Stability of Colloids in Chemically Reacting Network-Forming Solvent.

We consider a model for a two-component solution which consists of a chemically reacting solvent and a chemically inert hard-sphere solute. A directional associative interaction between the solvent molecules promotes the formation of a network. A multidensity associative Ornstein-Zernike equation is solved analytically using the Percus-Yevick approximation. On the basis of the analysis of the osmotic compressibility, an instability curve for different parameters of the model dispersion is constructed. It is shown that concentration and size of colloids affect the formation of associative complexes in a solvent and directs in such a way the location and behavior of the spinodal curves. The effective interaction between colloids is calculated and discussed. 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 Two-Dimensional Model for Equilibrium Partitioning of a Fluid Mixture through a Microporous Semipermeable Crystalline Membrane. A Monte Carlo Study.

We have considered a simple two-dimensional model for a system consisting of a two-component mixture of hard discs on one side of a microporous slit-like semipermeable membrane and one-component fluid of discs on the other side. The particles of a slit-like membrane are fixed according to either (11) or (10) crystal symmetry. The distance between these particles is chosen such that only one fluid component can permeate the membrane. Osmotic equilibrium in the system is then established. The entire system is confined, for technical convenience, to a wide slit-like pore with the membrane in the center. The walls of the wide pore are distanced from the external surfaces of the membrane to provide the bulk region where the density profiles appear to be constant. Monte Carlo canonical simulation results are presented for the density distributions of the fluid particles in the entire wide pore. We have observed that partitioning of the smaller particles essentially depends on the concentration of the larger particles on one side of the membrane. The osmotic pressure is calculated from the contact values of the density profiles on the walls of a wide pore using the contact theorem. The pressure also has been obtained via Boublik's equation of state for a mixture of hard discs using the bulk densities of species obtained from simulations. The values for the partition coefficients on the osmotic pressure are discussed. Copyright 1998 Academic Press.

Interaction between Colloids in Network Forming Solution.

The effective interaction between colloids in a solvent of hard spheres, which can polymerize with the formation of cross-like molecular clusters, is studied on the basis of the solution of the associative Percus-Yevick/Ornstein-Zernike equation for spatial correlations in an infinitely dilute solution. It is found that at low solvent density and low temperature, the effective intermolecular potential between the colloidal particles is attractive, thus facilitating a phase separation or precipitation of the colloids. A depletion of the local solvent density around the colloidal particle is observed. If the solvent consists of extended associative complexes, compared with hard spheres, the correlation between the colloid particles are of longer range and barrier for flocculation shifts to the larger separations. The effect of size ratio of the colloid spheres and of solvent on the potential of mean force is also discussed. 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