Large Colloidal Particles Research Articles

The Correlation Functions of a Suspension of Large Particles in Amorphous Polybutadiene: A Molecular Dynamics Simulation Study

The correlation functions of a suspension of Lennard-Jones large particles in a model 1,4-cis-polybutadiene solvent have been investigated by molecular dynamics simulations. We present the effects of temperature, the degree of polymerization, and the solvent/large particle density on the effective interactions between the large particles and the solvent. It is found that as the temperature increases, the structure between solvent–solvent, solvent–large particle, and large–large particle decreases. Additionally, as the bulk density or the chain length is increased, the attractive part of the large–large interaction becomes weaker and small. We believe that in part, this is due to the polymer having collapsed onto itself and entangling the large particles and lessening their interaction until they are actually in contact. Increasing the length of the polymer also entangles the large particles. However, we believe that this may be a general feature that is a characteristic of a polymer solvent containing macroscopically large colloidal particles even though entanglement should be of less significance.

Non-linear screening in concentrated colloidalsuspensions

In a charge-stabilized colloidal solution, the large colloidalparticles are surrounded by microions that are up to four ordersof magnitude smaller than the colloidal particles. Because of this sizeasymmetry, it is desirable to obtain an effective one-componentdescription of the mixture where the colloidal particle plus itsionic atmosphere is treated as one, dressed particle. The effectivepair potential between these dressed particles is a screenedCoulomb potential. The screening depends, of course, on thedensity distribution of the small ions around and between the bigcolloidal particles. If the colloidal charge and the concentrationof the ions is not too high, this distribution can beapproximately determined from the linearized Poisson-Boltzmannequation, and the resulting effective pair potentials are Yukawapotentials. In concentrated suspensions, however, the full,non-linear Poisson-Boltzmann equation must be solved to determinethe density distribution of the small ions. In this article, wesuggest a way to obtain effective pair potentials forthis case. We solve the non-linear Poisson-Boltzmann equationaround a colloidal particle that is displaced a certain distancefrom the centre of its Wigner-Seitz cell. From the resultingdensity profile of the ions, we determine the total force actingon the shifted particle as a function of the displacement. Fromthis function one can then estimate the non-linearlyscreened pair forces, and, thus, the effective pair potentials.

Hydrogen Peroxide Decomposition in Ammonia Solutions

The decomposition of in ammonia hydroxide peroxide mixtures (APM) have been studied as a function of metal‐ion weight concentration (0.1‐10 ppb range) and temperature. It has been found that the uncatalyzed self‐decomposition of is second order with an activation energy of 170 (±10) kJ/mol. Additionally, copper ions, which form mononuclear ammonia complexes catalyze the decomposition. Superimposed on the uncatalyzed reaction, a first order copper‐ion catalyzed decomposition with an activation energy of 84 (±2) kJ/mol has been measured. The iron‐ion catalyzed reaction is a mixture of first and second order. Due to the insolubility of Fe(III) it aggregates to large clusters or colloid particles. Higher concentrations of the large clusters shift the reaction kinetics from first order to second order. The first and second order iron‐ion catalyzed decomposition reactions have similar activation energies [65 (±4) and 71 (±4) kJ/mol] suggesting a similar primary reaction step. Furthermore, the effect of complexing agents has been evaluated. Substoichiometric amounts of diethylenetriamine‐NNN′N″N″‐penta(methylphosphonic) acid break down these iron clusters favoring the first order reaction at the expense of the second order reaction. Superstoichiometric amounts of the complexing agent completely inactivate the iron ions, but had no effect on the copper‐ion catalyzed decomposition reaction. © 1999 The Electrochemical Society. All rights reserved.

Theory for stabilization of magnetic colloid in liquid metal

This work suggests that van der Waals attraction of small colloidal particles to large colloidal particles can establish a cloud of the small particles that tends to shield the larger ones from agglomerating to each other. With consideration of the finite size of the small particles the system is analyzed thermodynamically with interest in stabilizing dispersions of magnetic particles in liquid metallic media.

Locating the Micellar Shear Plane and Its Relationship with the Debye Screening Length

The shear plane and shear potential of suspended nanoscale particles such as micelles and colloids are important parameters; however, reports detailing their determination are scarce and are restricted to relatively large colloidal and polymeric particles. In addition, for charged particles such as colloids and anionic or cationic micelles, the relationship between the Debye screening length and the location of the shear plane has not yet been established. In this paper, we have used the rotating disk electrode to determine micellar hydrodynamic radii ( ), as a function of electrolyte (KCl) concentration, for CTAC (cetyltrimethylammonium chloride) micelles at 298.0 ± 0.1 K. The results reveal an initial decrease in due to collapse of the shear plane toward the micellar hard-sphere surface followed by an increase in resulting from electrolyte-induced linear spherical expansion of the micellar particles. Calculation of the micellar hard-sphere radii from monomer volume considerations allows evaluation of the shear plane thickness and its associated ζ-potential. The ζ-potential was found to be 95 ± 10 mV and was invariant to changes in electrolyte concentration. In addition, estimations of the Debye screening length at the micellar charged surface allow direct comparison with the shear plane thickness. The relationship is shown to be a simple linear function with the shear plane collapsing toward the micellar surface as the double layer collapses with added electrolyte.

Resolution Effects in Dissipative Particle Dynamics Simulations

Dissipative Particle Dynamics (DPD) simulations were performed to investigate resolution or "coarse graining" effects on the simulation results. Fluid flow through a periodic array of spheres has been studied as a model for fluid filtration into a porous medium. In our model system, it appears that quantitatively correct results for the dimensionless drag can be obtained for relatively small system sizes. For higher solid volume fractions, it is necessary to increase the system size to avoid finite size and resolution effects. Simulations of colloidal spheres suspended in a DPD fluid show effective attraction between the large colloid particles, causing depletion aggregation. This effect may be expected as a consequence of the coarse-grained nature of the DPD fluid. By imposing a steady shear rate the aggregation can be suppressed. The results show that for dilute suspensions, the Brownian noise in the particle interactions causes an effective colloid polydispersity, which suppresses aggregation effects.

Self-diffusion coefficients and shear viscosity of model nanocolloidal dispersions by molecular dynamics simulation

The self-diffusion coefficients $D$ of both species in model nanocolloidal dispersions have been computed using molecular dynamics (MD) simulation, in which three-dimensional model spherical colloidal particles were in a molecularly discrete solvent. The effects of the relative density, size, and concentration of the two species were explored. Simulations were carried out at infinite dilution (a single colloidal particle) and at finite packing fractions (many colloidal particles) in the simulation cell using single interaction centers between the model colloidal particles and solvent molecules. The calculations used the Weeks-Chandler-Andersen (WCA) or Lennard-Jones (LJ), interaction potentials between all species. Nanocolloid particles with diameters up to \ensuremath{\sim}6 times the solvent molecule were modeled. At liquidlike densities the self-diffusion coefficients of the colloidal particles, ${D}_{c},$ for all sizes and packing fractions, statistically exhibited no mass dependence but a significant colloid particle size dependence. This can be interpreted in a systematic manner using a Mori series expansion. The first Mori coefficient (which is inversely proportional to particle mass) dominates the value of the self-diffusion coefficient for both species, and which also leads to a formal cancellation of the mass dependence at the order of the first Mori coefficient ${K}_{B1}$ (the self-diffusion coefficient is therefore determined by a ``static'' property to this order). The values of ${D}_{c}$ at each packing fraction are found to be approximately inversely proportional to the colloidal particle diameter, quantitatively following the same trend as the Stokes-Einstein equation, even for the small colloidal particle sizes and finite colloidal particle concentrations studied here. Another consequence of the dominance of the first Mori coefficient is that the normalized velocity autocorrelation function of the colloidal particle at a short time can be represented well at all state points and packing fractions by the analytic form $\ensuremath{\simeq}\mathrm{cos}({\ensuremath{\Omega}}_{0}t),$ where ${\ensuremath{\Omega}}_{0}=\sqrt{{K}_{B1}},$ which is the so-called Einstein frequency. LJ and WCA systems with otherwise the same system parameters manifest the same oscillation frequency, but the LJ oscillation amplitudes are larger and the values of ${D}_{c}$ are smaller. The self-diffusion coefficients and shear viscosities obey a volume fraction dependence similar to that found for much larger colloidal particles.

Hydrogen Peroxide Decomposition in Ammonia Solutions

The decomposition of in ammonia hydroxide peroxide mixtures (APM) have been studied as a function of metal‐ion weight concentration (0.1‐10 ppb range) and temperature. It has been found that the uncatalyzed self‐decomposition of is second order with an activation energy of 170 (±10) kJ/mol. Additionally, copper ions, which form mononuclear ammonia complexes catalyze the decomposition. Superimposed on the uncatalyzed reaction, a first order copper‐ion catalyzed decomposition with an activation energy of 84 (±2) kJ/mol has been measured. The iron‐ion catalyzed reaction is a mixture of first and second order. Due to the insolubility of Fe(III) it aggregates to large clusters or colloid particles. Higher concentrations of the large clusters shift the reaction kinetics from first order to second order. The first and second order iron‐ion catalyzed decomposition reactions have similar activation energies [65 (±4) and 71 (±4) kJ/mol] suggesting a similar primary reaction step. Furthermore, the effect of complexing agents has been evaluated. Substoichiometric amounts of diethylenetriamine‐NNN'N''N''‐penta(methylphosphonic) acid break down these iron clusters favoring the first order reaction at the expense of the second order reaction. Superstoichiometric amounts of the complexing agent completely inactivate the iron ions, but had no effect on the copper‐ion catalyzed decomposition reaction. © 1999 The Electrochemical Society. All rights reserved.

Influence of attractive forces on the solvent mediated potential of mean force between colloidal particles

The solvent induced potential of mean force between colloidal particles is computed by using the superposition approximation for the solvent–macrospheres triplet distribution function and the density functional theory for the solvent density profile at an isolated macrosphere. The influence of solute–solvent attractive forces is investigated for different diameter ratios and solvent bulk densities. For large colloidal particles, the mean force is shown to scale with the solvent–colloid hard core radius. At densities of liquid solvents, sufficient solvent–colloid attractions are found to strongly enhance the repulsive barriers in the potential of mean force and reduce the depth of the depletion well. At lower density, the mean force can become repulsive. Solvent–solvent attractions can suppress the oscillations in the potential and make it more attractive. These results are finally contrasted with those relative to purely repulsive interactions.

Irreversible deposition/adsorption processes on solid surfaces

In this article, we summarize the knowledge in the field of irreversible deposition processes of large molecules or colloidal particles on solid surfaces. An irreversible adsorption process is defined as a process in which, once adsorbed, a particle can neither diffuse along, nor desorb from the surface. We first introduce the basic tools used in these studies, one of the most important being the concept of available surface function. General results relative to these processes are then presented. We discuss, in particular, the connection between the reduced variance of the number density fluctuations of adsorbed particles and the available surface function. We then review the main models which were introduced in the literature to account for these phenomena. They can be divided in two classes: (i) the models which are based entirely on statistical and geometrical grounds. The best known and most widely studied of them is the Random Sequential Adsorption (RSA) model which is discussed in details. For the processes in which gravity plays an important role one uses the Ballistic Deposition (BD) model. We also present models which are aimed at accounting for the behavior lying between the ballistic deposition and the RSA. (ii) The second type of models corresponds to those which take explicitly the diffusion of the particles in the vicinity of the adsorption plane into account. The results relative to these models, called diffusional models, are discussed in details. Finally, the last part of the review is devoted to experimental results. We show, in particular, that the Langmuir model, which is the most widely used model in the literature to account for the protein adsorption kinetics, does not predict correctly the experimental observations. We present and discuss in a critical way experimental evidence which seems to indicate the validity of the RSA and ballistic models.

The effective interaction between colloidal hard sphere particles in a polymerizing solvent. Application of Wertheim's theory of association

The effective interaction between colloidal particles in a solvent of hard spheres, which can polymerize with the formation of chains and rings, is studied. Polymerization results in changes of the effective interaction between the colloid particles, compared with the effective interaction for unpolymerized solvents that has been extensively studied in previous publications. To describe the mixture, we use the associative Percus-Yevick approximation for Wertheim's Ornstein-Zernike integral equation. Both the infinite dilution and the nonvanishing concentration limits for the colloid species are considered. It is shown that, in the model of Wertheim for polymerization, the intercolloidal potential of mean force (PMF) depends primarily on the solvent density and to a lesser extent on the degree of polymerization. A depletion of the solvent density around the colloid particles is observed. If the solvent consists of longer chains, compared with hard spheres, the correlations between the colloid particles are of longer range. The oscillations of the PMF depend upon the average chain length but not so strongly as in the model with chains having a fixed number of beads; this is probably due to the flexibility of the chain conformations in Wertheim's model. We also study the effect of the size ratio of the colloid spheres and the solvent monomers. We observe that the polymer hypernetted chain closure is difficult to apply to the systems under study. This closure shares difficulties, especially near the critical point, that are well known in the study of the phase diagram of fluids with both repulsive and attractive interactions. For large colloidal particles, the mixed closure of Henderson is useful.

Small-Angle Neutron Scattering Study of Polyethylene-co-Methacrylate Ionomer Aqueous Solutions

A small-angle neutron scattering (SANS) study of aqueous solutions of polyethylene-co-methacrylic acid partially neutralized ionomers is presented. A series of polymers presenting different degrees of polymerization, degrees of neutralization, counterions, and molecular weights are studied. The modeling of the dilute solution spectra combined with the analysis of the large-angle asymptotic behavior of the concentrated solutions spectra allowed us to conclude that these polymers form very large colloidal particles which are well described as either finite length cylinders or prolate ellipsoids. It is shown that the most important parameter defining the size of the particles is the ionic content. The radius varies from 55 to 135 Å and is proportional to the average distance between ionized carboxylated groups while the length of the particles increases dramatically for low ionic content. A transition from almost spherical particles to cylinders was evidenced. The effect of the concentration and of the molecular weight can be considered as negligible while the effect of the counterion is a modification of the eccentricity of the particles. The SANS spectra of concentrated solutions present a well-defined maximum. The position of the scattering maximum can be calculated using space-filling arguments, and the shape of the interference term is well reproduced by a Hayter and Penfold analysis.

Simulating the rheology of dense colloidal suspensions using dissipative particle dynamics

The rheological properties of colloidal suspensions of spheres, rods, and disks have been studied using a mesoscopic simulation technique, known as dissipative particle dynamics (DPD). In DPD, a suspension is modeled as a system of large colloidal particles in a liquid of interacting point particles. For the calculation of hydrodynamic interactions, this method is computationally more efficient than conventional techniques using a continuum model for the solvent. Applying a steady-shear rate to the particulate suspensions, we have measured the viscosity as a function of shear rate and volume fraction of the suspended particles. The viscosity of a 30 vol % suspension of spheres displays characteristic shear-thinning behavior as a function of increasing shear rate. The values for the high- and low-shear viscosity are in good agreement with experimental data. For higher particulate densities good results are obtained for the high-shear viscosity, although the viscosity at low-shear rates shows a dependence on the size of the suspended spheres that we attribute to finite size effects. Dilute suspensions of rods and disks show intrinsic viscosities which are in excellent agreement with theoretical predictions. For concentrated suspensions of both rods and disks, the viscosity increases with the third power of the volume fraction. We find the same scaling behavior as predicted by Doi and Edwards [M. Doi and S. F. Edwards, The Theory of Polymer Dynamics (Oxford University Press, New York, 1986)] for rod suspensions in the semidilute regime. The DPD simulation technique emerges as a useful tool for studying the rheology of particulate suspensions.

Absorption spectra of large colloidal silver particles in aqueous solution

Calculations of the optical absorption spectra of spherical particles of silver with a radius (r) of 10 to 80 nm in water were performed. The single intense absorption band for small particles (r=10 nm) with a maximum at about 395 nm is gradually broadened and shifted to the long-wave region with an increase in the particle size (to 435 nm atr=40 nm). In the case of large particles, the band splits into several components absorbing light in the visible spectral region. The results are in good agreement with the optical characteristics of colloidal solutions of silver obtained upon radiochemical reduction of Ag+ ions in aqueous solution.

Size Characterisation of Wheat Proteins, Particularly Glutenin, by Asymmetrical Flow Field-Flow Fractionation

Asymmetrical flow field-flow fractionation was demonstrated to fractionate the most high molecular weight proteins of glutenin found in flour from wheat cultivars with excellent bread-making performances. The fractionation occurs in a flow of liquid on the basis of differences in diffusion coefficients, and therefore hydrodynamic diameters, and applies to large molecules and colloidal particles. Microgram quantities of glutenin were analysed and shown to have hydrodynamic diameters of 5 to 45 nm. Much larger sizes can be analysed because the method does not have an upper size limit as do gel electrophoresis and gel filtration. Fractionations can often be done in less than 5 minutes. Sonication in combination with a surfactant, SDS, dissolved proteins from freeze-dried extracts of flour obtained by sequential extraction using progressively increasing concentrations of dilute HCl. An early fraction showed a response around 8 nm hydrodynamic diameter, which corresponds to a molecular weight range of 23 000–128 000. This indicated the presence of gliadins. Later fractions gave responses up to 35 nm, which translated to the molecular weight range 440 000–11 million, which is consistent with the fact that these fractions contain glutenin. Reduction of a late-fraction sample shifted the band to 8 nm, which corresponds to glutenin subunits. The method appears to be a viable procedure to obtain rapid size fractionations of unreduced glutenin.

Effects of Particle Size and Polydispersity on the Depletion and Structural Forces in Colloidal Dispersions

The effective pair interaction (potential of the mean force) between large hard spherical colloidal particles in a dispersion containing small hard sphere particles has been studied theoretically. We calculated the effective interaction from the total correlation function by solving the Ornstein-Zernike equation. For a simple binary mixture of particles, the calculated effective interaction between two large (colloidal) particles is found to be oscillatory in general, including both the Asakura-Oosawa type attractive depletion well and the repulsive energy barrier (which is caused by the formation of small particle layers between these two large particles). A systematic study was conducted to examine the relationship between the effective interaction potential and the controllable parameters like the particle concentration and the size ratio. The existence of the structural energy barrier is expected to have dual effects on the stability of the large particle dispersion in general: stabilizing in a short time scale and destabilizing in a long time scale. The effect of the polydispersity of small particles on both depletion and the structural force between large particles is also addressed in our study, and we found that polydispersity affects the structural energy barrier more than it does the attractive depletion well. 45 refs., 11more » figs.« less

Competitive immunosorbent assays using ligand-enzyme conjugates bifunctional liposomes: theory and experiment.

Two models of immunoadsorbent assays are developed that describe the competitive adsorption of labeled antigen and unlabeled analyte to antibody binding sites immobilized on a solid surface. In the first model, a small labeled antigen and a small unlabeled analyte compete with only binding site limitations and no steric limitations. A multicomponent langmuir isotherm results that is sufficient to quantify competitive adsorption. This model can describe, with no adjustable parameters, the data of competitive assays for biotin using biotinylated horseradish peroxidase (B-HRP) over a wide range of anti-biotin antibody (ABA) surface densities. In the second model, the small unlabeled analyte competes with a large colloidal particle containing many antigens and enzyme labels attached to its surface. This model quantifies the steric interference that large particles can experience upon binding (large ligand effect) due to the lower probability of finding an available area of the right size to accommodate the larger adsorbent. This large ligand model also takes into account the increased probability of binding a large particle due to the larger number of antibody binding sites covered per collision. The resulting model is used to analyze the competitive assay data of biotin competing with liposomes to which many biotin and HRP molecules have been conjugated. This analysis is of interest because previous work has shown that these bifunctional liposomes can reduce the detection limit for antigens in bulk solution relative to assays performed with conventional small labeled antigens.

熱膨張抑制埋没材を用いたチタン鋳造 第３報 大粒子コロイダルシリカを練和液として用いた場合

We studied the effects of the size of colloidal silica in a mixing liquid for phosphatebonded thermal expansion inhibited investment on the surface properties and the reaction layers of pure titanium crowns.1. Cast crowns were easiest to separate from their molds when 40% large colloidal silica particles were used.2. The surface roughness of a crown when using large colloidal silica particles was observed half as much as that when using small colloidal silica particles.3. The surface hardness of crowns showed lower values when large colloidal silica particles were used than when small colloidal silica particles were used.4. The surface roughness of investment mold showed lower values when large colloidal silica particles were used than when small colloidal silica particles were used.5. Elemental analysis revealed that there was a Ti-poor reaction layers which contained O, P and Si in the surface of the crown.Additionally, this analysis showed that the lowest concentration of Si and P was in the surface of crown when 40% large colloidal silica particles were used. These results indicate that the surface properties and the reaction layer of pure titanium crowns made by the titanium casting procedure using phosphate-bonded investment when large colloidal silica particles were used were better than those of small particles.

Facilitated Transport of Charged Colloids in Geologic Media

AbstractDiffusion of a charged colloidal particle in a two-dimensional simple shear flow was studied by means of Monte Carlo calculations and the effects of a bounding wall and charge of a particle on convective diffusion were elucidated. Taking charge effects into account has a marked effect on the diffusion behavior of the particle, increasing the migration distance. Diffusion of latex colloidal particles in a quartz-powder packed cell was also studied by through-diffusion methods. For the large latex colloidal particles, the effective diffusion coefficients measured are larger than those estimated by Stokes-Einstein equation.

Present-day understanding of crystal growth from aqueous solutions

Recent results on the kinetics of crystal growth from aqueous solutions are reviewed, with the emphasis placed on the KDP group of crystals. Elementary parameters of spiral growth are discussed. Nonlinear dependences of step velocity on supersaturation, incompatible with the Burton-Cabrera-Frank theory, are analysed, and a new more general theory is presented. Morphological stability of stepped vicinal faces is considered, with due account of the influence of solution hydrodynamics and impurities. New experiments are described, in which reproducible growth of the ADP dipyramidal face by two-dimensional (2D) nucleation is found. Effective step energy is deduced. At low supersaturations the preferential 2D nucleation is demonstrated to occur on active sites, whereas at high supersaturations the 2D nucleation seems to be homogeneous. Secondary nucleation in aqueous solutions is analysed. New experiments demonstrating a direct correlation between secondary nucleation and basic crystal cracking are described and discussed. Therefore, the achievement of high supersaturations and thus high growth rates is closely related to high crystal perfection, and vice versa. Selected achievements in biological crystallization, primarily in nucleation and growth of protein, viruses and other crystals built up of large molecules or colloidal particles, are briefly discussed.