Stephen B. Johnson

Surface chemistry-rheology relationships in concentrated mineral suspensions

The rheological behaviour of concentrated mineral suspensions can be controlled by understanding and regulating the net particle-particle interaction force. This paper summarizes the experimental results from studies showing how the yield stress of concentrated aqueous suspensions of alumina, zirconia and kaolin can be controlled through regulation of pH and the addition of inorganic electrolytes, ionic surfactants and polyelectrolytes (polymers). The various kinds of inter-particle forces acting in aqueous suspensions of fine particles are reviewed and used to interpret the experimental results.

VOLUME FRACTION EFFECTS IN SHEAR RHEOLOGY AND ELECTROACOUSTIC STUDIES OF CONCENTRATED ALUMINA AND KAOLIN SUSPENSIONS

Abstract The electroacoustic and shear yield stress response of concentrated alumina and kaolin suspensions have been measured across a range of pH conditions and volume fractions. The inter-relationship between the surface properties as measured electrokinetically and the shear yield stress is shown to scale in a straight forward manner for alumina particles and a qualitative description has been developed from kaolin suspensions. The pair-wise interaction between particles over the range of volume fractions has been demonstrated to remain constant. The use of a semi-empirical electroacoustic volume fraction correction to the zeta potential, integral to the electroacoustic technique, appears reasonable on the basis of the results presented herein.

The electrokinetic and shear yield stress properties of kaolinite in the presence of aluminium ions

Abstract The electroacoustic ζ potential and shear yield stress ( τ y ) properties of kaolinite suspensions have been examined over a wide range of pH conditions and Al(III) concentrations. At zero or low Al(III) concentrations, the ζ potential data are dominated by the silica-like kaolinite face, while the corresponding τ y data show the characteristics of pH-dependent face and edge interactions. As the Al(III) concentration is raised, all ζ potential data become increasingly more positive, the electrokinetic isoelectric point progressively shifts to higher pH, and the τ y versus pH data change systematically from kaolinite-like to alumina-like behaviour. These findings are indicative of the adsorption of Al(III)-based hydrolysis products, as is predicted by the adsorption model of James and Healy, J. Colloid Interface Sci., 40 (1972) 65. A comparison of the properties of partially Al(III)-coated kaolinite particles in the presence of Cl − and SO 4 2− shows that the ζ potential results are more negative and the shear yield stress data are lower for the kaolinite–Al(III)–SO 4 2− system. These results can be rationalised in terms of the extent of surface coverage by Al(III) hydrolysis products and anion adsorption on the surface-adsorbed Al(III)-based precipitates.

Electrical double layer properties of hexadecyltrimethylammonium chloride surfaces in aqueous solution

Abstract The effect of aqueous KCl concentration on the electrical double layer properties of hexadecyltrimethylammonium chloride (CTAC) surfaces has been investigated. For self-assembled CTAC bilayers adsorbed onto amorphous silica surfaces, flat plate streaming potentials (ζs potentials), electrostatic potentials obtained from the electrophoretic mobility of colloid microspheres (ζe potentials) and diffuse layer potentials (ψd) derived from the force versus separation curves for the interaction of a colloid microsphere with a flat plate (measured with an atomic force microscope) have been analysed. Trends and differences in the ζs and ζe potentials have been attributed to incomplete electrical neutralisation of the silica surface by the inner adsorbed monolayer of CTAC, and differences in the silica surface charge density for the colloid and flat plate. It is considered that derived ψd values highlight the problem created by surface roughness for atomic force microscopy force curve analysis. CTAC micellar surface potentials (ψo), which have been determined by a solvatochromic acid-base indicator technique, are greater than the ζ potentials for the planar adsorbed CTAC surfaces. Analyses in terms of a site-binding model and classical electrical double layer theory suggest that the degree of counterion dissociation from the surfactant headgroups (α) is significantly different for highly curved and planar surfaces; α is in the range 0.10–0.15 for the adsorbed CTAC bilayers and in the range 0.41–0.66 for spherical CTAC micelles when activities are used in the calculation. The CTAC critical micelle concentration and the minimum area per CTAC molecule at a planar interface as a function of KCl concentration have been ascertained from air/aqueous solution surface tension curves. The hydrophobic contribution to the free energy of CTAC micellisation has been calculated on the basis that CTAC micelles behave as Nernstian objects in KCl solution with the surfactant monomer acting as a potential determining ion. The results for the CTAC KCl systems have been compared with the results obtained for similar hexadecyltrimethylammonium bromide KBr systems.

The Influence of Surface Chemistry on the Rheology and Flow of Flocculated Particulate Suspensions

Abstract The role of the surface chemistry of particles in controlling the rheology of flocculated particulate suspensions is discussed. The case is developed for the measurement of the shear yield stress of a suspension of alumina particles and a theoretical interpretation of the yield stress is presented which is able to describe the effect of particle size distribution, solid loading, pH and hence, electrokinetics of the suspension. Scaling of the data to the maximum yield stress at a given volume fraction provides a means of removing particle size and volume fraction related effects and gives information as to the mode of failure in yielding of suspensions and the mode of action of molecular additives.