J. M. Méndez-Alcaraz

Static structure factors of binary suspensions of charged polystyrene spheres: experiment against theory and computer simulation

The microscopic static structure of dilute binary colloidal suspensions, for compositions covering the full molar fraction range, are investigated using static light scattering experiments. The mixtures of liquid-like ordered suspensions are made from charged polystyrene particles having averaged radii of 42 nm and 65 nm. The data are compared with the results of integral equation theories using the extended re-scaled mean spherical approximation and the hypernetted chain approximation and Rogers-Young closure schemes, and to Monte Carlo computer simulations. The values of charges needed in the Yukawa-type pair potentials are determined from fits to the scattering data of the pure components, and they are kept constant for the mixtures. Of the various theoretical schemes used, the Rogers-Young method is found to be the most satisfactory. In order to fully describe the experimental data, it is necessary to take the intrinsic polydispersity of the two pure components into account.

Phase separation of binary liquid mixtures of hard spheres and Yukawa particles

By solving the Ornstein-Zernike equation together with the Rogers-Young closure relation for the correlation functions of binary liquid mixtures of hard spheres and Yukawa particles we calculate the concentration-concentration structure factor, Scc(k), for different values of the diameter ratio and molar fraction of the mixtures. For hard spheres much larger than the Yukawa particles, and for large values of the molar fraction of the latter, we find a divergence of the value of Scc(k) at k = 0, which signals the limit of thermodynamic stability with respect to phase separation. At the same time, from the form of the partial radial distribution functions of systems near to the instability we conclude that the phase separated systems are composed of one phase rich in hard spheres, whose structure may be interpreted in terms of bridge formation, immersed in an other rich in Yukawa particles.

Depletion forces in two-dimensional colloidal mixtures

Depletion forces in homogeneous and inhomogeneous binary colloidal mixtures in two dimensions are accounted for by a theoretical approach based on a contraction of the description of liquid mixtures, as well as by computer simulations. We study the depletion interactions in concentrated binary mixtures of additive and non-additive hard discs. The wall–particle depletion potential for a disc close to a hard wall with a concave curvature, or with a relief pattern, is obtained in the infinitely dilute limit, as well as the depletion potentials in mixtures of hard discs and hard plates.

Structural properties of colloidal suspensions

Structural properties of three- and two-dimensional colloids composed by hard spheres and/or by Yukawa particles, which can have different diameters and charges, are studied by solving the Ornstein-Zernike equation, together with Percus-Yevick, hypernetted chain and Rogers-Young approximations. From the partial radial distribution functions gij(r) the partial structure factors Sij(k) are determined, and with them the compressibility structure factor Sx(k), the measured structure factor SM(k) and the Bhatia-Thornton structure factors SNN(k), SNQ(k) and SQQ(k). As an effect of diameter and/or charge polydispersity on the structure of binary mixtures, the position and height of the main peak of SM(k), and its value at k = 0, change non-monotonously with the composition. In the case of binary mixtures of hard and Yukawa spheres the structure is given by two different scales. A liquid-solid phase transition induced by a change in the dimensionality was found for monodisperse systems.

Structure factors of charged bidispersed colloidal suspensions

In the present study, the effects of charge bidispersity on static structure factors of dilute charged aqueous colloidal suspensions, for compositions covering the full molar fraction range, are investigated using static light-scattering experiments and integral equation theory. The suspensions under study are made from different charged polystyrene particles with an average diameter of 100 nm. The experimentally-observed static structure factors are then compared with theoretical results obtained from the hypernetted chain approximation. The values of both charges are determined by fitting the scattering data of the two pure components and are kept constant for the mixtures. In order to fully describe the experimental data, the effects of the intrinsic polydispersity have to be taken into account. It is found that the height of the main peak of the measured static structure factor, SM(q), changes monotonically as a function of the composition.

Concentration profiles of a colloidal mixture near a charged wall

The structure of a model two‐component colloidal mixture in the vicinity of a charged wall is studied in the framework of the Derjaguin–Landau–Verweg–Overbeek potential and the hypernetted‐chain approximation for the particle–particle and wall–particle direct correlation functions as the closure for the bulk and wall–particle Ornstein–Zernike equations. It is found that for strongly repulsive walls the structure of the inhomogeneous mixture near the wall only depends on the bulk parameters. For neutral and attractive walls, a monolayer of colloidal particles adjacent to the wall is formed. Depending on the charge asymmetry between the two colloidal species, the composition of this monolayer indicates a preferential electrostatic adsorption of the more highly charged species. For large charge asymmetry between the two species, a distinct segregation effect is predicted.

Structure and effective interactions in parallel monolayers of charged spherical colloids

We study the microstructure and the effective interactions of model suspensions consisting of Yukawa-like colloidal particles homogeneously distributed in equally spaced parallel planar monolayers. All the particles interact with each other, but particle transfer between monolayers is not allowed. The spacing between the layers defines the effective system dimensionality. When the layer spacing is comparable to the particle size, the system shows quasi-three-dimensional behavior, whereas for large distances the layers behave as effective two-dimensional systems. We find that effective attractions between like-charged particles can be triggered by adjusting the interlayer spacing, showing that the distance between adjacent layers is an excellent control parameter for the effective interparticle interactions. Our study is based on Brownian dynamics simulations and the integral equations theory of liquids. The effective potentials are accounted for by exploiting the invariance of the Ornstein-Zernike matrix equation under contractions of the description, and on assuming that the difference between bare and effective bridge functions can be neglected. We find that the hypernetted chain approximation does not account properly for the effective interactions in layered systems.

Correlations among colloidal particles confined to a spherical monolayer

The internal structure of spherical colloidal monolayers of charged particles is studied here, both by means of Monte Carlo computer simulations and of an integral equation approach based on the application of the Ornstein–Zernike equation for spherical surfaces. The latter is complemented with a relatively fast and accurate numerical method for its solution, obtained by expanding the corresponding correlation functions in series of Legendre polynomials. It is found that the density correlations among the particles within the monolayer have some special features that differentiate them from the corresponding bulk corrections in open spaces. In particular, for a sufficiently small radius of the spherical monolayer, the distribution of particles around a particle fixed at one of the poles exhibits a peak at the opposite pole which is noticeably larger than the peaks immediately before it. It is also shown here that the introduction of a simple functional form with one adjustable parameter for the bridge fun...

The structure of binary mixtures of charged colloidal suspensions

Structural properties of mixtures of charged colloidal particles are studied by solving the Ornstein-Zernike equation together with the Rogers-Young closure relation. The partial radial distribution functions gij(r) are found to agree very well with results obtained from molecular dynamics simulations. From gij(r) partial structure factors Sij(k) are determined. They are used to obtain results for the experimentally accessible structure factor SM(k) and for the Bhatia-Thornton structure factors SNN(k), SCC(k) and SCC(k). The latter describe concentration ordering, charge ordering and their coupling. A systematic study of these quantities is presented as a function of relative composition of the two components. It is found that various features of SM(k), such as position and height of the main peak and its value at k = 0, change non-monotonously as a function of composition.

A Brownian dynamics algorithm for colloids in curved manifolds.

The many-particle Langevin equation, written in local coordinates, is used to derive a Brownian dynamics simulation algorithm to study the dynamics of colloids moving on curved manifolds. The predictions of the resulting algorithm for the particular case of free particles diffusing along a circle and on a sphere are tested against analytical results, as well as with simulation data obtained by means of the standard Brownian dynamics algorithm developed by Ermak and McCammon [J. Chem. Phys. 69, 1352 (1978)] using explicitly a confining external field. The latter method allows constraining the particles to move in regions very tightly, emulating the diffusion on the manifold. Additionally, the proposed algorithm is applied to strong correlated systems, namely, paramagnetic colloids along a circle and soft colloids on a sphere, to illustrate its applicability to systems made up of interacting particles.