Robert J. Hunter

Flow properties of coagulated colloidal suspensions: I. Energy dissipation in the flow units

Abstract The flow properties of electrically charged, but unstable, colloidal sols are investigated in the light of three possible models. Models which treat the ultimate flow unit at high shear rate as either a single particle or a hard (nondeformable) floc are shown to be incapable of providing a satisfactory description of the behavior. On the other hand an elastic floc model provides a consistent physical interpretation of both the linear region of the flow curve and the Bingham yield value for these plastic-pseudoplastic systems.

Flow properties of coagulated colloidal suspensions: III. The elastic floc model

Abstract The viscous flow behavior of a colloidal sol can be described in terms of the various contributions to the energy dissipation during flow. When this is done for a coagulated sol it is found that a satisfactory description of the known features of the basic shear diagram can be obtained by attributing most of the energy dissipation to two processes: (i) the viscous flow of the suspension medium around the flocs, which are the basic flow units, and (ii) the energy involved in stretching the flocs in order to break floc doublets apart so that the amount of structure in the system is preserved in spite of floc-floc collisions. This model can be analyzed in a simple manner and is able to generate all 10 of the known relationships between the flow parameters (Bingham yield value, critical shear rate, and plastic viscosity) and the colloidal properties (particle size, volume fraction, and interaction energy). It also predicts a linear dependence of critical shear rate on particle radius, but this has not yet been tested.

The effect of electrolyte concentration and co-ion type on the ζ-potential of polystyrene latices

Abstract High-frequency dielectric response measurements were performed on three monodisperse polystyrene latices at varying electrolyte concentrations. For one latex, the ζ-potential and electrokinetic charge obtained from these measurements were compared with results obtained from microelectrophoresis. It appears that the high-frequency dielectric response technique probes the whole of the diffuse double layer and gives an estimation of the Outer Helmholtz plane potential ψd. The results also showed that care must be taken when calculating the ζ-potential from the mobility in latex systems. To investigate the effect of co-ions on the ζ-potential, microelectrophoresis experiments were performed on a single latex at varying electrolyte concentrations using potassium fluoride, chloride, bromide, and iodide as the indifferent electrolyte. Little difference was found among these electrolytes, suggesting the absence of adsorption of co-ions into the inner Helmholtz plane. A model is proposed for the polystyrene/electrolyte interface that invokes localized surface roughness to explain the observed electrokinetic maximum.

The dependence of plastic flow behavior of clay suspensions on surface properties

Abstract A comparison is made between two current theories of pseudoplastic behavior (1, 2) which are based on a reaction-rate chemical equilibrium treatment. The Cross theory (2) is shown to be more general, to be better able to represent the behavior of the kaolinite system at low shear rates, and to be consistent with the “aggregate-collision” theory of Michaels and Bolger (4). This latter theory is extended to enable the specific surface properties of the kaolinite sol to be correlated with its rheological behavior, and a satisfactory description is found for the behavior of rapidly coagulating kaolinite sols with both positively and negatively charged surfaces. The behavior of slowly coagulating sols is less readily interpretable, but the proliminary results suggest that collisions between particles result in a high proportion (1%–10%) of particles surmounting the potential energy barrier and that the doublets so formed are readily broken down by the shear field. Aggregation in the secondary minimum, though it undoubtedly occurs, involves so little energy that it is not discernible at high shear rates.

The flow behavior of coagulated colloidal dispersions

Abstract A review is given of the work of the author and his collaborators in developing a discription of the viscous and elastic behavior of a coagulated colloidal dispersion in Coutte flow. The elastic floc model is described in some detail and the many tests to which it has been subjected are examined to establish its internal consistency and its ability to relationalize the considerable body of experimental data now available. The importance of control of colloid chemical interactions and, in particular, the zeta potential) is also emphasized.

The dependence of electrokinetic potential on concentration of electrolyte

Abstract The variation of the zeta potential of oxides, as a function of electrolyte concentration, has been investigated. Using typical values of the double-layer parameters (e.g., thickness and dielectric constant of the compact region), the simultaneous equations of the Gouy-Stern-Grahame model of the surface have been solved by numerical means, and the calculated potential at the Outer Helmholtz Plane (O.H.P.) compared with the observed zeta potential values. Reasonable agreement occurred in many cases, including situations involving specifically adsorbed ions, but deviations were most pronounced in monovalent electrolytes at low concentrations. Several variations were made to the parameters of the model to see which conditions gave the best simulation of the observed results. No combination of the parameters could produce agreement in all cases, unless the effective surface potential was reduced below the value indicated by elementary thermodynamic considerations. Placing the plane of shear at some distance from the O.H.P. did not produce any improvement in the agreement.

The interpretation of electrokinetic potentials

Abstract The electrical field around a charged particle influences both the viscosity and the permittivity of the dispersion medium. Both of these quantities are important in the calculation of the true ζ-potential from electrophoretic mobility measurements. A theoretical analysis is made of the effects of assuring ( 1 ) variable viscosity, ( 2 ) variable permittivity, and ( 3 ) both variable viscosity and variable permitivity in the double layer on the calculation of ζ-potentials. Comparison of the theoretical results with experiment shows that both effects are rather small under most conditions. Graphs are given of the corrections required to the Smoluchowski equation for the various values of B and f , the constants measuring the influence of the electrical field of the double layer on the permittivity and the viscosity, respectively. Haydons (1) experimental comparison of ζ-potentials and surface potential for positively and negatively charged micelles is examined on the assumption that the shear surface coincides with the plane of closest approach of the hydrated ions in the diffuse double layer. It is shown that the results are consistent with this hypothesis and that they do not require that the viscosity and permittivity near the surface depart significantly from their bulk values.

Measuring zeta potential in concentrated industrial slurries

Abstract The main practical and theoretical problems encountered in continuous monitoring of the zeta potential of concentrated industrial slurries are discussed. A suitable method must be able to make measurements in real time on flowing concentrates and provide a reliable assessment of the zeta potential from the measured parameters. It is shown that the electroacoustic method offers great advantages for this measurement and examples are given of its application in the study of oxides like titania and alumina and a bitumen-in-oil emulsion. In its application in computer chip manufacture (chemical mechanical polishing) a combination of electroacoustics and acoustic attenuation can widen the range and improve the reliability of the assessment of zeta potential and suspension stability.

Coagulation of Amphoteric Latex Colloids Reversibility and Specific Ion Effects

Coagulation studies of amphoteric latex sols of various i.e.p. values in LiNO3, KNO3 and CsNO3 solutions are reported for a wide range of salt concentrations and pH. Three different techniques for the study of coagulation phenomena all indicate reversibility, in that sols coagulated by pH or salt are able to be redispersed. K+ and Li+ counter ions are able to stabilize these sols in the high salt concentration region. With NO3– and Cs+, the expected narrow zone of coagulation at low salt (i.e.p. coagulation) expands into the usual broad coagulation zone at high salt, where coagulation is observed at all pH values. The stabilizing effect of Li+ and K+ counter ions is attributed to a hydration barrier at the interface.

The dielectric response of concentrated latices

Abstract Dielectric response measurements can, in principle, provide important information on the concentration of colloidal particles, their shape and charge, and the composition of the background electrolyte. In this paper we present measurements of the effect of particle volume fraction on the dielectric response of concentrated latices in the frequency range 1–10 MHz, a range where the theory predicts large dielectric dispersion effects. It is found that the measurements on suspensions with a volume fraction of 0.3 or more are in accordance with a result obtained using a cell model. As the volume fraction decreases the departures from the formula increase, but these may be due to errors in our measurement of the particle contribution to the complex conductivity at these low volume fractions.