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S Amokrane

Publications and source records attributed to S Amokrane.

11 recordsLinked to original sources
PubMed2006

Phase transitions in highly asymmetric binary hard-sphere fluids: Fluid-fluid binodal from a two-component mixture theory.

Fluid-fluid binodals of binary hard-sphere mixtures are computed from the recently proposed fundamental measure functional-mean spherical approximation closure of the two-component Ornstein-Zernike equation. The results, especially in the dense fluid region that was not accessible by previous theoretical methods, are compared with the corresponding ones for the one-component fluid of big spheres with effective potential obtained from the same closure. The general trends are those expected for hard-sphere potentials but small difference are detectable. The overall agreement found validates the equivalence of the two descriptions for size ratios R = 8.5 or greater.

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PubMed2005

Structure of highly asymmetric hard-sphere mixtures: an efficient closure of the Ornstein-Zernike equations.

A simple modification of the reference hypernetted chain (RHNC) closure of the multicomponent Ornstein-Zernike equations with bridge functions taken from Rosenfeld's hard-sphere bridge functional is proposed. Its main effect is to remedy the major limitation of the RHNC closure in the case of highly asymmetric mixtures--the wide domain of packing fractions in which it has no solution. The modified closure is also much faster, while being of similar complexity. This is achieved with a limited loss of accuracy, mainly for the contact value of the big sphere correlation functions. Comparison with simulation shows that inside the RHNC no-solution domain, it provides a good description of the structure, while being clearly superior to all the other closures used so far to study highly asymmetric mixtures. The generic nature of this closure and its good accuracy combined with a reduced no-solution domain open up the possibility to study the phase diagram of complex fluids beyond the hard-sphere model.

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PubMed2005

Potential of mean force in confined colloids: integral equations with fundamental measure bridge functions.

The potential of mean force for uncharged macroparticles suspended in a fluid confined by a wall or a narrow pore is computed for solvent-wall and solvent-macroparticle interactions with attractive forces. Bridge functions taken from Rosenfeld's density-functional theory are used in the reference hypernetted chain closure of the Ornstein-Zernike integral equations. The quality of this closure is assessed by comparison with simulation. As an illustration, the role of solvation forces is investigated. When the "residual" attractive tails are given a range appropriate to "hard sphere-like" colloids, the unexpected role of solvation forces previously observed in bulk colloids is confirmed in the confinement situation.

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PubMed2004

When mixtures of hard-sphere-like colloids do not behave as mixtures of hard spheres.

The validity of the concept of "hard-sphere-like" particles for mixtures of colloids is questioned from a theoretical point of view. This concerns the class of pseudobinary mixtures in which the nonsteric interactions between the colloids are "residual" (with very small range and moderate strength). It is shown that contrary to common expectation, such interactions may have unexpected consequences on the theoretical phase diagram. The distinction between this situation and true solute-solvent mixtures is emphasized.

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PubMed2003

Is the binary hard-sphere mixture a good reference system for sterically stabilized colloids?

The relevance of the hard-sphere mixture model as a starting point for the study of sterically stabilized colloids is discussed. Two physical situations are distinguished: true molecular solvent-colloid mixtures, and pseudobinary mixtures of two supramolecular objects. For the former, the limitation of the hard-sphere mixture model are recalled. Its potential use as a reference system for perturbation treatments is then analyzed. The accuracy of the latter is tested numerically. This study shows that the hard-sphere mixture is, in general, not a good reference system for sterically stabilized colloids in molecular solvents. For pseudobinary mixtures, the potential of mean force between the bigger solutes induced by the smaller ones is considered. The influence of a very short-range heteroattraction is discussed.

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PubMed2002

Attractive forces in sterically stabilized colloidal suspensions: from the effective potential to the phase diagram.

The potential of mean force for macroparticles at infinite dilution is computed for several models of solvent-solvent and solvent-macroparticle interactions by using the reference hypernetted chain (RHNC) integral equations with Rosenfeld's density functional theory bridge functions. The phase diagram of the associated effective fluid is obtained from the RHNC free energy for the fluid branch and the perturbation theory for the solid one. The computation of the effective potential and of the fluid branch is tested by comparison with Monte Carlo simulation. The important modifications with respect to the pure hard spheres that were previously reported are confirmed. The possibility of inverting the relative stability of the fluid-fluid and the fluid-solid transitions by appropriate combination of the interaction parameters is shown. The importance of a fine description of the interactions is illustrated in the example of the role of the range of the solvent-solvent interaction potential.

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PubMed2002

Shape of the liquid-vapor coexistence curve for temperature and density dependent effective interactions.

The asymmetry of the coexistence curve that is observed in several micellar systems is discussed in relation with the dependence of the effective interaction on temperature and density. Standard results for the diameter of the coexistence curve in the van der Waals theory are generalized so as to deal with this combined dependence. The qualitative trends so deduced are assessed by comparison with coexistence curves of Yukawa fluids computed with integral equation theories. The role of the variables used to plot the coexistence curve and the nonlinear behavior of its diameter beyond the critical region are discussed in relation with the decrease of the interaction strength with density. The possibility of using the asymmetry of the coexistence curve as an indicator of the state dependence of the effective interaction is finally discussed.

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PubMed2002

Validity of the perturbation theory for hard particle systems with very-short-range attraction.

Motivated by recent studies of colloidal systems at the effective one-component level, the validity of the first-order perturbation theory of classical systems of hard particles interacting via short-range potentials is investigated. The influence of the physical parameters on the accuracy of the perturbation theory is examined. It is shown that this simple method is intrinsically appropriate to describe the fluid-solid transition. Concerning the fluid-fluid one, the first-order perturbation theory provides acceptable results when the interaction range is not too small. For very-short-range potentials it systematically leads to an unphysical fluid-fluid transition. In the case of the depletion interaction between hard sphere solutes such a transition is found even at moderate size asymmetry. It is finally shown that the perturbation theory is not more appropriate for an extended solid than for a liquid with the same density, thus making difficult a quantitative description of the isostructural solid-solid transition.

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PubMed2000

Phase diagram of highly asymmetric binary mixtures: A study of the role of attractive forces from the effective one-component approach

The phase diagram of an asymmetric solute-solvent mixture is investigated at the level of the effective one-component fluid. The solvent is taken into account by computing the potential of mean force between solute particles at infinite dilution for different models of solvent-solvent and solute-solvent short range interactions. Fluid-fluid and fluid-solid coexistence lines are determined from the free energy in the reference hypernetted chain theory for the fluid branch and from a variational perturbation theory for the solid one. The phase boundaries so determined compare well with recently published Monte Carlo data for mixtures of pure hard spheres. The influence of solute-solvent and solvent-solvent short range attractive forces is then investigated. When compared with pure hard core interactions, these forces are found to produce dramatic changes in the phase diagram, especially on the solvent packing fractions at which a dense fluid of solutes can be stable and on the separation of the fluid-fluid and fluid-solid coexistence lines. Finally, the connection of these results with the behavior of some colloidal suspensions is emphasized.

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