Peter J. Scales

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.

Chemical and physical control of the rheology of concentrated metal oxide suspensions

Abstract Experimental and theoretical work defining the effect of the surface chemistry and particle physics on the rheology of metal oxide suspensions is reviewed. The influence of variables, including solids concentration, particle size and size distributions are examined at various inter-particle interaction conditions controlled by pH, electrolyte concentration, and/or addition of various additives for both flocculated and well-dispersed suspensions. The rheological properties examined include shear viscosity, shear and compressive yield stress, shear thinning and shear thickening. Data from various studies on metal oxide suspension systems at well-defined surface chemistry conditions are interpreted through analyses, primarily of pair interactions and suspension microstructure. Better insight has been gained concerning the governing rules of the surface chemistry and various physical factors on the rheological properties of metal oxide suspensions.

Rheological evidence of adsorbate-mediated short-range steric forces in concentrated dispersions

In order to examine quantitatively the effect of short-range forces on the state of dispersion in dense suspensions, the effect of a series of anionic adsorbates on the rheology of a model zirconia concentrated suspension has been studied in detail. The anionic additives include sulfate, phosphate, pyrophosphate and polyphosphates as well as simple organic acid anions such as lactate, malate and citrate. The adsorbates shifted the pH of maximum static yield stress and, equivalently, the pH of zero zeta potential (ζ), to lower pH values. The additives also lowered the magnitude of the maximum yield stress at ζ= 0. The results are all consistent with the adsorbates producing a steric barrier equivalent to the size of the adsorbed molecule along the attractive van der Waals interaction. Separately the polyphosphate adsorbates are shown to assume a flat orientation and do not appear to produce a thick electro-steric barrier to coagulation.

The yield stress of concentrated flocculated suspensions of size distributed particles

An investigation of shear yield stress is made on well-characterized alumina suspensions of different distributed particle sizes at the vicinity of the particle isoelectric point (IEP) across a wide range of volume fractions. Experimental results are compared with recently developed models [; ] and structural effects on the yield stress are examined. The models predict the magnitude order of the yield stress below a volume fraction of approximately 0.42, suggesting that interparticle forces play a dominant role in determining the network strength in this concentration region. Deviations between experimental results and theoretical predictions are explained in terms of structural effects being controlled by a competition between weak particle–particle linkages and geometric resistance on the network strength. At higher volume fraction, the effect of geometric resistance on the deformation of suspensions becomes more pronounced. A number of models for the yield stress of size distributed suspensions are then proposed. Results suggest that the effect of polydispersity of particles on the yield stress of suspensions can be well characterized by a surface area average diameter and the broad size distributed suspension exhibits a higher yield stress than the narrow size distributed suspension of the same volume average diameter.

Interparticle forces arising from adsorbed polyelectrolytes in colloidal suspensions

The effects of molecular weight (MW) and concentration of polyacrylic acid (PAA) on the yield stress τy of ZrO2 suspensions were evaluated. The Mw range studied was from 1656 to 750000. The effect of adsorbed polyelectrolyte on the forces between particles was determined by focussing on the pH at which zero zeta-potential (ζ=0) is observed in the presence and absence of polymer. At this ζ=0 condition a maximum in yield stress (τymax) is observed. Low molecular weight PAA imposes a steric (repulsive) interaction energy which is gradually replaced by a bridging (attractive) energy as the molecular weight is increased. The shift in τymax and the pH at which ζ=0 both show systematic changes with Mw as electrostatic, steric, van der Waals and bridging forces change in relative importance.

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.

Effects of citrate adsorption on the interactions between zirconia surfaces

The adsorption of the trivalent citrate anion on zirconia has been studied as a function of additive concentration and solution pH. The effects of citrate adsorption on the surface properties of the zirconia were monitored by using adsorption isotherms, FTIR spectroscopy and electrokinetic measurements. Microscopic information about the adsorbed species and its surface conformation were obtained with an atomic force microscope (AFM). The force–distance profiles were obtained at AFM scan rates equivalent to those expected for Brownian collision rates between colloidal particles. For a given concentration of added adsorbate, the adsorbed amount of citrate was seen to increase with decreasing solution pH. This was attributed to increased Coulombic repulsions between the citrate and the surface at higher pH. The effective pHiep after adsorption was seen, from the electrokinetic data, to move towards lower pH values as the adsorbed amount increased. A limiting value of pH 3–3.2 was observed at an added citrate concentration of 10–3 mol dm–3. AFM force–distance measurements demonstrated the presence of an electrosteric barrier to flocculation in the presence of adsorbed citrate. A short-range steric barrier of ca. 10 A per surface was seen under conditions where citrate was adsorbed, this barrier was in addition to any long-range electrostatic component of the overall force profile. Best fits to these long-range electrostatic parts of the data were used to calculate a diffuse-layer interaction potential. Comparison of these potentials with the electrokinetic data showed good agreement.

Lipid profile remodeling in response to nitrogen deprivation in the microalgae Chlorella sp. (Trebouxiophyceae) and Nannochloropsis sp. (Eustigmatophyceae).

Many species of microalgae produce greatly enhanced amounts of triacylglycerides (TAGs), the key product for biodiesel production, in response to specific environmental stresses. Improvement of TAG production by microalgae through optimization of growth regimes is of great interest. This relies on understanding microalgal lipid metabolism in relation to stress response in particular the deprivation of nutrients that can induce enhanced TAG synthesis. In this study, a detailed investigation of changes in lipid composition in Chlorella sp. and Nannochloropsis sp. in response to nitrogen deprivation (N-deprivation) was performed to provide novel mechanistic insights into the lipidome during stress. As expected, an increase in TAGs and an overall decrease in polar lipids were observed. However, while most membrane lipid classes (phosphoglycerolipids and glycolipids) were found to decrease, the non-nitrogen containing phosphatidylglycerol levels increased considerably in both algae from initially low levels. Of particular significance, it was observed that the acyl composition of TAGs in Nannochloropsis sp. remain relatively constant, whereas Chlorella sp. showed greater variability following N-deprivation. In both algae the overall fatty acid profiles of the polar lipid classes were largely unaffected by N-deprivation, suggesting a specific FA profile for each compartment is maintained to enable continued function despite considerable reductions in the amount of these lipids. The changes observed in the overall fatty acid profile were due primarily to the decrease in proportion of polar lipids to TAGs. This study provides the most detailed lipidomic information on two different microalgae with utility in biodiesel production and nutraceutical industries and proposes the mechanisms for this rearrangement. This research also highlights the usefulness of the latest MS-based approaches for microalgae lipid research.

A Quantitative Analysis of Microalgal Lipids for Optimization of Biodiesel and Omega-3 Production

The lipid characteristics of microalgae are known to differ between species and change with growth conditions. This work provides a methodology for lipid characterization that enables selection of the optimal strain, cultivation conditions, and processing pathway for commercial biodiesel production from microalgae. Two different microalgal species, Nannochloropsis sp. and Chlorella sp., were cultivated under both nitrogen replete and nitrogen depleted conditions. Lipids were extracted and fractionated into three major classes and quantified gravimetrically. The fatty acid profile of each fraction was analyzed using GC–MS. The resulting quantitative lipid data for each of the cultures is discussed in the context of biodiesel and omega‐3 production. This approach illustrates how the growth conditions greatly affect the distribution of fatty acid present in the major lipid classes and therefore the suitability of the lipid extracts for biodiesel and other secondary products. Biotechnol. Bioeng. 2013; 110: 2096–2104.

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.