Binary Colloidal Suspensions Research Articles

Brownian motion in binary colloidal suspensions

The statics and dynamics of dilute binary suspensions of charged polystyrene spheres have been investigated by static and dynamic light scattering experiments (SLS and DLS). The samples under study consist of a small amount of large spheres immersed in a system of small spheres. The SLS data of the measured static structure factor are compared with a theoretical approach, the so-called extended resealed mean spherical approximation, for colloidal mixtures. Dynamic light scattering at large wavenumbers was used to measure the self-scattering function which, in general, contains information on the Brownian motion of both species. The experimentally observed self-scattering functions are compared with theoretical calculations based on the single exponential approximation and good agreement has been found within the experimentally accessible time window. It is demonstrated that for appropriately chosen system parameters, the Brownian motion of only the large spheres is observed.

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.

Order in a simple colloidal mixture suspension

Preliminary SANS (small angle neutron scattering) measurements from a well characterized binary colloidal suspension of 91 nm polystyrene latex and 54 nm silica particles in water are reported for the Q range 0.02< Q (nm −1)<0.12, where Q is the scattering wave vector. The polystyrene-to-silica number ratio was 1.7 to 1 and the mixture volume fraction φ=0.15. Key measurements were the scattered intensity of the mixture, and the partial scattered intensities of the polystyrene and silica in the mixture. The partial scattered intensity of one species was obtained by contrast matching the other species in the appropriate mixture of H 2O and D 2O. Partial structure factors and pair distribution functions are compared with computer simulation data for a soft sphere mixture and with the haro sphere expressions of Aschroft and Langreth.

Clustering of binary colloidal suspensions: Theory

Recent experiments on binary colloidal suspensions have shown that particles of the first kind (e.g., alumina) can be induced to flocculate by the presence of particles of the second kind (e.g., polyacrylate), within a certain range of the density of the second species of particles. This is similar to the depletion flocculation and depletion stabilization of polymer—colloidal systems. We have thoroughly examined the binary-particle systems with numerical simulations (Monte Carlo) as well as with analytical equilibrium calculations [cluster variation method (CVM)]. The simulations show a peak in the flocculation rate as the number density of the second species particles is varied, in agreement with the experiments. The CVM calculations show a monotonic increase in cluster size as the particle 2 density is increased. Moreover, the simulations show an aging phenomenon at high particle 2 densities, i.e., the growth of the cluster size in the initially restabilized region, which has also been well observed in the experiment. We further show from free energy calculations that the flocculated state, including the seemingly restabilized state at high particle 2 densities, is metastable and the underlying thermodynamically stable state is phase separation. Therefore, the restabilization at high particle 2 densities is due to kinetics, namely the slowdown in particle movements, but not to thermodynamic reasons. Our calculations together with the experiments on binary colloids may shed some light on the understanding of polymer—colloidal systems.

Clustering of binary colloidal suspensions: Experiment

The stability of binary colloidal suspensions is examined and compared to that of particle—polymer systems. In particle—polymer systems, it is known that the addition of free polymers within a concentration range of polymers to a stabilized dispersion leads to flocculation. Similar to particle—polymer systems, we have observed that in binary colloidal suspensions, particles of the first kind can be induced to flocculate by the presence of particles of the second kind, within a certain range of concentration of the second kind of particles. Moreover, we have observed the growth of clusters of particles 1 at a later time in the initially restabilized regime. This slow growth of clusters in the initially restabilized suspensions is associated with the slowing of particle movement due to higher particle 2 concentrations. The aging phenomenon (growth of clusters) that occurs at high particle 2 concentrations clearly indicates that the seemingly restabilized suspension at high particle 2 concentrations is not due to thermodynamics, but to slow kinetics associated with slow particle movements at high densities. A similar aging phenomenon was observed with a particle—polymer (α-Al 2O 3/PAA) system in the seeming restabilization regime at high polymer concentrations.

Contrast matched studies of a sheared binary colloidal suspension

A method is proposed to measure by neutron scattering the partial scattered intensities from a suspension mixture in an H2O/D2O solvent that will match out one of the components. Total and partial neutron-scattered intensities from an aqueous colloidal suspension mixture (volume fraction ϕ = 0·15) of 91 nm polystyrene latex and 54 nm silica particles in water are reported. Results are given for the mixture at rest and under shear. We comment on the standard assumption that structure factors of components in a mixture should scale with the particle radii.

Structure of a binary colloidal suspension under shear

Neutron scattering intensities from an aqueous mixture suspension of 91 nm polystyrene latex particles and 54 nm silica particles are reported in the range 0.02 < Q/nm–1 < 0.2, where Q is the momentum transfer. The suspension was dense at a mixture volume fraction of 0.15, and the polystyrene/silica particle ratio was ca. 1.7 : 1. Results are given for the suspension at rest and under shear. The sheared data were obtained with a concentric cylinder shearing apparatus constructed and tested at the SANS facility of the National Institute of Standards and Technology and the pulsed neutron facility, LANSCE, of the Los Alamos National Laboratory. The design and operation of the cell is described. The shear-influenced behaviour of the mixture is compared with and contrasted to that of a pure polystyrene suspension that can form a crystal lattice in equilibrium, but which melts to a liquid-like structure under shear.A method is proposed to measure, by contrast matching or variation, the polystyrene and silica partial scattered intensities from the mixture suspension in H2O–D2O solvents of different scattering-length densities. Estimates of the partial structure factors are given.

Stability of a Binary Colloidal Suspension and its effect on Colloidal Processing

The stability of a colloidal suspension plays an important role in colloidal processing of materials. The stability of the colloidal fluid phase is especially vital in achieving high green densities. By colloidal fluid phase, we refer to a phase in which colloidal particles are well separated and free to move about by Brownian motion, By controlling parameters such as pH, salt concentration, and surfactants, one can achieve high packing (green) densities in the repulsive regime where the suspension is well dispersed as a colloidal fluid, and low green densities in the attractive regime where the suspensions are flocculated [1,2]. While there is increasing interest in using bimodal suspensions to improve green densities, neither the stability of a binary suspension as a colloidal fluid nor the stability effects on the green densities have been studied in depth as yet. Traditionally, the effect of using bimodal-particle-size distribution has only been considered in terms of geometrical packing developed by Furnas and others [3,4]. This model is a simple packing concept and is used and useful for hard sphere-like repulsive interparticle interactions. With the advances in powder technology, smaller and smaller particles are available for ceramic processing. Thus, the traditional consideration of geometrial packing for the green densities of bimodal suspensions may not be enough. The interaction between particles must be taken into account.

The split in the second peak in the structure factor of binary colloidal suspensions: glass-like order

Structure factors of dilute binary colloidal suspensions of various compositions, particle diameter ratios and ionic impurity concentrations are investigated using polarised light scattering. For the first time, a split in the second peak of the measured structure factor is observed, suggesting the existence of glass-like order in the binary suspensions. The near-forward-scattered intensity from the colloidal suspension exhibiting a split second peak fluctuates much more slowly than that from the liquid-like ordered phase, corroborating the existence of glass-like order in the binary mixtures. The time evolution of the structure indicates that a liquid-like order continuously evolves into glass-like order as the strength of inter-particle interaction is increased by reducing the impurity concentration. The glass-like order is unstable towards formation of the crystalline state above a critical volume fraction (which depends on the impurity concentration) of the particles.