Colloidal Particles In Concentrated Suspensions Research Articles

Charge fluctuations in charge-regulated systems: dependence on statistical ensemble.

We investigate charge regulation of nanoparticles in concentrated suspensions, focusing on the effect of different statistical ensembles. We find that the choice of ensemble does not affect the mean charge of nanoparticles, but significantly alters the magnitude of its fluctuation. Specifically, we compared the behaviors of colloidal charge fluctuations in the semi-grand canonical and canonical ensembles and identified significant differences between the two. The choice of ensemble-whether the system is isolated or is in contact with a reservoir of acid and salt-will, therefore, affect the Kirkwood-Shumaker fluctuation-induced force inside concentrated suspensions. Our results emphasize the importance of selecting an appropriate ensemble that accurately reflects the experimental conditions when studying fluctuation-induced forces between polyelectrolytes, proteins, and colloidal particles in concentrated suspensions.

Sedimentation velocity and potential in a concentrated colloidal suspension: Effect of a dynamic Stern layer

The standard theory of the sedimentation velocity and potential of a concentrated suspension of charged spherical colloidal particles, developed by H. Ohshima on the basis of the Kuwabara cell model (J. Colloid Interf. Sci. 208 (1998) 295), has been numerically solved for the case of non-overlapping double layers and different conditions concerning volume fraction, and ζ-potential of the particles. The Onsager relation between the sedimentation potential and the electrophoretic mobility of spherical colloidal particles in concentrated suspensions, derived by Ohshima for low ζ-potentials, is also analyzed as well as its appropriate range of validity. On the other hand, the above-mentioned Ohshima's theory has also been modified to include the presence of a dynamic Stern layer (DSL) on the particles’ surface. The starting point has been the theory that Mangelsdorf and White (J. Chem. Soc. Faraday Trans. 86 (1990) 2859) developed to calculate the electrophoretic mobility of a colloidal particle, allowing for the lateral motion of ions in the inner region of the double layer (DSL). The role of different Stern layer parameters on the sedimentation velocity and potential are discussed and compared with the case of no Stern layer present. For every volume fraction, the results show that the sedimentation velocity is lower when a Stern layer is present than that of Ohshima's prediction. Likewise, it is worth pointing out that the sedimentation field always decreases when a Stern layer is present, undergoing large changes in magnitude upon varying the different Stern layer parameters. In conclusion, the presence of a DSL causes the sedimentation velocity to increase and the sedimentation potential to decrease, in comparison with the standard case, for every volume fraction. Reasons for these behaviors are given in terms of the decrease in the magnitude of the induced electric dipole moment on the particles, and therefore on the relaxation effect, when a DSL is present. Finally, we have modified Ohshima's model of electrophoresis in concentrated suspensions, to fulfill the requirements of Shilov–Zharkhik's cell model. In doing so, the well-known Onsager reciprocal relation between sedimentation and electrophoresis previously obtained for the dilute case is again recovered but now for concentrated suspensions, being valid for every ζ-potential and volume fraction.

Dynamic electrophoretic mobility of spherical colloidal particles in concentrated suspensions: approximation of nonoverlapping double layers

An approximate expression for the dynamic electrophoretic mobility μ of spherical colloidal particles in concentrated suspensions in an oscillating electric field is derived on the basis of Kuwabara’s cell model. The analysis is constrained to low zeta potentials and nonoverlapping electrical double layers of adjacent particles. The dynamic mobility depends on the frequency ω of the applied electric field and the particle volume fraction φ as well as on the reduced particle radius κa and the zeta potential ζ. The approximation of nonoverlapping double layers is found to be valid for κa>50 (where κ is the Debye–Huckel parameter and a is the particle radius). Even for κa>10 the approximation works well for φ<0.15.

Dynamic mobility of spherical colloidal particles in concentrated suspensions: moderately thick double layers

A simple approximate expression is derived for the dynamic electrophoretic mobility μ of spherical colloidal particles in concentrated suspensions in an applied oscillating electric field. This expression is applicable when the particles have low zeta potentials, very low relative permittivity and moderately thick double layers. It is found that the present approximation produces the correct dependence of the mobility magnitude on the particle volume fraction φ. That is, the mobility magnitude exhibits a maximum when plotted as a function of φ, in contrast to the case of infinitesimally thin double layers, in which case the mobility magnitude increases as φ increases, showing no maximum.

Sedimentation Potential in a Concentrated Suspension of Spherical Colloidal Particles

A theory of sedimentation in a concentrated suspension of spherical colloidal particles proposed by Levineet al. (J. Colloid Interface Sci.57, 424 (1976)) is further developed to derive a simple convenient expression for the sedimentation potential applicable for low zeta potentials and nonoverlapping electrical double layers of adjacent particles. An Onsager relation between the sedimentation potential and electrophoretic mobility of spherical colloidal particles in concentrated suspensions is derived.

Dynamic Electrophoretic Mobility of Spherical Colloidal Particles in Concentrated Suspensions

A theory for the dynamic electrophoretic mobility μ of spherical colloidal particles in concentrated suspensions in an oscillating electric field is proposed on the basis of Kuwabara's cell model. The dynamic mobility depends on the frequency ω of the applied electric field and the particle volume fraction φ as well as on the reduced particle radius κa(where κ is the Debye–Hückel parameter andais the particle radius) and the zeta potential ζ. A mobility formula which involves numerical integration is obtained for particles with zero permittivity and low ζ. It is found that the mobility magnitude decreases with decreasing κaas in the static case (ω = 0) and as in the single particle case (φ → 0) and that it decreases with increasing ω as in the single particle case. However, the φ dependence of the mobility magnitude is much more complicated. Namely, for small κathe mobility magnitude decreases with increasing φ as in the static case. For large κait increases with increasing φ. For moderate κaand not very low ω the mobility magnitude may exhibit a maximum. In all cases the ω dependence of the mobility magnitude becomes less as φ increases, that is, the dynamic mobility at any ω approaches the static mobility as φ increases. An accurate mobility formula without involving numerical integration applicable for all κaat zero particle permittivity and low ζ is also derived. This formula is applicable even for high ζ at κa→ ∞ unless the dynamic relaxation effect becomes appreciable.

Electrophoretic Mobility of Spherical Colloidal Particles in Concentrated Suspensions

A general mobility expression is derived for a swarm of identical spherical colloidal particles in concentrated suspensions on the basis of S. Kuwabara's cell model [J. Phys. Soc. Japan14,527 (1959)]. This expression, which is a function of κa(κ = Debye–Hückel parameter anda= particle radius) and the particle volume fraction (or the porosity), tends to a mobility expression derived by S. Levine and G.H. Neale [J. Colloid Interface Sci.47,520 (1974)] for low ζ potentials and to that derived by M.W. Kozak and E.J. Davis [J. Colloid Interface Sci.129,166 (1989)] for the case of nonoverlapping double layers. A simple approximate mobility expression is derived with relative errors less than 4%.

Dynamics of concentrated colloidal suspensions

Studies of the dynamics of colloidal particles in concentrated suspensions by dynamic light scattering, viscometry and computer simulation are described. Recent experiments have revealed a relationship between structural relaxation and viscosity, and an interesting scaling of the intermediate scattering functions.

Three-dimensional imaging of submicrometer colloidal particles in concentrated suspensions using confocal scanning laser microscopy

Confocal scanning laser fluorescence microscopy (CSLM) has been used as a novel and versatile tool to study the interparticle structure of colloidal particles in the bulk of concentrated suspensions. Micrographs showing individual particles were made of a “colloidal glass”, a “colloidal crystal”, and a thermotropic reversible gel. The imaged particles consisted of newly developed monodisperse, fluorescent silica spheres. The fluorophore fluorescein isothiocyanate was chemically incorporated deep inside the l-octadecanolcoated particles. Such model spheres in combination with the improved resolution and optical sectioning of CSLM enable three-dimensional imaging and other measurements that are very difficult with other microscopic techniques or indirect methods like scattering.

Electrical double-layer interaction between two eccentric spherical surfaces

The nonlinear Poisson—Boltzmann equation has been solved numerically for a model system in which a spherical colloid is placed eccentrically in the core of a spherical shell of colloidal dimension. The proposed model simulates systems such as colloidal particles entrapped in near-spherical cavities in porous media, colloidal particles in concentrated suspensions, macromolecules (e.g., proteins) solubilized in reverse micelles, and drug particles encapsulated within liposomes in drug delivery systems. To describe the electrical double-layer interaction in the above systems of interest, the interaction energy has been calculated by first solving numerically the Poisson—Boltzmann equation for the model system, as well as for each of its component surfaces (the surface of the shell and the outer surface of the particle in the core). The equation has been solved in bispherical coordinates for the model system. Both constant surface potential and constant surface charge types of boundary conditions have been treated. The effects of electrical double-layer interaction on the aforementioned systems have been predicted based on the results of interaction energy calculations.

A fibre-optic sensor for flocculation detection by electric field light scattering

A laser-based apparatus is described that is able to differentiate between the isolated and flocculated state of colloidal particles in concentrated suspensions. It is based on the principle that the intensity of the scattered light changes when anisodiametric particles are made to rotate and align in an electric field. The system is demonstrated by measurements on kaolinite clay suspensions and can be readily adapted to monitor and control suspensions deflocculation in ‘on-line’ industrial processes.