Non-hydrodynamic transverse collective excitations in hard-sphere fluids.

Collective excitations in hard-sphere fluids were studied in a wide range of wave numbers and packing fractions η by means of molecular dynamics simulations. We report the observation of non-hydrodynamic transverse excitations for packing fractions η≥0.395 in the shape of transverse current spectral functions. Dispersion of longitudinal excitations in the whole range of packing fractions shows a negative deviation from the linear hydrodynamic law with increasing wave numbers even for dense hard-sphere fluids where the transverse excitations were observed. These results do not support a recent proposal within the "Frenkel line" approach that the positive sound dispersion in liquids is defined by transverse excitations. We report calculations of the cutoff "Frenkel frequencies" for transverse excitations in hard-sphere fluids and discuss their consistency with the estimated dispersions of shear waves.

An improved first-order mean spherical approximation theory for the square-shoulder fluid.

The theory, which utilizes an exponential enhancement of the first-order mean spherical approximation (FMSA) for the radial distribution functions of the hard-core plus square-well fluid, is adopted to study the properties of the simplest model of the core-softened fluids, i.e., the hard spheres with a square-shoulder interaction. The results for structure and thermodynamic properties are reported and compared against both the Monte Carlo simulation data as well as with those obtained within the conventional FMSA theory. We found that in the region of low densities and low temperatures, where the conventional FMSA theory fails, the exponential-based FMSA theory besides being qualitatively correct also provides with a notable quantitative improvement of the theoretical description.

Direct correlation function for complex square barrier-square well potentials in the first-order mean spherical approximation.

The direct correlation function of the complex discrete potential model fluids is obtained as a linear combination of the first-order mean spherical approximation (FMSA) solution for the simple square well model that has been reported recently [Hlushak et al., J. Chem. Phys. 130, 234511 (2009)]. The theory is employed to evaluate the structure and thermodynamics of complex fluids based on the square well-barrier and square well-barrier-well discrete potential models. Obtained results are compared with theoretical predictions of the hybrid mean spherical approximation, already reported in the literature [Guillen-Escamilla et al., J. Phys.: Condens. Matter 19, 086224 (2007)], and with computer simulation data of this study. The compressibility route to thermodynamics is then used to check whether the FMSA theory is able to predict multiple fluid-fluid transitions for the square barrier-well model fluids.

Direct correlation function of the square-well fluid with attractive well width up to two particle diameters.

Analytical expression for direct correlation function of the square-well fluid with an attractive well width up to two particle diameters (2 < lambda < or = 3) is reported. This result is obtained within the first-order mean-spherical approximation (FMSA) and represents the nontrivial extension of the recent study due to Tang [J. Chem. Phys. 127, 164504 (2007)], where the width of square-well attraction was limited by one particle diameter (1 < lambda < or = 2). Prediction of the FMSA theory is validated by direct comparison against Monte Carlo simulation data. Additionally, an impact of the increase in the range of attraction on the parameters of the critical point of the square-well fluid is discussed using the compressibility route to thermodynamics.

Entropically driven ordering in a binary colloidal suspension near a planar wall.

The local ordering of a binary hard-sphere mixture with a size ratio 1:10 near a planar wall is investigated by means of integral equation theory. We find that when the bulk volume fraction of the smaller particles is greater than 15%, the larger particles (at a bulk volume fraction of 1% and higher) become highly localized on the wall surface, forming a quasi-two-dimensional surface-localized monolayer. Our results are discussed and compared against computer simulation data with an effective one-component Hamiltonian that is based on sphere-sphere and sphere-wall depletion potentials.

Partitioning of Polymerizing Fluids in Random Microporous Media: Application of the Replica Ornstein-Zernike Equations.

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.

Adsorption of a Hard Sphere Fluid in Disordered Microporous Quenched Matrix of Short Chain Molecules: Integral Equations and Grand Canonical Monte Carlo Simulations.

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 Molecular Theory of the Hydration Force in an Electrolyte Solution.

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.

Liquid-Vapor Coexistence in the Screened Coulomb (Yukawa) Hard Sphere Binary Mixture in Disordered Porous Media: The Mean Spherical Approximation.

The thermodynamics of a two-component fluid with a hard core interaction and screened Coulomb (Yukawa) interaction between particles, similar to the primitive model of an electrolyte solution, adsorbed in a disordered matrix of hard spheres, is studied by using replica Ornstein-Zernike integral equations and the mean spherical approximation (MSA). The gas-liquid transition is localized. The coexistence curve is investigated dependent on the range of interaction between fluid species, on matrix density, and on fluid-matrix attraction. We have observed shrinking of the coexistence envelope with increasing matrix density. The critical temperature of adsorbed mixture decreases with increasing matrix density. The critical density is less affected; however, it also decreases slightly. The critical temperature is sensitive to the fluid species-matrix attraction and depends nonmonotonously on their strength. For a given matrix microporosity, it increases slightly and then decreases with augmenting strength of fluid-matrix attraction. The critical density is less affected by this attraction. However, it decreases for the model with a sufficiently long-range tail of fluid-matrix attraction. Copyright 1998 Academic Press.

Monte Carlo Study of Chemically Associating Polymerizing Two-Dimensional Fluids

NVT Monte Carlo simulations are reported for chemically associating two-dimensional fluids which can polymerize for certain interaction energies, due to the presence of two attractive sites per monomeric hard disc. The sites are fixed inside a hard core at a given valence angle and mutual penetration of discs is permitted. The type of products of polymerization depends on the parameters of the model; we observe the formation of small associates, as well as of extended chains, bent chains, and rings with different number of monomers. The values of valence angles and of association energy are of primary importance. The dependence of the structural properties of the model on fluid density and association energy is investigated. We performed detailed analysis of the clusters formed due to association in terms of fractions of singly and doubly bonded particles, of average numbers of chains and rings, and of their size. We also obtain the average end to end distance, the radius of gyration, and the persistent length of the products of polymerization. The pressure is calculated from the density profiles of particles of the polymerizing fluid near a hard "wall." The data can be used to develop the equation of state for chemically associating two-dimensional fluids. Copyright 1997 Academic Press. Copyright 1997Academic Press

Adsorption of a Hard Sphere Fluid in a Disordered Polymerized Matrix: Application of the Replica Ornstein-Zernike Equations

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.