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Performance of the SAFT-VR equation of state developed for the hard sphere based simple fluids, namely the square-well, Sutherland and Yukawa fluids, is examined by comparing its results with simulation data and an augmented van der Waals (vdW) equation based on a Yukawa (Y) reference. Its shown that both for the equilibrium vapor–liquid data and data along selected isotherms in the liquid and supercritical fluid phases the vdW(Y) equation provides better results, particularly when going to lower temperatures.

The vapour–liquid phase diagram of long-range Yukawa fluids with the decay parameter zσ ≤ 1 is calculated by means of an augmented van der Waals theory based on the reference system that includes both the repulsive and short-range attractive interaction. Comparison with Monte Carlo simulation data, as well as with the traditional van der Waals approach based on a purely repulsive hard-sphere reference system and other semi-analytical theoretical approaches, is made.

In this study, we show that the inclusion of a short-range part of the total attractive interaction into a reference system allows a natural extension of the traditional first-order perturbation theory of simple fluids to practically all thermodynamic states. This theory is applied to the thermodynamic functions and the liquid–vapor coexistence curve of the Lennard-Jones-like medium range Yukawa fluid and to the Sutherland fluid. Comparison with computer simulation data and the second-order Barker–Henderson perturbation theory is discussed.

The layering of macroions confined to a wedge slit formed by two uncharged hard walls is studied using a canonical Monte Carlo method combined with a simulation cell that contains both wedge-shaped slit and bath regions. The macroion solution is modeled within a one-component fluid approach that in an effective way incorporates the double layer repulsion due to simple electrolyte ions as well as the discrete nature of an aqueous solvent. The layer formation under a wedge confinement is analyzed by carrying out separate simulation runs for a set of consecutive wedge segments designed to represent a single wedge slit. As the wedge thickness progressively increases, the sequence of regions along the wedge film with distinct features of macroion layering has been established. This sequence comprises (i) a wedge region of the thickness smaller than the macroion diameter that is free of macroions; (ii) a region with a one-dimensional macroion chain along the wedge corner at a wedge thickness of a one macroion diameter; (iii) a region comprising a low-ordered macroion monolayer that extends until the wedge thickness slightly above two macroion diameters; (iv) a region comprising a pair of well-defined two-dimensional configurations of macroions segregated on each of the wedge walls; and (v) a free-of-macroions wedge region between two surface monolayers that now originates from an electrostatic repulsion imposed by the surface macroions, which is followed by (vi) a well-defined macroion monolayer film between two surface monolayers, a less defined bilayer film, a three-layer film, and so on up to the bulk solution. Once formed, the macroion surface monolayers persist for all remaining wedge thicknesses up to the bulk, forming in such a way effective charged wedge boundaries. Such a formation of the macroion surface monolayers on the uncharged confining walls is related to the haloing mechanism for regulating the stability in colloidal suspensions [Tohver et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8951] and is discussed as well. Finally, the estimated boundary of the free of macroion region between surface monolayers correlates well with the location of the boundary of the so-called "vacuum" phase that has been observed experimentally in an aqueous suspension of charged polystyrene spheres bounded by electrostatically repulsive glass walls [Pieransky et al. Phys. Rev. Lett. 1983, 50, 900].

A convenient approximation is proposed for the correlation function of a pair of large hard-sphere particles in a bidisperse or binary hard-sphere mixture. This approximation is based on the solution of the Ornstein–Zernike/Percus–Yevick integral equations for a hard-sphere mixture. From this result, we obtain the potential of mean force and interaction force for a pair of colloidal spheres in a bidisperse hard-sphere fluid.

The approach to treat the strong attractive interaction between fluid particles adsorbed into porous media is reported. Results are illustrated by the calculations of liquid/vapor phase diagrams for Lennard-Jones fluid in the presence of quenched disorder. Perspectives of the approach from the comparison with computer simulation data available from the literature are discussed.

Canonical ensemble molecular dynamics and Monte Carlo simulations have been performed to study the vapor/liquid coexistence in a hard-core fluid with an attractive Yukawa interaction. Coexisting densities and pressure along the vapor/liquid coexistence line for different ranges of attractive interaction have been evaluated and found to agree well with the Gibbs ensemble Monte Carlo data reported in the literature. To obtain surface tension, the normal and tangential components of the pressure tensor have been calculated during simulations by using a hybrid molecular dynamics algorithm (which combines the hard-core and continuous forces) and by using an original numerical algorithm for the hard-core contribution to the virial in Monte Carlo simulations. We found that surface tension is strongly dependent on the range of attractive interaction, i.e., it drops when the attraction becomes short-ranged. The relation of the attractive hard-core Yukawa potential to the spherically-truncated Lennard-Jones potenti...

By use of the analytic result for the Laplace transform of the radial distribution function for two large hard spheres dispersed in a fluid of the smaller hard spheres, simple equations for the film interaction energy and film disjoining pressure, due to the structural and depletion forces, are derived. The proposed equations satisfy some known exact results and explicitly express the energy and pressure as a function of the film thickness and volume fraction of the small hard spheres. The predicted results for the film energy and film disjoining pressure are compared with existing computer simulation data. The proposed simple analytical expressions can be applied to understand the stability of liquid films containing colloidal particles and the stability of suspensions, foams, and emulsions.

We have studied the structure and adsorption of a hard sphere fluid in a rigidly fixed array of permeable matrix species. The surface of each matrix particle is represented by a barrier of finite height and width, such that the entire model describes partitioning of a fluid in a set of permeable membranes. We have applied the grand canonical Monte Carlo (GCMC) simulations and replica Ornstein—Zernike (ROZ) equations for partly quenched systems complemented by the Percus—Yevick (PY) closure as our theoretical tools. The pair distribution functions of species and the adsorption isotherms are discussed dependent on the parameters of the model. It is shown that the theory provides adequate description of the behaviour of fluid species in the interior of the matrix particles, inside the barriers, and close to the interface. On the other hand, the coordination numbers for a fluid in the matrix interior and the adsorption isotherms from the ROZ-PY theory are in excellent agreement with computer simulation data.

A model of hard spheres adsorbed in a disordered quenched matrix of chain molecules is studied by using the replica Ornstein-Zernike equations and grand canonical Monte Carlo simulations. The pair distribution functions and the adsorption isotherms are obtained and discussed. The theory agrees well with simulation data. The Percus-Yevick and the hypernetted chain approximations are almost equally adequate for the description of the structure and thermodynamics of adsorbed hard sphere fluid. It is shown that the excluded volume effects of chain matrix, prepared by chemical association mechanism and then quenched, have predominant influence on the adsorption of a hard sphere fluid at fixed matrix packing fraction in matrices of chains with 4, 8, and 16 hard sphere beads. The partitioning coefficient is weakly dependent on the fluid chemical potential at fixed matrix packing. It, however, substantially decreases with decreasing microporosity of the matrix. Copyright 1999 Academic Press.

A model, consisting of a pair of large macroions in a dipolar hard sphere-point ion electrolyte, is considered in order to evaluate the hydration force (solvent-mediated) contribution to the force between colloidal particles, which is missing in the DLVO theory. Using the mean spherical approximation (MSA), an explicit expression for this force is obtained. It is shown that the force consists of the hard-core exclusion term that was proposed recently by Henderson and Lozada-Cassou (HLC) [J. Colloid Interface Sci. 121, 486 (1988)], and a dipole alignment contribution that originates from the orientational ordering of the solvent molecules near the colloidal particles. The long-range asymptotic form of the total force is given by a Coulomb contribution and is described by the Poisson-Boltzmann or Derjaguin-Landau-Verwey-Overbeek (DLVO) result. The hydration force is short-ranged and extends about ten solvent layers and is responsible for the oscillations of the total force. The total force that we obtain is similar to the semiempirical result of HLC. The comparison with the experimental results for a 10(-3) M KCl electrolyte solution is discussed. Copyright 1999 Academic Press.

The adsorption isotherms for diatomic fluids in disordered porous media have been obtained from grand canonical Monte Carlo computer simulation. A disordered porous medium, or matrix, is prepared by quenching an equilibrium configuration of spherical hard core molecules. In addition, canonical Monte Carlo simulations have been performed for equilibrium binary mixtures that represent counterparts of quenched-annealed systems in question. For the same chemical potential of diatomic molecules at a fixed density of matrix species, the structure and the density of the diatomic fluid in the binary mixture and in the quenched medium are very similar. This behaviour holds also for systems with attractive interactions in the region of high temperatures. Observed similarity of the structural and thermodynamic properties of the mixture and the corresponding annealed fluid-quenched matrix system permits the use of the thermodynamics of the mixture to evaluate some properties of quenched-annealed fluids. Calculations ...

The replica Ornstein–Zernike (ROZ) equations, supplemented by the hypernetted chain and mean spherical closures, were solved for an ionic fluid adsorbed in a disordered charged matrix. To obtain the numerical solution of the ROZ equations we performed renormalization of the initial equations. Both the matrix and adsorbed fluid were modeled as charged hard spheres in a dielectric continuum, i.e., in the so-called restricted primitive model. As a result, the pair distribution functions between fluid ions and for fluid-matrix correlations were obtained. Structural properties were studied as a function of the matrix density, the concentration of adsorbed electrolyte and for different prequenching conditions. The isothermal compressibility, excess internal energy, and the chemical potential were calculated and discussed with respect to of the model parameters. Comparison with the Monte Carlo computer simulations of Bratko and Chakraborty [J. Chem. Phys. 104, 7700 (1996)] indicates that the theory yields qualitatively correct results for the model system.

A two-dimensional model for a chemically associating fluid adsorbed in a disordered porous media has been investigated. The model is studied by means of the associative Replica Ornstein-Zernike equations, with Perens-Yevick and hypernetted chain approximations, extended for associating fluids. The effects of disordered media density and strength of association interaction on the adsorption isotherms and structural properties of adsorbed fluid are explored. To investigate the effect of association interaction on the liquid-gas phase transition, the additional out-core interparticle interaction in the form of square-well attraction is included. This term is considered in the mean field approximation. It permits us to analyze qualitative trends of the behavior of the critical density and temperature dependence on the combined effect of association and of quenched disorder in the model of dimerizing discs with square-well attraction. Our preliminary conclusion concerning this unexplored problem is that the presence of disorder and of association have opposite effect on the critical temperature. Manipulating the matrix density and bonding between particles may keep almost unaltered the critical temperature.

The pair distribution functions and the adsorption isotherms for a hard-core dimerizing fluid in disordered hard-sphere matrices have been determined using the associative extension of replica Ornstein-Zernike equations. Some grand canonical Monte Carlo simulation results for tangent hard-sphere dumbbells adsorbed in hard-sphere matrices are presented. Theoretical predictions for the structure from the solutions of integral equations with the associative hypernetted chain (HNC) and Percus-Yevick (PY) approximation agree with those from the simulations. However, the HNC closure has been found to work better than the PY one.

Monte Carlo simulations are reported for the distribution functions, the fraction of unbonded and bonded particles, and the pressure of a chemically associating fluid of hard discs of a circularly symmetric attractive square well. Depending on the choice of the bonding length, dimers or chain molecules of different length can be formed. The dependence of the structural properties of the model on the fluid density and association energy is discussed.

We performed Monte-Carlo computer simulations of a fluid of chemically reacting, or overlapping, hard spheres near a hard wall. The model of the interparticle potential is that introduced by Cummings and Stell. This investigation is directed to the determination of the structure of the fluid at the wall, and the orientation of the dimers in particular. In addition, we applied the singlet Percus–Yevick, hypernetted chain and Born–Green–Yvon equations to calculate the total density profiles of the particles. A comparison with the Monte-Carlo data indicates that the singlet Percus–Yevick theory is superior and leads to results that are in reasonable agreement with simulations for all the parameters investigated. We also calculated the average numbers of dimers formed in the bulk part of the system and the results are compared with different theoretical predictions.

The associative analogue of the integral equation theory developed by Madden and Glandt is applied to a model of methane adsorbed in a silica xerogel. The integral equations are solved in the assoc...

A model of a chemically reacting fluid in a slit-like pore is studied. The density profiles of the fluid particles inside the pore, the adsorption isotherms, and the solvation force acting between the walls of the pore are investigated within the integral equation method. The Percus-Yevick and hypernetted chain approximations for the fluid particle-pore correlations are used. The model for the bulk chemically reacting fluid is that proposed by Cummings and Stell for a mixture of overlapping hard spheres. The investigations are performed for pores of different width, at different densities of the bulk fluid which is in an equilibrium with the confined fluid and for different degrees of bulk fluid association. The bulk fluid is described by using the Percus-Yevick approximation. A wide range of thermodynamic states and different bonding lengths between the reactants are investigated. It is shown that the effect of a chemical reaction in the bulk fluid produces qualitative changes in the behaviour of the den...

A two-Yukawa representation of the EXP6 fluids at supercritical temperatures and high pressures has been developed and examined using molecular simulations. A uniquely defined mapping of the repulsive part of the EXP6 potential curve onto the two-Yukawa potential is used. Two ranges of temperatures, one encountered in geochemical applications (T(geo) range) and the other at conditions of detonations (T(det) range), are considered and it is shown that the local structures of both fluids are practically identical. Deviations between the EXP6 and two-Yukawa potential functions at intermediate separations lead to differences in the thermodynamic properties of the two fluids at lower temperatures of the T(geo) range; at higher temperatures and in the high T(det) temperature range both the structural and thermodynamic properties of the EXP6 and two-Yukawa fluids are practically identical.

A model for the formation of diatomic molecules from the association of spherically symmetric atoms is reported. The model incorporates a flexible intracore chemical bond mimicked by Gaussian potential and Lennard-Jones interatomic interactions. To calculate the correlation functions necessary to describe the chemical equilibrium and phase diagrams, the second-order Percus-Yevick approximation is applied. Some structural properties of the model are discussed in terms of binary and triplet distribution functions.

We have investigated a model for a polymerizing fluid in which each of the particles has two bonding sites, such that chains can be formed via a chemical association mechanism. The fluid model is considered to be in a random quenched microporous matrix. The matrix species are assumed to be either impermeable to adsorbed fluid particles or permeable, such that the surface of the matrix particles represents a permeable membrane of finite width. We have studied the influence of the matrix species on the formation of chains due to association. The model is investigated by means of the associative replica Ornstein-Zernike equations with the Percus-Yevick closure and the ideal chain approximation. We have observed that the average chain length is longer in the presence of an impermeable matrix than in the case where the matrix is absent. Matrix is therefore conducive to the growth of the polymerizing species in micropores. There is a decrease in the average chain length with increasing permeability of matrix species. This behavior reaffirms the attenuating role of the permeable matrix species as a whole. Copyright 1999 Academic Press.

A model of hard spheres adsorbed in disordered porous media is studied using the associative replica Ornstein–Zernike (ROZ) equations. Extending previous studies of adsorption in a hard sphere matrices, we investigate a polymerized matrix. We consider an associating fluid of hard spheres with two intracore attractive sites per particle; consequently chains consisting of overlapping hard spheres can be formed due to the chemical association. This is the generalization of the model with sites on the surface of Wertheim that has been studied in the bulk by Chang and Sandler. The matrix structure is obtained in the polymer Percus–Yevick approximation. We solve the ROZ equations in the associative hypernetted chain approximation. The pair distribution functions, the fluid compressibility, the equation of state and chemical potential of the adsorbed fluid are obtained and discussed. It is shown that the adsorption of a hard sphere fluid in a matrix at given density, but consisting of longer chains of overlapping hard spheres, is higher than the adsorption of this fluid in a hard sphere matrix.

A theoretically based closed-form analytical equation for the radial distribution function, g(r), of a fluid of hard spheres is presented and used to obtain an accurate analytic representation. The method makes use of an analytic expression for the short- and long-range behaviors of g(r), both obtained from the Percus-Yevick equation, in combination with the thermodynamic consistency constraint. Physical arguments then leave only three parameters in the equation of g(r) that are to be solved numerically, whereas all remaining ones are taken from the analytical solution of the Percus-Yevick equation.

A model for a chemically associating fluid, adsorbed in a disordered porous media, is proposed. The formation of the associates occurs through the directional bonding between the fluid particles. For simplicity, we restrict our attention to the dimerization of particles. In the absence of association, this model reduces to that of Kaminsky and Monson (KM) for the adsorption of methane in a xerosilica gel. This model is studied by means of the replica Ornstein–Zernike ROZ equations, with the hypernetted chain approximation, extended for associating fluids. It follows from a comparison with the computer simulation data that this theory yields a very good description of the structural properties of the KM model. The influence of the fluid density, the matrix packing fraction, and the association energy on the dimerization in the disordered matrix is studied. The fluid compressibility for the KM model and for the dimerizing fluid in a disordered matrix is obtained via the compressibility equation.