Publications (22)49.84 Total impact
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ABSTRACT: We perform systematic simulation experiments on model systems with softsphere repulsive interactions to test the predicted dynamic equivalence between softsphere liquids with similar static structure. For this we compare the simulated dynamics (mean squared displacement, intermediate scattering function, αrelaxation time, etc.) of different softsphere systems, between them and with the hardsphere liquid. We then show that the referred dynamic equivalence does not depend on the (Newtonian or Brownian) nature of the microscopic laws of motion of the constituent particles, and hence, applies independently to colloidal and to atomic simple liquids. Finally, we verify another more recently proposed dynamic equivalence, this time between the longtime dynamics of an atomic liquid and its corresponding Brownian fluid (i.e., the Brownian system with the same interaction potential).  [Show abstract] [Hide abstract]
ABSTRACT: We perform systematic simulation experiments on model systems with softsphere repulsive interactions to test the predicted dynamic equivalence between softsphere liquids with similar static structure. For this we compare the simulated dynamics (mean squared displacement, intermediate scattering function, {\alpha}relaxation time, etc.) of different softsphere systems, between them and with the hardsphere liquid. We then show that the referred dynamic equivalence does not depend on the (Newtonian or Brownian) nature of the microscopic laws of motion of the constituent particles, and hence, applies independently to colloidal and to atomic simple liquids. In addition, we verify another more recentlyproposed dynamic equivalence, this time between the longtime dynamics of a Brownian fluid and its corresponding atomic liquid (i.e., the atomic system with the same interaction potential).  [Show abstract] [Hide abstract]
ABSTRACT: Using the generalized Langevin equation formalism and the process of contraction of the description we derive a general memory function equation for the thermal fluctuations of the local density of a simple atomic liquid. From the analysis of the longtime limit of this equation, a striking equivalence is suggested between the longtime dynamics of the atomic liquid and the dynamics of the corresponding \emph{Brownian} liquid. This dynamic equivalence is confirmed here by comparing molecular and Brownian dynamics simulations of the selfintermediate scattering function and the longtime selfdiffusion coefficient for the hardsphere liquid. 
Article: Coalescence in Double Emulsions
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ABSTRACT: Coalescence processes in double emulsions, waterinoilinwater, are studied by optical microscopy. The time evolution of such systems is determined by the interplay of two coalescence processes, namely, between inner water droplets and between the inner water droplets and the continuous external water phase. The predominance of one of those processes over the other, regulated by the relative amount of hydrophilic and lipophilic surfactants, leads to different evolutions of the system. We present here results for a class of systems whose evolution follows a master behavior. We also implemented a computer simulation where the system is modeled as a spherical cavity filled with smaller Brownian spheres. Collisions between spheres allow coalescence between them with probability P(i), whereas collisions between a sphere and the wall of the cavity allow coalescence with the external phase with probability P(e). The phenomenology observed in the experimental systems is well reproduced by the computer simulation for suitable values of the probability parameters.  [Show abstract] [Hide abstract]
ABSTRACT: We show that the kinetictheoretical selfdiffusion coefficient of an atomic fluid plays the same role as the shorttime selfdiffusion coefficient D_S in a colloidal liquid, in the sense that the dynamic properties of the former, at times much longer than the mean free time, and properly scaled with D_S, will indistinguishable from those of a colloidal liquid with the same interaction potential. One important consequence of such dynamic equivalence is that the ratio D_L/ D_S of the longtime to the shorttime selfdiffusion coefficients must then be the same for both, an atomic and a colloidal system characterized by the same interparticle interactions. This naturally extends to atomic fluids a wellknown dynamic criterion for freezing of colloidal liquids[Phys. Rev. Lett. 70, 1557 (1993)]. We corroborate these predictions by comparing molecular and Brownian dynamics simulations on (soft and hardsphere) model systems, representative of what we may refer to as the "hardsphere" dynamic universality class.  [Show abstract] [Hide abstract]
ABSTRACT: We have employed Monte Carlo simulations and a coarse grain model in order to analyze the final structure and morphology of complexes arising from the interaction between fullyflexible polycations and polyanions with different chain stiffness. Different morphologies, like globules, toroids and rods, are obtained depending on chain stiffness. It was observed that longer chains yield more frequently toroids than rods, as compared with shorter chains. However, the size of toroids does not depend entirely on the chain length. This suggests that the final structure of the toroids is highly dependent on the intrinsic rigidity of chain rather than on the electrostatic contributions. 
Article: Overcharging and charge reversal in the electrical double layer around the point of zero charge
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ABSTRACT: The ionic adsorption around a weakly charged spherical colloid, immersed in sizeasymmetric 1:1 and 2:2 salts, is studied. We use the primitive model (PM) of an electrolyte to perform Monte Carlo simulations as well as theoretical calculations by means of the hypernetted chain/mean spherical approximation (HNC/MSA) and the unequalradius modified GouyChapman (URMGC) integral equations. Structural quantities such as the radial distribution functions, the integrated charge, and the mean electrostatic potential are reported. Our Monte Carlo "experiments" evidence that near the point of zero charge, the smallest ionic species is preferentially adsorbed onto the macroparticle, independently of the sign of the charge carried by this tiniest electrolytic component, giving rise to the appearance of the phenomena of charge reversal (CR) and overcharging (OC). Accordingly, colloidal CR, due to an excessive attachment of counterions, is observed when the macroion is slightly charged and the coions are larger than the counterions. In the opposite situation, i.e., if the counterions are larger than the coions, the central macroion acquires additional likecharge (coions) and hence becomes "overcharged," a feature theoretically predicted in the past [F. JiménezAngeles and M. LozadaCassou, J. Phys. Chem. B 108, 7286 (2004)]. In other words, here we present the first simulation data on OC in the PM electrical double layer, showing that close to the point of zero charge, this novel effect surges as a consequence of the ionic size asymmetry. We also find that the HNC/MSA theory captures well the CR and OC phenomena exhibited by the computer experiments, especially as the macroion's charge increases. On the contrary, even if URMGC also displays CR and OC, its predictions do not compare favorably with the Monte Carlo data, evidencing that the inclusion of hardcore correlations in Monte Carlo and HNC/MSA enhances and extends those effects. We explain our findings in terms of the energyentropy balance. In the field of electrophoresis, it has been generally agreed that the charge of a colloid in motion is partially decreased by counterion adsorption. Depending on the location of the macroion's slipping surface, the OC results of this paper could imply an increase in the expected electrophoretic mobility. These observations aware about the interpretation of electrokinetic measurements using the standard PoissonBoltzmann approximation beyond its validity region.  [Show abstract] [Hide abstract]
ABSTRACT: Monte Carlo simulations of a spherical macroion, surrounded by a sizeasymmetric electrolyte in the primitive model, were performed. We considered 1:1 and 2:2 salts with a size ratio of 2 (i.e., with coions twice the size of counterions), for several surface charge densities of the macrosphere. The radial distribution functions, electrostatic potential in all the space and at the Helmholtz surfaces, and integrated charge are reported. We compare these simulational data with original results obtained from the OrnsteinZernike integral equation, supplemented by the hypernetted chainhypernetted chain (HNCHNC) and hypernetted chainmean spherical approximation (HNCMSA) closures, and with the corresponding calculations using the modified GouyChapman and unequalradius modified GouyChapman theories. The HNCHNC and HNCMSA integral equations formalisms show good concordance with Monte Carlo "experiments," whereas the notable limitations of pointion approaches are evidenced. Most importantly, the simulations confirm our previous theoretical predictions of the nondominance of the counterions in the sizeasymmetric spherical electrical double layer [J. Chem. Phys. 123, 034703 (2005)], the appearance of anomalous curvatures at the outer Helmholtz plane, and the enhancement of the charge reversal and screening at high colloidal surface charge densities due to the ionic size asymmetry. 
Article: Overcharging and charge reversal in the electrical double layer near the point of zero charge
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ABSTRACT: The ionic adsorption around a weakly charged spherical colloid, immersed in sizeasymmetric 1:1 and 2:2 salts, is studied. We use the primitive model of an electrolyte to perform Monte Carlo simulations as well as theoretical calculations by means of the hypernetted chain/mean spherical approximation (HNC/MSA) and the unequalradius modified GouyChapman (URMGC) integral equations. Structural quantities such as the radial distribution functions, the integrated charge, and the mean electrostatic potential are reported. Our Monte Carlo "experiments" evidence that near the point of zero charge the smallest ionic species is preferentially adsorbed onto the macroparticle, independently of the sign of the charge carried by this tiniest electrolytic component, giving rise to the appearance of the phenomena of charge reversal and overcharging. Accordingly, charge reversal is observed when the macroion is slightly charged and the coions are larger than the counterions. In the opposite situation, i.e. if the counterions are larger than the coions, overcharging occurs. In other words, in this paper we present the first simulational data on overcharging, showing that this novel effect surges close to the point of zero charge as a consequence of the ionic size asymmetry. Further, it is seen that the inclusion of hardcore correlations in HNC/MSA leads to spatial regions near the macroion's surface in which the integrated charge and/or the mean electrostatic potential can decrease when the colloidal charge is augmented and vice versa. These observations aware about the interpretation of electrophoretic mobility measurements using the standard PoissonBoltzmann approximation beyond its validity region.  [Show abstract] [Hide abstract]
ABSTRACT: We study the structure of charged colloidal suspensions under confinement and determine a state diagram for the occurrence of electrostatic adsorption onto the confining walls, an effect that results in the accumulation of particles on the bounding surfaces and that could be relevant in experiments. We use Monte Carlo simulations to quantify this structural transition and perform theoretical calculations based on integral equations. Overall, our results provide a guide for experimentalists dealing with charged colloidal systems to determine the relevance of this purely electrostatic effect. 
Article: Dynamic arrest within the selfconsistent generalized Langevin equation of colloid dynamics
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ABSTRACT: This paper presents a recently developed theory of colloid dynamics as an alternative approach to the description of phenomena of dynamic arrest in monodisperse colloidal systems. Such theory, referred to as the selfconsistent generalized Langevin equation (SCGLE) theory, was devised to describe the tracer and collective diffusion properties of colloidal dispersions in the short and intermediatetime regimes. Its selfconsistent character, however, introduces a nonlinear dynamic feedback, leading to the prediction of dynamic arrest in these systems, similar to that exhibited by the wellestablished mode coupling theory of the ideal glass transition. The full numerical solution of this selfconsistent theory provides in principle a route to the location of the fluidglass transition in the space of macroscopic parameters of the system, given the interparticle forces (i.e., a nonequilibrium analog of the statisticalthermodynamic prediction of an equilibrium phase diagram). In this paper we focus on the derivation from the same selfconsistent theory of the more straightforward route to the location of the fluidglass transition boundary, consisting of the equation for the nonergodic parameters, whose nonzero values are the signature of the glass state. This allows us to decide if a system, at given macroscopic conditions, is in an ergodic or in a dynamically arrested state, given the microscopic interactions, which enter only through the static structure factor. We present a selection of results that illustrate the concrete application of our theory to model colloidal systems. This involves the comparison of the predictions of our theory with available experimental data for the nonergodic parameters of model dispersions with hardsphere and with screened Coulomb interactions.  [Show abstract] [Hide abstract]
ABSTRACT: This letter presents a remarkably simple approach to the firstprinciples determination of the ergodic/nonergodic transition in monodisperse colloidal suspensions. It consists of an equation for the longtime asymptotic value ° of the mean squared displacement of the colloidal particles, whose finite real solutions signal the nonergodic state, and determines the nonergodic parameter f(k). We illustrate its concrete application to three simple model colloidal systems, namely, hardspheres, hardspheres plus repulsive (screened Coulomb) Yukawa interaction, and hardsphere plus attractive Yukawa tail. The results indicate that this is quite a competitive theory, similar in spirit to, but conceptually independent from, the wellknown mode coupling theory.  [Show abstract] [Hide abstract]
ABSTRACT: We study the structural and thermodynamic properties of three discrete potential fluids: the square well (SW), the square well–barrier (SWB), and the square well–barrier–well (SWBW) fluids by means of the Ornstein–Zernike (OZ) integral equation and the HMSA (hybrid mean spherical approximation) closure relation. The radial distribution functions, structure factors, and pressure of the systems are calculated as a function of the strength of the attractive and repulsive parts of the potential in an extended range of densities, mainly covering the range 0.1 ≤ ρ* ≤ 0.9. We find that far away from the liquid–vapour coexistence region the HMSA theory is an accurate approach that compares well with Monte Carlo simulations. We also find that when the attractive parts of the potential dominate over the repulsive part the structure factor at low q values shows a considerable increase, which suggests the formation of largescale domains that locally exhibit fluidlike structure.  [Show abstract] [Hide abstract]
ABSTRACT: Brownian dynamics simulations were performed to study the structure of polyelectrolyte complexes formed by two flexible, oppositely charged polyelectrolyte chains. The distribution of monomers in the complex as well as the radius of gyration and structure factor of complexes and individual polyelectrolytes are reported. These structural properties were calculated for polyelectrolyte chains with equal number of monomers, keeping constant the bond length of the negative chain and increasing the bond length of the positive chain. This introduces an asymmetry in the length of the chains that modulates the final structure of the complexes. In the symmetric case the distribution of positive and negative monomers in the complex are identical, producing clusters that are locally and globally neutral. Deviations from the symmetric case lead to nonuniform, asymmetric monomer distributions, producing net charge oscillations inside the complex and large changes in the radius of gyration of the complex and individual chains. From the radius of gyration of the polyelectrolyte chains it is shown that the positive chain is much more folded than the negative chain when the chains are asymmetric, which is also confirmed through the scaling behavior of the structure factors.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we present a theory for the structure of a suspension of highly charged spherical polyions near the surface of a much larger but more weakly charged spherical colloid. Starting from a level of description involving effective screened Coulomb potentials between the particles of both species, we calculate the radial distribution function of the polyions around isolated big colloids. Our theory is based on the use of the Ornstein–Zernike integral equation, complemented with the hypernetted chain approximate closure, whose numerical solutions are selectively contrasted with the results of Monte Carlo simulations generated for this purpose. Our results indicate that, under certain conditions, our model suspension of polyions is predicted to adsorb onto the surface of the larger colloid, even if both species carry charges of the same sign. Under extreme conditions, the structure of the collection of adsorbed particles corresponds strictly to a monolayer, strongly bound to the surface of the larger particle by electrostatic forces. The complex formed by one large particle plus its adsorbed polyions then bears a large effective charge, which may enhance the electrostatic stability of the former. At some threshold condition, however, this monolayer structure becomes loose enough to be better described as a halo. One might thus associate this threshold condition with the threshold for the stability of the colloidal species formed by the larger particles. We discuss the possible connection of our theoretical results with the recent experimental observations that motivated this work.  [Show abstract] [Hide abstract]
ABSTRACT: 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 function greatly enhances the fit between the theoretical approach and the simulation data. © 2003 American Institute of Physics.  [Show abstract] [Hide abstract]
ABSTRACT: The pair correlation function g(r) between likecharged colloidal particles in quasitwodimensional geometries is measured by optical microscopy for a wide range of particle concentrations and various degrees of confinement. The effective pair potential u(r) is obtained by deconvoluting g(r) via Monte Carlo computer simulations. Our results confirm the existence of a longrange attractive component of u(r) and the appearance of an extra attractive term under stringent confinement.  [Show abstract] [Hide abstract]
ABSTRACT: In this work we present a study of the local structure of a model colloidal suspension highly confined inside a cylindrical pore. Such a study is based in Monte Carlo computer simulations, using the repulsive part of the DerjaguinLandauVerweyOverbeek potential as the pair interaction between particles. The structural properties calculated here are the concentration profile n(rho), the axial pair correlation function g(z), and the axialangular pair correlation function g(z,straight phi). The behavior of these quantities is analyzed as a function of the density of colloidal particles in the restricted space, and as a function of the size of the pore.  [Show abstract] [Hide abstract]
ABSTRACT: The concentration profiles of a model colloidal mixture inside a charged cylindrical pore are studied theoretically and by computer simulations. The theoretical structure of the colloidal mixture is obtained by employing the basic chemical equilibrium equations from which the concentration profiles can be expressed in terms of a set of integral equations. The pair potential between particles is taken to be the repulsive part of the DerjaguinLandauVerweyOverbeek potential. Simulation results are also presented in order to assess the accuracy of the theoretical predictions.  [Show abstract] [Hide abstract]
ABSTRACT: A simple theoretical scheme is employed to calculate the equilibrium local concentration profile of a model suspension of highly charged colloidal particles inside a cylindrical pore in the regime corresponding to thick and narrow pores. In addition, we report results of Brownian dynamics simulations for some of the systems presented here. We have found that the local concentration profile in this geometry scales in quite a similar way as in the vicinity of a confining charged wall.
Publication Stats
256  Citations  
49.84  Total Impact Points  
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Institutions

19972013

Universidad Autónoma de San Luis Potosí
 Instituto de Física
San Luis, San Luis Potosí, Mexico


1995

Universidad Politécnica de San Luis Potosí
San Luis, San Luis Potosí, Mexico
