Virial expansions and augmented van der Waals approach: Application to Lennard-Jones-like Yukawa fluid

We argue that recently proposed [Melnyk et al., Fluid Phase Equilibr., 2009, Vol. 279, 1] a criterion to split the pair interaction energy into two parts, one of which is forced to be responsible the most accurate as possible for excluded volume energy in the system, results in expressions for the virial coefficients that improve the performance of the virial equation of state in general, and at subcritical temperatures, in particular. As an example, application to the Lennard-Jones-like hard-core attractive Yukawa fluid is discussed.Comment: 12 pages, 6 figure

Improved first order mean spherical approximation for simple fluids

A perturbation approach based on the first-order mean spherical approximation (FMSA) is proposed. It consists in adopting a hard-sphere plus short-range attractive Yukawa fluid as the novel reference system, over which the perturbative solution of the Ornstein-Zernike equation is performed. A choice of the optimal range of the reference attraction is discussed. The results are compared against conventional FMSA/HS theory and Monte-Carlo simulation data for compressibility factor and vapor-liquid phase diagrams of the medium-ranged Yukawa fluid. Proposed theory keeps the same level of simplicity and transparency, as the conventional FMSA/HS approach does, but shows to be more accurate.Comment: 8 pages, 3 figure

Mean spherical approximation for the Lennard-Jones-like two Yukawa model: Comparison against Monte Carlo data

Monte Carlo simulation studies are performed for the Lennard-Jones like two Yukawa (LJ2Y) potential to show how properties of this model fluid depend on the replacement of the soft repulsion by the hard-core repulsion. Different distances for the positioning of hard core have been explored. We have found, that for temperatures that are slightly lower and slightly higher of the critical point temperature for the Lennard-Jones fluid, placing the hard core at distances that are shorter than zero-potential energy is well justified by thermodynamic properties that are practically the same as in original LJ2Y model without hard core. However, going to extreme conditions with the high temperature one should be careful since presence of the hard core provokes changes in the properties of the system. The later is extremely important when the mean spherical approximation (MSA) theory is applied to treat the Lennard-Jones-like fluid.Comment: 11 pages, 13 figure

Phase Behavior of Colloidal Superballs: Shape Interpolation from Spheres to Cubes

The phase behavior of hard superballs is examined using molecular dynamics within a deformable periodic simulation box. A superball's interior is defined by the inequality $|x|^{2q} + |y|^{2q} + |z|^{2q} \leq 1$, which provides a versatile family of convex particles ($q \geq 0.5$) with cube-like and octahedron-like shapes as well as concave particles ($q < 0.5$) with octahedron-like shapes. Here, we consider the convex case with a deformation parameter q between the sphere point (q = 1) and the cube (q = 1). We find that the asphericity plays a significant role in the extent of cubatic ordering of both the liquid and crystal phases. Calculation of the first few virial coefficients shows that superballs that are visually similar to cubes can have low-density equations of state closer to spheres than to cubes. Dense liquids of superballs display cubatic orientational order that extends over several particle lengths only for large q. Along the ordered, high-density equation of state, superballs with 1 < q < 3 exhibit clear evidence of a phase transition from a crystal state to a state with reduced long-ranged orientational order upon the reduction of density. For $q \geq 3$, long-ranged orientational order persists until the melting transition. The width of coexistence region between the liquid and ordered, high-density phase decreases with q up to q = 4.0. The structures of the high-density phases are examined using certain order parameters, distribution functions, and orientational correlation functions. We also find that a fixed simulation cell induces artificial phase transitions that are out of equilibrium. Current fabrication techniques allow for the synthesis of colloidal superballs, and thus the phase behavior of such systems can be investigated experimentally.Comment: 33 pages, 14 figure

Potential of mean force between ions in infinitely diluted simple short-range models of aqueous electrolytes

As an attempt to assess the effect of the long-range electrostatic interactions in solutions of electrolytes, a simple short-range model (SSM) of electrolytes made up of primitive water and primitive ions (i. e., ions whose Coulombic interaction with water has been replaced by a triangular-well interaction) has been considered to compute the potential of mean force. The sizes of the primitive ions have been set so as to approximate realistic NaCl, LiI, and CsCl electrolytes. It is shown that despite the missing longrange Coulombic interaction the model captures the basic features of real electrolytes while the indirect, i.e. water mediated, potential of mean force in the SSM is in qualitative agreement with that of realistic models.Обчислення потенціалу середньої сили для спрощеної моделі з короткодіючими взаємодіями (кулонівська взаємодія між іонами і водою замінена взаємодією трикутної ями) розглядається як спроба визначити ефект далекодіючих електростатичних взаємодій у розчинах електролітів. Розміри таких розчинених іонів вибирались таким чином, щоб апроксимувати реальні NaCl, LiI, та CsCl розчини електролітів. Показано, що, незважаючи на нехтування далекодіючими кулонівськими взаємодіями, модель відображає основні характеристики реальних розчинів електролітів, тоді як непряма взаємодія між іонами опосередкована водою, тобто потенціал середньої сили якісно добре узгоджений з результатами реалістичних моделей

Virial coefficients and vapor-liquid equilibria of the EXP6 and 2-Yukawa fluids

Virial coefficients B₂ through B₄ and the vapor-liquid equilibria for the EXP6 and 2-Yukawa (2Y) fluids have been determined using numerical integrations and Gibbs ensemble simulations, respectively. The chosen 2Y models have been recently determined as an appropriate reference fluid for the considered EXP6 models.Вiрiальнi коефiцiєнти в iд B₂ до B₄ i фазова рiвновага пара - рiдина у EXP6 та 2-Юкава (2Y) плинах розрахованi, вiдповiдно, з допомогою чисельного iнтегрування та на основi комп’ютерного експерименту з використанням ансамблю Гiбса. Вибранi 2Y модельнi системи нещодавно були запропонованi як базиснi для EXP6 плинiв, що розглядаються

Higher order glass-transition singularities in colloidal systems with attractive interactions

The transition from a liquid to a glass in colloidal suspensions of particles interacting through a hard core plus an attractive square-well potential is studied within the mode-coupling-theory framework. When the width of the attractive potential is much shorter than the hard-core diameter, a reentrant behavior of the liquid-glass line, and a glass-glass-transition line are found in the temperature-density plane of the model. For small well-width values, the glass-glass-transition line terminates in a third order bifurcation point, i.e. in a A_3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail) singularity at a critical well width. Close to the A_3 and A_4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.Comment: 19 pages, 12 figures, Phys. Rev. E, in prin

Molecular Modeling and Simulation: Force Field Development, Evaporation Processes and Thermophysical Properties of Mixtures

To gain physical insight into the behavior of fluids on a microscopic level as well as to broaden the data base for thermophysical properties especially for mixtures, molecular modeling and simulation is utilized in this work. Various methods and applications are discussed, including a procedure for the development of new force field models. The evaporation of liquid nitrogen into a supercritical hydrogen atmosphere is presented as an example for large scale molecular dynamics simulation. System-size dependence and scaling behavior are discussed in the context of Kirkwood-Buff integration. Further, results for thermophysical mixture properties are presented, i.e. the Henry’s law constant of aqueous systems and diffusion coefficients of a ternary mixture