Christian Regnaut

Hard core Yukawa fluid with temperature and density dependent interaction: Phase diagram of the AOT/water/decane microemulsion

A fluid with an interaction potential consisting of a hard core and an attractive Yukawa tail is considered. The strength of the attraction is taken to depend both on density and temperature in order to take into account the state dependence of the effective interaction suggested by the analysis of structural data of several colloidal suspensions, in particular microemulsions of AOT reverse micelles. The thermodynamics of this fluid is investigated starting from the inverse temperature expansion of the free energy in the mean spherical approximation. The temperature and density dependence of the interaction is incorporated in consistent expressions for the pressure, isothermal compressibility, and chemical potential. The phase diagram predicted by this method is in agreement with experimental data on AOT reverse micelles analyzed in the effective one component approach.

SURFACE LAYERS OVERLAP AND EFFECTIVE ADHESION IN REVERSE MICELLES : A DISCUSSION FROM THE ADHESIVE SPHERES MIXTURE MODEL

Structural data on reverse micelles from the literature are analyzed from the adhesive spheres mixture model in the Percus–Yevick approximation. The solvent is modeled by a fluid of pseudoadhesive spheres with same compressibility and coordination number as a square well fluid. The attractive part of the direct intermicellar potential is taken proportional to the overlap volume of interpenetrating surface layers. The micelle–solvent coupling is characterized by a square or triangular well and a constraint on the packing fractions. A very good correlation is then found between the micelles stickiness parameters computed from the actual potentials via the equality of second virial coefficients and those determined from a fit to experiment. The change in effective adhesion observed with micelles of different surface composition and in the same solvent can then be associated to the change in the direct intermicellar interaction at nearly unchanged solvent–micelle interactions. The interpretation of the role o...

On the analysis of the effective adhesion in colloidal dispersions in the sticky hard sphere model

Structural trends in multicomponent mixtures of adhesive spheres are analyzed by using the Baxter formalism and the Percus–Yevick approximation (PYA). The Orsnstein–Zernike (OZ) equations in q space are cast in a form which allows a fully analytical expression of the effective adhesiveness coefficient of the large (solute) spheres in the asymptotic limit of vanishing size ratio and no solute self‐stickiness. This allows a simple discussion of the factors which determine the effective solute adhesiveness and suspension stability: while the steric effect and like particles stickiness are found to favor suspension instability, hetero stickiness is found to act in one side or in the opposite depending on the concentration of the smaller species. These qualitative predictions are paralleled with studies on solvent effects in ordinary colloidal solute–solvent systems and with the behavior of pseudobinary systems such as colloid–polymer or bidisperse colloidal mixtures. Results from the literature for hard spher...

On some empirical expressions for the contact values of the pair distribution functions and fluid-fluid phase separation in hard sphere mixtures

The fluid-fluid binodal of asymmetric hard sphere mixtures obtained from approximate expressions of the virial pressure is investigated. Also the behaviour of the Gibbs free energy following from particular combinations of standard expressions of the contact values of the pair distributions functions is examined. A recently proposed parametrization of the latter in the colloidal limit is then discussed and compared with existing simulation data for the binodal of the effective fluid.

Effective interaction between reverse micelles: A study from the potential of mean force at infinite dilution

The effective interaction potential between reverse micelles is computed by adding the potential of mean force at infinite dilution to the direct interaction between micelles. The former is computed from an integral equations approach and the latter is modeled by van der Waals and electrostatic contributions. This model is used to discuss the characteristics of the effective interaction—range, strength, and temperature dependence—deduced in previous work from the effective one-component approach. From a detailed analysis of the various contributions to the effective interaction, it is concluded in agreement with other studies, that the relative long range of the interaction is attributed to electrostatic contributions. The moderate interaction strength is found to result from strong cancellations between direct interactions (short range repulsion plus van der Waals attraction between surfactant films) and indirect ones (“depletion” plus “solvation” effects). In contrast with previous studies which attribu...

Adhesive limit of the GMSA as a simple model of hard sphere fluids

The adhesive limit of the GMSA is shown to give an accurate description of the structure of the pure hard sphere fluid if the adhesiveness coefficient is suitably determined. The alternative Percus-Yevick solution is found to represent pseudo adhesive spheres with an effective attraction which depends linearly on the packing fraction. The percolation threshold of such a fluid is found close to the random close packing value of the hard sphere fluid. This simple analytical model can be extended to mixtures of hard spheres with moderate difference of size.

Comment on the contact values in asymmetric hard-sphere mixtures (Mol. Phys. 107, 685 (2009); Mol. Phys. 106, 607 (2008))

We comment on the equation of state of highly asymmetric hard-sphere mixtures obtained from the virial equation using parameterized contact values of the radial distributions functions

Demixion in simple liquid metals alloys comparative investigation of non local and local pseudopotentials: example of LiNa

Using perturbation theory and classical molecular dynamics simulations, we study the static structure and demixing behaviour of the liquid LiNa alloy from the pseudopotential approach and different classes of models. We find that the norm conserving pseudopotential does not lead to demixing while various local models, with few adjustable parameters correctly predict the structure and spinodal unstability in the alloy. Transferability of the pseudopotential to the alloy is improved if the parameters are fitted to some bulk or structural properties of the pure metal. We find that demixion can be predicted when the structure factors of the pure liquid Li and Na are reasonably reproduced from such pseudopotentials.