Electrostatic adsorption of colloidal particles on the walls of a planar slit: Simulation versus theory

Nonadditivities of the Particle Sizes Hidden in Model Pair Potentials and Their Effects on Physical Adsorptions

It is important to understand the mechanism of colloidal particle assembly near a substrate for development of drug delivery systems, micro-/nanorobots, batteries, heterogeneous catalysts, paints, and cosmetics. Understanding the mechanism is also important for crystallization of the colloidal particles and proteins. In this study, we calculated the physical adsorption of colloidal particles on a flat wall mainly using the integral equation theory, wherein small and large colloidal particles were employed. In the calculation system, like-charged electric double-layer potentials were used as pair potentials. In some cases, it was found that the small particles are more easily adsorbed. This result is unusual from the viewpoint of the Asakura-Oosawa theory, and we call it a "reversal phenomenon". Theoretical analysis revealed that the reversal phenomenon originates from the nonadditivities of the particle sizes. Using the knowledge obtained from this study, we invented a method to analyze the size nonadditivity hidden in model pair potentials. The method will be useful for confirmation of various simulation results regarding the adsorption and development of force fields for colloidal particles, proteins, and solutes.

Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with atomic force microscopy (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the nonexponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits extraction of the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.

Structuring of colloidal silica nanoparticle suspensions near water-silica interfaces probed by specular neutron reflectivity

Structuring of aqueous suspensions of colloidal silica nanoparticles near an isolated planar silica-water interface is studied by specular neutron reflectivity. The reflectivity data clearly show that the suspensions develop a damped, oscillatory concentration profile in the normal direction to the interface. The wavelengths of these oscillations agree well with those independently determined by direct force measurements in the slit-geometry. The reflectivity data further demonstrate that the oscillatory structure persists over several layers and that the first particle layer is separated from the interface by a particle-free region.

Nanoparticles confined to a spherical surface in the presence of an external field: Interaction forces and induced microstructure

The structural response of a set of charged nanoparticles confined to move on a spherical surface under the influence of an external field is studied by Brownian Dynamics (BD) simulations and by an integral equation approach (IEA). Considering an identical nanoparticle as the source of the external field, we analyze the force exerted by the N confined particles on the external one, as well as the corresponding potential energy, focusing on their dependence on the distance of the external particle to the center of the sphere r₀. The connection of the force and potential to the equilibrium local distribution of the adsorbed particles, that is, the microstructure within the spherical monolayer induced by the external nanoparticle, which is also dependent on r₀, is elucidated by this analysis. It is found that the external particle needs to surmount a considerable potential barrier when moving toward the spherical surface, although much smaller than the one generated by a uniform surface distribution with an equivalent amount of charge. This is understood in terms of the correlation hole within the confined monolayer induced by the external particle. Another interesting conclusion is that the IEA provides an accurate, almost quantitative, description of the main features observed in the BD results, yet it is much less computationally demanding. The connection of these results with the overall chemical equilibrium of charged surfactant nanoparticles in the context of Pickering emulsions is also briefly discussed.

Effective electrostatic interactions in colloid-nanoparticle mixtures

Interparticle interactions and bulk properties of colloidal suspensions can be substantially modified by the addition of nanoparticles. Extreme asymmetries in size and charge between colloidal particles and nanoparticles present severe computational challenges to molecular-scale modeling of such complex systems. We present a statistical mechanical theory of effective electrostatic interactions that can greatly ease large-scale modeling of charged colloid-nanoparticle mixtures. By applying a sequential coarse-graining procedure, we show that a multicomponent mixture of charged colloids, nanoparticles, counterions, and coions can be mapped first onto a binary mixture of colloids and nanoparticles and then onto a one-component model of colloids alone. In a linear-response approximation, the one-component model is governed by a single effective pair potential and a one-body volume energy, whose parameters depend nontrivially on nanoparticle size, charge, and concentration. To test the theory, we perform molecular dynamics simulations of the two-component and one-component models and compute structural properties. For moderate electrostatic couplings, colloid-colloid radial distribution functions and static structure factors agree closely between the two models, validating the sequential coarse-graining approach. Nanoparticles of sufficient charge and concentration enhance screening of electrostatic interactions, weakening correlations between charged colloids and destabilizing suspensions, consistent with experiments.

State diagram for the electrostatic adsorption of charged colloids on confining walls: simulation and theory

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.

Particle migration induced by confinement of colloidal suspensions along the gravitational direction

We confine charged spheres in cells with the smallest dimension along the direction of gravity ĝ. The particles are density mismatched with the surrounding medium and sediment along ĝ with typical Péclet numbers of Pe approximately 10(-3). After a certain time, we find that the number of particles N increases near both upper and lower plates until a characteristic time tau is reached; above this time N plateaus. We attribute the observed phenomenology to collective particle motions driven by gravity and mediated by hydrodynamic interactions; these could yield formation of swirls made of particles with correlated velocities that could eventually drive the particles towards the upper plate. The characteristic time for these migrations scales with plate-to-plate separation Lz as tau approximately Lz1.2, exactly as the characteristic decay time of velocity fluctuations in sedimentation processes [S. Y. Tee, Phys. Rev. Lett. 89, 054501 (2002)], despite that in these experiments the smallest cell dimension is perpendicular to ĝ and 7<Pe<50. In the absence of gravitational field, the observed particle migrations disappear, emphasizing the key role played by gravity in these experiments.

Entropic forces in dilute colloidal systems

Depletion forces can be accounted for by a contraction of the description in the framework of the integral equations theory of simple liquids. This approach includes, in a natural way, the effects of the concentration on the depletion forces, as well as energetic contributions. In this paper we systematically study this approach in a large variety of dilute colloidal systems composed of spherical and nonspherical hard particles, in two and in three dimensions, in the bulk and in front of a hard wall with a relief pattern. We show by this way the form in which concentration and geometry determine the entropic interaction between colloidal particles. The accuracy of our results is corroborated by comparison with computer simulations.

Energetic contributions to wall-particle depletion forces

Recently, depletion forces were accounted for by a contraction of the description based on the integral equations theory of simple liquids [Phys. Rev. E 61, 4095 (2000)]. The extension of those results to the case of inhomogeneous systems is reported here. Besides, the energetic contributions to the wall-particle depletion forces are studied, as they arise as soon as charge is put on some of the components of a binary mixture of hard spheres on the front of a hard wall. By charging the small particles the amplitude of the depletion attraction between wall and large particles is reduced, and can even become a repulsion. A similar effect is observed if an attractive interaction between wall and small particles is present.

Axial and angular correlations between colloidal particles in narrow cylindrical pores

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 Derjaguin-Landau-Verwey-Overbeek 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 axial-angular 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.