Aggregation In Colloidal Suspensions Research Articles

Structural Formation of Dendritic Polymer Brushes: Analysis Using an Explicit Solvent Model

Abstract Dendritic brushes, which provide higher stability against the aggregation of colloidal suspensions, were investigated using multicolor domain decomposition Monte Carlo simulations. Solvent molecules were explicitly considered, and the profiles of the monomer density and perimeter density distributions were calculated. The results show that the power-law dependence on grafting density depends weakly on the spacer length, the distance between neighboring branch points in dendrons. No apparent evidence was found for backfolding in the dendritic polymer brushes.

Retarded hydrodynamic properties of fractal clusters

Fractal clusters are commonly encountered when working with the stability and the aggregation of colloidal suspensions. In spite of the number of studies that have focused on their stationary hydrodynamic properties, no information is currently known on their retarded hydrodynamic properties. The objective of this work is to close this gap. Clusters with a broad range of fractal dimension values, generated via Monte-Carlo simulations have been analyzed. A rigorous model based on multipole expansion of time-dependent Stokes equations has been developed, and then the full cluster resistance matrix as a function of the frequency has been computed. An attempt has been made to extend Basset, Boussinesque and Oseen equations to fractal clusters, but it was found that the corresponding hydrodynamic radius needs to be a function of frequency. In the case of translational motion, the cluster hydrodynamic radius loses any structural information at high frequencies, becoming independent of the fractal dimension, but depending only on its mass. A simplified model, based on an extension of Kirkwood–Rieseman approach has also been developed. This allows one to perform calculations for clusters with arbitrary masses and fractal dimensions, with good accuracy and very low computational time. It is the first time that the frequency dependence of hydrodynamic properties of complex non-spherical objects has been investigated.

Aggregation in Colloidal Suspensions: Evaluation of the Role of Hydrodynamic Interactions by Means of Numerical Simulations

Numerical simulations constitute a precious tool for understanding the role of key parameters influencing the colloidal arrangement in suspensions, which is crucial for many applications. The present paper investigates numerically the role of hydrodynamic interactions on the aggregation processes in colloidal suspensions. Three simulation techniques are used: Brownian dynamics without hydrodynamic interactions, Brownian dynamics including some of the hydrodynamic interactions, using the Yamakawa-Rotne-Prager tensor, and stochastic rotation dynamics coupled with molecular dynamics. A system of monodisperse colloids strongly interacting through a generalized Lennard-Jones potential is studied for a colloid volume fraction ranging from 2.5 to 20%. Interestingly, effects of the hydrodynamic interactions are shown in the details of the aggregation processes. It is observed that the hydrodynamic interactions slow down the aggregation kinetics in the initial nucleation stage, while they speed up the next cluster coalescence stage. It is shown that the latter is due to an enhanced cluster diffusion in the simulations including hydrodynamic interactions. The higher the colloid volume fraction, the more pronounced the effects on the aggregation kinetics. It is also observed that hydrodynamic interactions slow down the reorganization kinetics. It turns out that the Brownian dynamics technique using the Yamakawa-Rotne-Prager tensor tends to overestimate the effects on cluster diffusion and cluster reorganization, even if it can be a method of choice for very dilute suspensions.

Large Counterions Boost the Solubility and Renormalized Charge of Suspended Nanoparticles

Colloidal particles are ubiquitous in biology and in everyday products such as milk, cosmetics, lubricants, paints, or drugs. The stability and aggregation of colloidal suspensions are of paramount importance in nature and in diverse nanotechnological applications, including the fabrication of photonic materials and scaffolds for biological assemblies, gene therapy, diagnostics, targeted drug delivery, and molecular labeling. Electrolyte solutions have been extensively used to stabilize and direct the assembly of colloidal particles. In electrolytes, the effective electrostatic interactions among the suspended colloids can be changed over various length scales by tuning the ionic concentration. However, a major limitation is gelation or flocculation at high salt concentrations. This is explained by classical theories, which show that the electrostatic repulsion among charged colloids is significantly reduced at high electrolyte concentrations. As a result, these screened colloidal particles are expected to aggregate due to short-range attractive interactions or dispersion forces as the salt concentration increases. We discuss here a robust, tunable mechanism for colloidal stability by which large counterions prevent highly charged nanoparticles from aggregating in salt solutions with concentrations up to 1 M. Large counterions are shown to generate a thicker ionic cloud in the proximity of each charged colloid, which strengthens short-range repulsions among colloidal particles and also increases the corresponding renormalized colloidal charge perceived at larger separation distances. These effects thus provide a reliable stabilization mechanism in a broad range of biological and synthetic colloidal suspensions.

Predicting Aggregation Rates of Colloidal Particles from Direct Force Measurements

Direct force measurements between negatively charged colloidal latex particles of a diameter of 1 μm were carried out in aqueous solutions of various inorganic monovalent and multivalent cations with the multiparticle colloidal probe technique based on the atomic force microscope (AFM). The observed force profiles were rationalized within the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). In the presence of monovalent and divalent cations, this theory was capable to describe the force profiles correctly down to distances of a few nm. At shorter distances, however, a strong non-DLVO attraction was identified. For more highly charged cations, an additional and more long-ranged non-DLVO attractive force is observed, and it was interpreted by surface charge heterogeneities. On the basis of these force profiles, the aggregation rates, which were independently measured by light scattering, can be predicted relatively well. The main conclusion of this study is that, in the present system, direct force measurements do capture the principal interactions driving aggregation in colloidal suspensions.

Aggregation in colloidal suspensions : evaluation of the role of hydrodynamic interactions by means of numerical simulations

Aggregation in colloidal suspensions : evaluation of the role of hydrodynamic interactions by means of numerical simulations

Aggregation in colloidal suspensions: Effect of colloidal forces and hydrodynamic interactions

The forces acting in colloidal suspensions and affecting their stability and aggregation kinetics are considered. The approximations used for these forces in numerical simulations and the importance of the balanced account for both colloidal forces and hydrodynamic interactions are discussed. As an example the results of direct numerical simulations of kinetics of aggregation either with account for hydrodynamic interaction between particles or without it are compared by varying the parameters of the interaction potential between particles and fraction of solid. Simulations are based on the Langevin equations with pairwise interaction between particles and take into account Brownian, hydrodynamic and colloidal forces. It is confirmed that the neglecting of hydrodynamic interaction results in an accelerated growth of aggregates. The results of numerical simulations of aggregation kinetics are compared with well known analytical solutions.

Aggregation in colloidal suspensions and its influence on the suspension viscosity

Influence of aggregate formation on rheological properties of concentrated suspensions is discussed and reviewed. Methods of Stokesian and Brownian dynamic simulations of aggregation are discussed and the results of simulations for both Brownian and non-Brownian particles are presented.

Reaction‐limited aggregation in presence of short‐range structural forces

AbstractA geometrically discretized sectional population balance model for reaction‐limited aggregation of colloidal suspensions is presented. The two important model parameters are collision frequency factor and collision efficiency factor. The collision frequency factor is derived from physically realistic arguments proposed for collision of fractal aggregates. The collision efficiency factor is computed as a function of total interaction energy between particles, including short‐range structural repulsion forces. The irregular and open structure of aggregates is taken into account by incorporating their mass fractal dimension. The characteristic time constant of reaction‐limited aggregation, derived from dynamic scaling of mean aggregate size‐aggregation time data, is found to correlate with electrolyte concentration. The population balance model is tested with published experimental data for aggregation of γ‐alumina suspensions in the presence of different electrolytes. It is shown that the slow kinetics of aggregation under certain conditions of pH and electrolyte concentration require inclusion of short‐range structural repulsion forces along with van der Waals attraction and electrical double layer repulsion forces in an extended DLVO theory. The model predictions are in good agreement with experimental data for time evolution of mean aggregate diameter in the reaction‐limited aggregation regime. © 2005 American Institute of Chemical Engineers AIChE J, 2005

The Fractal Structure of Cohesive Sediment Aggregates

The forces between particles and the structure of cohesive sediment aggregates to a large extent determine various mechanical and rheological properties of mud flocs and mud beds. The unifying concept of self-similar fractal geometry is adopted to model the complex and much variable structure of cohesive sediment aggregates. The fractal aggregate model, which has been applied to aggregation in colloidal suspensions over more than a decade, is related to Krone's (1963) notion or orders of aggregation. However, self-similarity on all scales implies that a unique relationship exists between aggregate size and number of primary particles that form the aggregate. The consequences of self-similar fractal structure concerning density and strength of flocs, and permeability, yield strength and viscosity of mud beds are analysed in some detail. Comparison of the results obtained with experimental data from the literature supports the fractal model.

The fractal structure of cohesive sediment aggregates

Abstract The forces between particles and the structure of cohesive sediment aggregates to a large extent determine various mechanical and rheological properties of mud flocs and mud beds. The unifying concept of self-similar fractal geometry is adopted to model the complex and much variable structure of cohesive sediment aggregates. The fractal aggregate model, which has been applied to aggregation in colloidal suspensions over more than a decade, is related to Krone's (1963) notion or orders of aggregation. However, self-similarity on all scales implies that a unique relationship exists between aggregate size and number of primary particles that form the aggregate. The consequences of self-similar fractal structure concerning density and strength of flocs, and permeability, yield strength and viscosity of mud beds are analysed in some detail. Comparison of the results obtained with experimental data from the literature supports the fractal model.

Aggregation of colloidal suspensions by fibrous surfaces: I. The influence of the fiber surface

In an earlier paper (1) it was shown that the aggregation of a commercial D.D.T. formulation was accelerated by the addition of fibers, and that hydrophobic fiber surfaces were much more effective than hydrophilic ones. This investigation has now been repeated and extended using a simple D.D.T. suspension containing only pure materials. The earlier assumption that the phenomenon is an aggregation process has been confirmed and the rate of aggregation has again been shown to depend on the wetting properties of the fiber surface. Some possible mechanisms are discussed.

Aggregation of colloidal suspensions by fibrous surfaces: II. The effects of added salts and soaps

The addition of soaps or simple salts changes the stability of the D.D.T. suspension and also the rate of aggregation by fibrous surfaces. Low salt concentrations cause an immediate partial coagulation of the suspension, but further coagulation is generally very slow. The same salt concentrations produce very marked increases in the rate of aggregation of D.D.T. by hydrophobic fiber surfaces. This is attributed to decreases in the electrical repulsive forces around the fibers and the particles. Added soaps usually lower the stability of the suspension (without added fibers), the lowering being much more marked at soap concentrations above the c.m.c. This observation and the formation of large D.D.T. crystals suggest a process of crystal growth by transport of D.D.T. in the soap micelles. When fibers are also present the soaps are shown to be adsorbed on the fibers, rendering them more hydrophobic, and hence increasing the rate of aggregation of the D.D.T. by the fiber surfaces.