Yee-Kwong Leong

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.

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.

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.

Surface chemistry effects on concentrated suspension rheology

Abstract Two brown coals of different surface charge densities and ionic strengths were evaluated for their rheological properties. Rheological behavior ranging from low viscosity Newtonian to high viscosity pseudoplastic yield, depending on the surface chemistry, was observed. By appropriately varying the ionic strength or surface charge density it was possible to convert the rheological behavior of one brown coal to that of the other. Yield stress behavior occurs when the surface charge density is low or the ionic strength is high. Conversely, the suspension is Newtonian at high surface charge density or low ionic strength. A minimum viscosity occurs when the suspension is close to the point of transition from attractive to repulsive particle interaction (i.e., just dispersed). The knowledge gained from this fundamental investigation was subsequently exploited in the development of a coal-water suspension fuel.

SURFACE CHEMISTRY AND RHEOLOGICAL PROPERTIES OF ZIRCONIA SUSPENSIONS

The pH and nature of anions were found to have a very significant effect on the rheological properties of zirconia suspensions. The variation in rheological properties with pH and anion type correlated well with the change in surface properties of zirconia. Zirconia suspensions with maximum yield stress and viscosity occurred at and near the isoelectric point (IEP). At a pH where the magnitude of the electrophoretic mobility of the zirconia is high, a low viscosity dispersed suspension was obtained. The zirconia suspensions were dispersed with potassium hydroxide at high pH while hydrochloric acid or nitric acid was used for dispersion at low pH. Sulfuric acid however did not disperse the suspensions at low pH because of significant sulfate adsorption on the zirconia particle. Polycarboxylate additives (polyacrylate) shifted the pH of maximum yield stress of zirconia to a lower pH. The degree of pH shift increased with increasing concentration of polycarboxylate; an effect attributed to the adsorption of ...

Rheology of brown coal-water suspensions

Techniques for measuring the fundamental flow properties of as-mined Victorian brown coal suspensions are developed. Flow properties are presented for Morwell, Yallourn and Loy Yang coals as a function of concentration for fixed particle size distribution. Even at the relatively low solids concentration of 20 to 30 percent by weight, the suspensions exhibit complex non-Newtonian characteristics. Generally at high concentration, the suspensions are thixotropic with a shear rate dependent viscosity and exhibit a yield stress. The inherent thixotropy of the coals can be exploited and it is technically but perhaps not economically feasible to transport the as-mined coals in a pipeline. The power requirement to pump the coals in a pipeline is found to be 10 to 30 times that required to pump black coal in the Black Mesa pipeline in the U.S.A. The differences in the rheological properties of the three coals is related to the surface properties of the coal particle — expressed in terms of carboxylate and inorganic cation content, and to differences in the pore volume of the coals. The flow characteristics of one coal can be converted to that of another simply by varying either the carboxylate or ionic content of the suspension.

A new method of processing the time-concentration data of reaction kinetics

Experimental data of reaction kinetics are usually in the form of concentration versus time. For kinetics investigation it is more convenient to have the data in the form of reaction rate versus concentration. Converting time-concentration data into concentration-reaction rate data is an ill-posed problem in the sense that if inappropriate methods are used the noise in the original data will be amplified leading to unreliable results. This paper describes a conversion procedure, independent of reaction rate model or mechanism, that manages to keep noise amplification under control. The performance of this procedure is demonstrated by applying it to several sets of published kinetic data. Since these data are accompanied by their rate equations, the computed rates are used to obtain the unknown parameters in these equations. Comparison of these parameters with published figures and the ease with which they are obtained highlights the advantages of the new procedure.

Ageing and collapse of bentonite gels—effects of Li, Na, K and Cs ions

The state of bentonite gels at the start of the ageing experiment must be well-defined, and this required the gels to be at a constant surface chemistry condition. This is achieved by allowing the freshly prepared gels to rest for a day. At this state, the yield stress is constant, provided that the gel is at an equilibrium breakdown state after stirring prior to each measurement. This point is also the yield stress at zero aged time. Ageing study then commenced, and the behaviour is generally characterised by an increasing yield stress with wait time. Alkali metal ion type and concentration affect the gel ageing and stability behaviour significantly. The ageing behaviour is most pronounced at low salt concentrations for the smallest and most strongly hydrated cations, Li +and Na +. The yield stress at any given aged time and its rate of increase are generally larger. Coarsening of these suspensions was observed. The opposite is true for the weakly hydrated K +and Cs +ions. At high concentrations of 0.5 and 1.0 M Cs, K and Na ions, the gels became unstable over time and phase-separated. The stability time of these weak gels was found to increase with decreasing cation size, Na > K > Cs. This stability time displayed a very strong quantitative correlation with the hydration bond length. Coarsening was also expected, but not observed due to the lack of integrity of these weak aggregates during particle size measurement. The recovery or ageing behaviour was fitted with both the Nguyen–Boger and Leong models.

Inter-particle forces arising from adsorbed bolaformsurfactants in colloidal suspensions

The effects of adsorbed bolaform surfactants, [HOOC(CH 2 ) n COOH], on the yield stress–pH behaviour of concentrated aqueous ZrO 2 suspensions are reported. Both the bolaform concentration and its chain length, n, increase the yield stress. A ten-fold increase in the maximum yield stress, τ ymax , is recorded for n10. The results are explained in terms of a particle bridging model. The increase in τ ymax reaches a limit at the critical dimer concentration.

Effects of Gum Arabic macromolecules on surface forces in oxide dispersions

Abstract The effects of Gum Arabic (GA) on the zeta potential-pH and yield stress-pH behaviour of α-Al 2 O 3 , TiO 2 and ZrO 2 dispersions were evaluated. GA is amphoteric in character as it contains protein fractions. Its macromolecule has a molecular weight ranging from 50 000 to several million. In aqueous solution, the GA molecule assumed a very compact structure. Upon adsorption it will form a relatively thick protrusion from the surface that may act as a steric layer or participate in bridging. At charge neutral condition, GA increased the maximum yield stress, τ y max , at pH >5.0 by as much as 60–80% for both α-Al 2 O 3 and ZrO 2 dispersions. This strengthening of the network structure was attributed to bridging. In contrast GA decreased the τ y max of TiO 2 dispersions at pH