperature. A comparison between theory and simulation results is also given for the width of the interface and for the coexistence curves for the different interaction ranges. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The formation of cavities in ionic liquids in the vicinity of the liquid binodal curve is investigated by means of Monte Carlo simulations of the restricted primitive model (RPM). Analysis of the cavity size distribution functions provides a quantitative view of the hole sizes arising in ionic liquids when approaching the coexistence region. Cavities of sizes 0.1–1 nm are formed, the larger cavities being favored by the Coulombic forces. The mean cavity size grows with the square root of the temperature. We compute the reversible work needed to create a cavity in the ionic liquid and it is used to estimate the surface tension of the ionic liquid–vapor interface. The accuracy of theoretical approaches based on the scaled particle theory and Ornstein–Zernike equation to estimate the cavity work of formation in ionic liquids is discussed. We find that both simulations and integral equations predict density depletion with increasing cavity size, suggesting the existence of surface drying in ionic liquids. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]