review of the measurement of zeta potentials in concentrated aqueous suspensions using...

Advances in Colloid and Interface Science106 (2003) 55–81

0001-8686/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0001-8686(03)00105-2

Review of the measurement of zeta potentialsin concentrated aqueous suspensions

using electroacoustics

R. Greenwood*School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Abstract

This paper reviews the technique of electroacoustics as it has been applied to aqueoussuspensions of inorganic particles. It starts by charting the development of the techniquefrom its earliest beginnings in the 1930s to the present day. The technique has become wellestablished in the last decade with the advent of the Acoustosizer and the Acoustosizer II.Illustrations of how the technique can be used are given based on the author’s ownexperience, especially the measurement of iso electric points, the adsorption of polyelectro-lytes, the effect of ionic strength, the effect of powder surface area and the dissolution ofmaterial from powders. Some new data on(a) the adsorption of different molecular weightsof polyacrylic acid onto alumina and(b) titanium dioxide suspensions are also presented.� 2003 Elsevier B.V. All rights reserved.

Keywords: Electroacoustics; Zeta potential; Suspensions; Polyelectrolytes

1. Introduction

Over the last twenty years electroacoustics has proved to be a powerful tool forthe measurement of zeta potentials in concentrated aqueous suspensions. This paperfirst reviews the history behind the technique, and in particular, the application ofthe technique to aqueous suspensions of inorganic particles especially ceramic ones.The paper then reviews what type of experiments have been carried out usingelectroacoustics, using some previously published examples from the author’s ownwork. Finally, some new data are presented on the effect of molecular weight of

*Tel.: q44-121-414-7234; fax:q44-121-414-3626.E-mail address: [email protected](R. Greenwood).

56 R. Greenwood / Advances in Colloid and Interface Science 106 (2003) 55–81

polyacrylic acid adsorbing onto alumina under different salt concentrations. For anexcellent review of the theory behind the technique and the study of non-inorganicparticles see Hunterw1x. Although the first reported reference to the phenomena ofelectroacoustics was approximately 70 years ago, it was only since O’Brienw2–4xformulated his theories in the early 1990s that led to the development of commercialmachines that the technique has taken off.

There is an increasing trend within the ceramics industry towards finer and finerparticle sizes, as this leads to better strengths in the final product, shorter sinteringtimes and lower sintering temperatures. However, this reduction in particle size doeshave some associated problems. As the particle size range enters that of the colloidalsize range, i.e. sub micron, the particles will aggregate together due to the attractivevan der Waals force. If these aggregates persist during processing they can causeflaws in the final sintered product. Not only are these flaws potentially athseticallyunappealing, they also weaken the strength of the product. From the Griffithsw5xequation the strength of a ceramic body is inversely proportional to the flaw size.If the size of these flaws can be minimised to that of an individual particle then themechanical properties of the final product can be vastly increased. This techniqueis known as colloidal processingw6x and involves optimising the processingconditions for a powder from a fundamental understanding of the interparticleforces. As well as reducing the mechanical properties these flaws can also reducethe thermal, optical and electrical properties of the final product. The Acoustosizerand Acoustosizer II(Colloidal Dynamics USA) are especially suited for studyingthe colloidal processing of advanced ceramic materials.

Electroacoustics can refer to two types of processes. Firstly, if a sound wavepasses through a colloidal suspension it creates a macroscopic potential differencecalled the ultrasonic vibration potential or UVP effect. Secondly, the opposite effectoccurs when an alternating electric field is applied to a colloidal suspension, thisgenerates a sound wave, which is called the electrokinetic sonic amplitude or ESA.In both cases the applied field and the response occur at the same frequency.

The first reported reference to these phenomena was by Debyew7x in 1933, whogave a theoretical treatment of the UVP effect when applied to an electrolytesolution in water, which he termed the Ionic Vibration Potential, IVP. As the soundwave passed through the electrolyte solution it produced a separation of charge dueto differences in the effective masses and frictional coefficients of the solvatedanions and cations. The resulting sum of these tiny dipoles leads to a macroscopicpotential difference, which depends on the sound wave frequency. However, it wasnot until over a decade later, in which the experimental verification of the IVP wasestablished by Yaeger et al.w8x. Meanwhile, it had been established that much largereffects could be described theoretically in colloidal suspensions, the so-calledcolloidal vibration potential, CVPw9–11x. This effect was initially over estimatedby Enderbyw12x, but improved calculations were later made by Booth and Enderbyw13x. However, this was limited to the lower frequency range(<1 MHz) and suchapproaches have little modern value as today’s machines work at much higherfrequencies ranges of 0.1–10 MHz.

57R. Greenwood / Advances in Colloid and Interface Science 106 (2003) 55–81

The early work on the IVP has been reviewed by Zana and Yeagerw14,15x whoalso described the CVP and its relationship to proteins and polyelectrolytes. Morerecently, Marlow et al.w16x have reviewed the CVP and its extension to moreconcentrated suspensions. Since then there has been very little theoretical work onthe CVP, but there has been extensive development of the ESA effect since itsdiscovery in the early 1980sw17x. The ESA technique has transformed thecharacterisation of colloidal suspensions especially concentrated ones, because itallows simultaneous measurement of both the zeta potential and the particle size.Until the advent of this technique, the only methods for measuring zeta potentialswere for dilute systems only, e.g. electrophoresis. Early work on the ESA onlyappeared in conference proceedings or in the manufacturer’s technical notes, so theearly reviews were limitedw18–20x. The time taken from the first theoreticaltreatment of the UVP effect and its scientific exploitation reflects the technicaldifficulties involved. These were due to the fact that the devices used to generatethe sound waves require the application of electric fields that interfere with themeasurement of the resulting potentials. Additionally, measurement of the smallelectroacoustic signals required the development of low noise, high frequencyamplifiers. Nevertheless, substantial advances were made and by the 1980s com-mercial CVP devices were in development. However, one group of engineersconceived the idea of reversing the procedure and listening for the sound waveinstead. This they were able to develop quickly, building on the previous experiencegained from the CVP devices, but also due to the inherent advantages of the ESAtechniques over the alternative CVP. The resulting machine the ESA 8000 is still incommercial use, although it only operates at one frequency and requires the inputof a particle size to calculate the zeta potential.

The CVP is developed as the sound wave travels through the colloidal suspension,it moves the particles and their associated double layers in slightly different waysprovided there is a density difference between the particles and the medium. Theparticle charge becomes slightly separated from its counter charge in solution,creating an array of dipoles, which increase and decrease in magnitude on the samelength scale as the sound wave amplitude. It is the sum of these separate dipolefields that gives rise to the macroscopic potential, as long as the electrodes are notexactly one or more wavelengths apart. For a given sound wave intensity themaximum effect occurs when the electrodes are separated by an odd number ofwavelengths. In the ESA effect the applied electric field causes the charged particlesto oscillate backwards and forwards at the same frequency and this generates tinyacoustic dipoles associated with each particle(provided there is a density differencebetween the particle and the medium). These dipoles cancel one another throughoutthe body of the suspension except near the electrodes where the acoustic dipolesgenerate a sound wave, which can emerge from the suspension and move down thedelay rod where it is detected by the transducer. The transducer then detects theamplitude and phase angle of this sound wave as a function of the applied frequency.The phase angle measures the time lag between the applied field and the subsequentparticle motion. This is zero at low frequencies and increases with increasingfrequency and increasing particle size.


The first theoretical treatment of the ESA effect was provided by O’Brienw21xwho showed that the CVP and the ESA were reciprocally linked. An analogy canbe drawn with the link between electrophoresis and electro-osmosis established byMazur and Overbeekw22x. The initial analysis was limited to dilute solutions, butwas subsequently extended to systems of arbitrary concentration by O’Brienw3x,provided that the particles are small compared to the sound wave length. A conditionthat is always satisfied for colloidal particles for frequencies less than 20 MHz. Anexperimental verification of this reciprocal effect for polyelectrolytes and electrolyteswas provided by O’Brien et al. in 1994w23x. It is important to note the differencebetween the CVP and the ESA. The quality required is the dynamic mobility. TheESA technique gives this directly, whereas the alternative route requires not onlythe CVP, but also the complex conductivity. CVP devices only operate at onefrequency and the conductivity measurement is performed only in the low frequencylimit, which leads to an inherent error in estimating the dynamic mobility especiallyfor low conductivity systems due to neglect of the imaginary part of the conductivity.

The problem of relating particle mobility to the surface properties of a particlehas been examined since the start of the twentieth century when von Smoluchowskisolved the problem for d.c. electrophoresis in the special case where the doublelayer (k ) is thin compared to that of the particle radius(a), see Hunterw24x.y1

Smoluchowski’s solution breaks down forka-50 and large zeta potentials. TheAcoustosizer operates in the frequency range 0.3–11 MHz allowing particle sizingfrom 0.1 to 10mm. For smaller particles the zeta potential can be calculated, butthe inertia forces are too small to allow sizing. The dynamic mobility spectrum ismeasured over a frequency range to give the mobility spectrum, from which theparticle size and zeta potential can be obtained.

If the real part of the dynamic mobility is plotted against the imaginary part thenan Argand diagram can be obtained where each point represents a value of thedynamic mobility. The magnitude of the mobility is thus given by the distance fromthe origin and the phase lag is measured by theta. For the small particles themagnitude does not change greatly with frequency and the phase lag increasesslightly. The larger particles, however, show a smaller magnitude at the lowestfrequency and both the magnitude and phase angle change significantly withincreasing frequency. To determine the size and charge, it is then a question ofobtaining the best fit to the spectrum using the O’Brien theory. The limiting phaselag is 458 and when the sign on the particle changes the mobility points move fromone quadrant of the graph to the opposite one, i.e. 135–1808. This 1808 phasechange allows an unequivocal and accurate determination of the iso electric point.

The initial device that was developed the Matec ESA 8000 could operate in eitherESA or CVP mode, but only worked at one frequency(1 MHz). At the time theorydid not exist for relating the ESA to the particle properties through the dynamicmobility. One could only measure the relative ESA signal and standardise itssignificance by calibrating with a colloidal sol whose properties were assumed tobe constant, e.g. colloidal silica. However, it did work in aqueous and non-aqueousmedia and was accurate to approximately 2% and up to volume fractions of 0.05,


but as it only operates at one frequency it could not measure particle size. Theselimitations led to the development of the Acoustosizer.

The Acoustosizer measures the magnitude and phase of the dynamic mobilityspectrum of any particle concentration in the size range 0.1–10mm. For the smallestmeasurable size the inertia effect is only significant at the highest frequencies,whilst for the largest particle size the amplitude at the lowest frequency can still bedetected. Generally, the amplitude of the particle motion is extremely small. At 10MHz a 1 mm particle in a field of 10 Vycm moves a few picometres. TheAcoustosizer cell is manufactured from highly chemical resistant epoxy resin witha capacity of 400 ml. The contents of the cell can be agitated with a propellermixer. Three probes are inserted into the cell to measure the temperature, conduc-tivity and pH and four thermally conducting ceramic rods are immersed into thesuspension to provide temperature adjustment, by both positive(Joule heating) andnegative (Peltier effect). All of which are monitored and recorded during ameasurement. Calibration is with an electrolyte. All electrolytes give an ESA signalparticularly if the cation and anion differ significantly in mass andyor frictionaldrag i.e. radius. O’Brien et al.w25x have chosen the potassium salt of alphadodecatungstosilicic acid as a standard calibration. The octadecahydrate(K wSiW O x18ØH O) is an effective standard as long as it is pure. The Acousto-2 12 40 2

sizer II has been developed for both laboratory and pilot plant environments withthe capacity for in stream monitoring. Particle sizing is carried out by a combinationof electroacoustics and ultrasonic attenuation covering the size range from 0.02 to10 mm. Again the pH, conductivity and temperature are recorded although thesample volume can be as small as 20 ml and up to 150 ml. Instead of stirring thecolloidal suspension it is transported by a peristaltic pump, which means themaximum working viscosity is 0.25 Pas. Another improvement is the ability towork at a much higher conductivity, i.e. 5 Sym.

Recently, another commercial device for measuring zeta potentials in concentratedsuspension has been launched onto the market by Colloidal Dispersions, MountKisco, New York, USA. This calculates the zeta potential from the colloid vibrationcurrent (CVI), see Dukhin and co workersw19,26–28x. Much the same set ofexperiments has been carried out with this device as the ESA8000yAcoustosizer,e.g. optimal dispersant dosage to prevent aggregationw29x and measurement of isoelectric pointsw30x. However, here only work carried out by the Acoustosizer andits predecessor will be discussed.

2. Literature review

2.1. ESA 8000

Although the ESA8000 was extremely limited in its ability to measure theabsolute mobilities and zeta potential it proved to be of great use in moderatelyconcentrated(5% vyv) suspensions of minerals in aqueous media. Early resultswere to correlate ESA measurements with electrophoresisw3,16,18,31,32x. Manyearly papers on ESA measurements, however, failed to take into account the particle


size distribution of the material resulting in some strange results with zeta potentialsas high as several 100 mV! Scales and Jonesw33x demonstrated the importance ofparticle size and polydispersity for adequate correlation of electroacoustic data tothat obtained by electrophoresis. The earliest publications investigated; cetyl pyri-dinium chloride adsorption on kaolinitew34x and its charge reversal by hydrolysablemetal ionsw35x and the charge reversal of silicaw36x. In 1992, the National Instituteof Standards and Technology(N.I.S.T.) published the proceedings of a specialsymposium on the ESA techniquew37x. This covered a wide range of topicsincluding; colloidal diamond particles, corrections for background electrolyte effectsin mineral oxide systems, effect of pH on silicon nitride and yttrium oxide,adsorption of polyacrylate on calcium carbonate, effect of pH on gamma aluminaand coal, effect of pH on non-ionic polyacrylamide treated hematite and correlationof the zeta potential with viscosity measurements, cement in the presence ofplasticisers, rotogravure inks and applications to phosphors and toners. Otherparticles studied include ZnO and ZnSw38x, mullite particles mixed with a latexbinder w39x, silicon carbide platelets mixed with aluminaw40x and pigment particlesw41x. The technique has also been applied to the paper making processw42–44x.Paik et al. w45x reported that the particle surface charge of barium titanate wasinfluenced by the solids concentration of the suspension causing a shift in the i.e.p.These papers reflect the broad spectrum of applications the device can be used forrather than any development of theory. The ESA technique is also able to cope withunusual particle geometries, e.g. Texterw46x studied the adsorption of sodiumoleoylmethyltaurine onto a ceramic powder of parallelepiped particle morphologyand obtained fair agreement with adsorption isotherms for monolayer coverage.

Numerous researchers have also concentrated on the processing of that importantceramic material; silicon nitridew47–52x. Paik et al.w53x studied the adsorption ofa PMMA dispersant and a polyvinylalcohol binder onto silicon nitride. The PMMAcaused the effective i.e.p. of the system to be moved to lower pH, whereas the PVAhad no shifting effect but did reduce the magnitude of the zeta potential.

The ESA technique was also used by Walldalw54x to study the flocculationbetween two cationic polyelectrolytes and negatively charged silica particles.Polyacrylamide was more efficient than polyamide in neutralising the charge on theparticles. A similar study was carried out by Baltar and Oliveiraw55x usingpolyacrylamide as a flocculant for silica. The ESA technique was used by Smithand Haberw56x to study structure formation of alumina suspensions in terms of itsfiltration behaviour. A well-dispersed suspension cast slower than a flocculated oneat the same volume fraction.

One type of colloidal particle missing from the above list appears to be polymerlatex particles, which do not appear to have been investigated as widely as inorganicparticles. Shubin et al.w57x studied carboxylate latex using d.c. electrophoresis,ESA measurements and dielectric relaxation behaviour. They were able to rationalisetheir data by introducing the concept of anomalous surface conduction by whichions in the Stern layer are able to contribute to the electrical conductivity. Klingbielet al. w58x investigated polymethylmethacrylate(PMMA) lattices, whilst James et


al. w31x investigated polystyrene and PMMA lattices amongst other particles. O’Brienw2x verified his electroacoustic theory using a monodisperse latex and a cobaltphosphate sol. However, beyond these few isolated examples, latex particles havebeen massively under utilised in terms of electroacoustic experiments.

2.2. Acoustosizer

The earliest studies with an Acoustosizer were also published by the N.I.S.T.w59xand initial ceramicyminerals work was described by O’Brienw60x. The latter paperdescribes how the sample is titrated over a pH range to determine the iso electricpoint, an important characteristic of many colloidal systems(see experimentalsection). Stable oxides show a reversible titration curve with respect to the zetapotential. The size of the particles is a minimum at pH values away from the i.e.p.as they are well dispersed. Near the i.e.p. the size appears larger, but this is to someextent an artefact. As a system passes through its i.e.p. the system flocculates andthe particle size is undefined. The size of the aggregates depends on the time scaleand stirring speed. In 1995 O’Brienw61x developed a formula to calculate thedynamic mobility of a porous particle, which is relevant to studying flocculatedsystems.

As with the ESA technique there are numerous studies comparing electroacousticmeasurements with other techniques, e.g. Pettersson et al.w62x investigated theadsorption of polyacrylic acid and polymethacrylic acid onto alumina, zirconia andyttria doped zirconia using a combination of adsorption isotherms, particle sizemeasurements and adsorbed layer thickness measurements. Beattie and Djerdjevw63x compared electroacoustic results from alumina suspensions in the presence ofpolymethyacrylate dispersants with viscosity measurements and sedimentation vol-umes. Electroacoustic measurements have also been compared with electrophoresisand streaming potential experiments by Knosche et al.w64x in the case of silica,titania and alumina powders and by Rasmusson and Wallw65x for alumina modifiedsilica nanoparticles. Another powder that has been investigated by electroacousticsis titanium nitride. Shih et al.w66x studied the adsorption of polymethacrylic acidat different pH values with rheology and zeta potential measurements. The adsorbedamount increased as the pH decreased, an effect reported by numerous researchersw67–69x.

O’Brien and Rasmussonw70x measured the zeta potential and size of bentonite.It is normally very difficult to obtain reliable information on bentonite due to itsunusual shape and viscosity characteristics, but it behaved well in electroacousticanalysis. Another mineral system of great interest is the metal sulphides. Prestidgeand Rowlandsw71x compared electrophoretic and electroacoustic measurements ofthe zeta potential for zinc sulphide and lead sulphide. These systems are complicateddue to the possibility of surface oxidation, which produces time effects, but canalso depend significantly on the surface area to volume ratio. It is also possible tostudy semi conducting particles, e.g. silicon particles w72x by making a slightmodification to the theory. Another area of potential danger is in studying materialsthat display anomalous surface conduction, in particular kaolinite. Surface conduc-


tion can lead to erroneous sizing, which can be detected in the phase lag data.Instead of increasing from low values to a limiting value ofy458, it may gothrough a maximum at low frequency and even turn into a phase lead over part ofthe range, see Subhin et al.w57x and O’Brien and Rowlandsw73,74x.

The Acoustosizer is in principle capable of working at elevated salt concentrations,but the factory calibration only covers the range from 0.01 to 1 Sym i.e. in theorder of 0.5 mM to 0.1 M salt. To extend the range requires standards of knownESA and higher conductivity. Caesium chloride solutionsw75x are suitable for thispurpose and extend the range to approximately 20 Sym, which is the conductivityof 3 M NaCl. Early work at high salt concentrations was carried out by Kosmulskiand Rosenholmw76x, but unfortunately they did not know the limitations of themachine, so their results under estimate the zeta potentialw77,78x. Rowlands et al.w75x studied the effect of high salt concentrations on the behaviour of a suspensionof aluminium hydroxide. The i.e.p. of the material shifted from 9.1 for dilute saltconcentrations, but moved to over 11 for 0.5 M NaCl and showed no i.e.p. at allfor 3 M NaCl. Thus, what is normally considered an indifferent electrolyte shows adistinct tendency to specific adsorption at high enough concentrations, somethingthat has always been considered possible in the current descriptions of the oxide–solution interface. They also came up with the idea of determining the i.e.p. fromthe Argand diagram of real and imaginary components to the dynamic mobility.

Johnson et al.w79,80x published two papers on the binding of monovalentelectrolyte ions on alpha alumina at high electrolyte strengths using the techniquesof yield stress rheology measurements and electroacoustics. At high(approx. 1 moldm ) electrolyte concentration, the monovalent cations bind to the negative aluminay3

in the order Li )Na )K fCs . By contrast Br , Cl , I and NO anionsq q q q y y y y3

adsorb to an almost identical extent over the range of concentrations and pHconditions investigated. The cation binding sequence was consistent with thestructure makingystructure breaking model proposed in the 1960sw81,82x. Similarwork on alumina and zirconia has been carried out by Franksw83,84x and co-workers investigating IO , BrO . Cl , NO , ClO ions. The adsorption of othery y y y y

3 3 3 4

electrolytes namely cobalt chloride, molybdenum chloride and ammonium hepta-molybdate onto silica and alumina has been investigated by Deboer et al.w85x.

The importance of correcting for the background electrolyte signal has beendemonstrated by Desai et al.w86x. As mentioned previously for a solid in suspensionthe measured ESA signal is a combination of that due to the particles and that ofthe background electrolyte. Under certain circumstances this background must besubtracted. They noted that the i.e.p. of two silica suspensions at an ionic strengthof 0.1 M was 4.7 and 5.6, but with the background correction the particles did notchange sign until at least a pH of 2.

One of the most common substances for dispersing colloidal samples is thesodium salt of polyacrylic acid. This adsorbs very strongly on positively chargedsurfaces via electrostatic forces and will adsorb on poorly charged negative surfacesthrough hydrogen bonding and van der Waals forces. Fortunately, polyacrylic acidadsorbs so strongly on most surfaces that it lies almost flat, with negligible loops


and tails. As a result, the zeta potential is strongly affected, which can be easilydetected by electroacoustic measurements. Djerdjevw87x studied the adsorption ofpolyacrylic acid onto alumina over a range of pH values and obtained a three-dimension surface of how the zeta potential varies as a function of the coverageand the pH. The experiment was carried out by addition of successive amounts ofpolyacrylic acid at the end of each titration run. The whole process taking only afew hours. Kaolinite, silicon nitride and calcium carbonate all respond well topolyacrylic acid as a dispersing agentw88x. Kaolin has also been investigated,alongside alumina, by Johnson and Scalesw80,89x using both electroacoustics andrheology.

Polysulphonic acid seems to behave in a similar manner and is used to dispersecolloidal materials. Ukigai et al.w90x used electroacoustics to study the adsorptionof polysulphonates on coal water slurries at volume fractions of 65%. They wereable to identify the molecular weight of polysulphonate that generated a maximumin the zeta potential and a minimum in the particle size. A low molecular weightpolycarboxylate dispersant has been investigated by Costa et al.w91x.

Pradip et al.w92x investigated the dispersion of three different ceramic powders,i.e. alumina, zirconia and silicon nitride using two different polyelectrolytes; onecationic and one anionic. All the powders could be dispersed when the working pHwas at least two pH units away from the effective i.e.p. of the system in thepresence of the dispersant. Premachandran and Malghanw93x, however, recommendat least 2.5 to 3 pH units difference in their study of silicon nitride, silicon carbide,alumina, aluminium nitride and yttria powders. They also recommend cationicdispersants over anionic ones as they produce suspensions with higher stability, witha narrower particle size distribution over a wider range of pH values.

As previously mentioned, one of the most commonly investigated powders issilicon nitride. Hackleyw94x used the ESA 8000 and the Acoustosizer to study theadsorption of polyacrylic acid onto silicon nitride. In an earlier paper, Hackley andco-workersw95x used soxhlet extraction to clean five silicon nitride powders andobtained a pristine i.e.p. of 9.7 for unoxidised silicon nitride. In another paper bythe same group of workers Hackley and Malghanw47x noted that fluoride wasspecifically adsorbed onto the surface of silicon nitride powders. Galassi et al.w96xnoted that although the milling of a silicon nitride powder did not alter the i.e.p. itdid alter the magnitude of the zeta potential. A follow-up paperw97x on the additionof sintering aids noted that they dramatically altered the surface chemistry and thei.e.p. Silicon nitride has also been studied by Laarz and Bergstromw98,99x usingelectroacoustics in conjunction with rheological measurements. They comparedcopolymers of PEO and methacrylic acid with polyacrylic acid and demonstratedthat the grafted PEO chain had a minor influence on the colloidal stability, whereasthe polyacrylic acid extended perpendicular to the surface. They also attributed theeffect of excess polyelectrolyte to the increased suspension viscosity due to increasedionic strength caused by the release of associated counter ions of the polymerfunctional groups.

Titania powders have been investigated by Greenwood and Kendallw100x theynoted that due to an organic coating on one of the titania powders the adsorbed


amounts were very low(in the order of micrograms per m). This organic coating2

could be removed by calcining the powder, which then lead to increased adsorbedamounts. Roncari et al.w101x investigated a magnesia doped mullite suspension andnoted that the magnesia undergoes pronounced solubilization that strongly affectsthe suspension properties. The dissolution of material from powders has also beennoted for doped zirconia powders i.e. ceriumw102x and yttrium w100,103x. This isfurther discussed in Section 3.7.

Maier et al.w104x made an early attempt to measure the adsorption of polyethyleneoxide onto silica dispersions. They were able to relate the drop in zeta potential toa shift in the position of the plane of shear, as had been suggested by Koopal andLyklema w105x. Carasso and O’Brienw106,107x have also investigated this effectand developed a model which can distinguish effects on the mobility due to thefluid motion within the polymer layer itself and also those due to a shift in theplane of shear. The model allows an estimation of how much the shear plane hasbeen shifted by the adsorbed layer i.e. the adsorbed layer thickness. A PVA sampleof molecular weight 50 000 Daltons reduced the zeta potential by approximately30%. The same results were obtained for a series of nonylphenylethoxylates ofvarying molecular weights. The adsorbed layer becoming thicker with increasingconcentration and molecular weight. Similar results were obtained by Miller andBerg w108x studying the adsorption of PVA onto titania.

Thus, it has been shown that the Acoustosizer has been used alongside numerousother techniques to study concentrated suspensions of ceramic particles. The mostcommonly studied subjects are the iso electric point of the powder and the adsorptionof polyelectrolyte dispersants onto these powders. The technique is robust enoughthat even the dispersion of cement can be studied, e.g. Hodne and Saasenw109xrelated the consistency of cement slurries to the zeta potential, despite the nature ofthe material and its high pH)12. In the next section, a review of some of theexperiments that have been carried out by the author using electroacoustics to studythe manipulation of the interparticle forces is presented. The exact experimentaldetails if not reported are given in the references.

3. Examples of electroacoustic experiments

3.1. Measurement of iso electric points (i.e.p.)

For purely electrostatically stabilised systems it is important to know the exactlocation of the i.e.p., i.e. the pH value at which the particles have zero zeta potential.At this pH value there are no repulsive forces whatsoever and the particles will beheavily flocculated due to the dominance of the attractive van der Waals forces.Normally, this is an undesirable state of affairs and systems are designed such thatthe suspension pH is well away from the i.e.p. The further away the suspension pHfrom the i.e.p. the greater the surface charge and hence the greater the zeta potential.The magnitude of the repulsive force between particles is directly related to themagnitude of the zeta potential. As a rough rule of thumb working approximately2–3 pH units away from the i.e.p. in either direction is sufficient to generate a large


Fig. 1. Effect of NaCl concentration on the iso electric point of SDK160 alumina. DiamondssDoubledistilled water, Triangless1 mm NaCl, Circless50 mm NaCl, Open symbolssincreasing pH, Closedsymbolssdecreasing pH.

Fig. 2. Effect of KCl concentration on the iso electric point of SDK160 alumina.esDouble distilledwater,ns1 mm KCl, ss50 mm KCl, Open symbolssdecreasing pH, Closed symbolssincreasingpH.

enough zeta potential for stabilisation. A zeta potential of greater than"30 mV isconsidered sufficient for stabilisation and in fact this usually occurs approximately2–3 pH units away from the i.e.p. so the rough rule has some basis. However, it isalso well known that electrostatic stabilisation is insufficient to stabilise particles athigh volume fractions due to extensive double layer overlap, it is therefore necessaryto adsorb polymers or polyelectrolytes onto the particles to stabilise the particlesvia steric or electrosteric mechanisms, respectively. Electrosteric stabilisationdescribes the combined stabilisation mechanisms of electrostatic and stericstabilisation.

Figs. 1 and 2 show typical examples of the effect of altering the pH of an aluminasuspension(SDK 160, volume fraction 0.11, stirrer speed 300 rpm) at different


electrolyte strengths. Fig. 1 demonstrates the effect of varying the backgroundelectrolyte strength from that of double distilled water to 1 and 50 mM NaCl, whilstFig. 2 demonstrates the same effect but for KCl. At low pH the suspensions arepositively charged, whilst at high pH the suspensions are negatively charged. Thei.e.p. can be identified from the pH value at which the zeta potential is zero. Fig. 1shows that the i.e.p. with no salt present is approximately 7.4, which is shifted to7.9 and 8.2 when the background electrolyte is changed to 1 mM NaCl and 50 mMNaCl, respectively. The same effect is seen for the KCl, the i.e.p. is shifted from7.4 to 7.9 and 8.1, respectively. This shift in i.e.p. is due to specific adsorption ofcations onto the alumina particles. If the electrolyte was inert it would be expectedthat the i.e.p. would be independent of the salt concentration. The same effect hasbeen noted by Burkew110x for a Sumitomo alumina powder. The type of acidybasethat is used to alter the pH of a suspension can also have a major effect, Greenwoodand Kendallw111x noted that the i.e.p. of an alumina powder was independent ofthe base used, but the degree of hystersis varied depending on the polarisability ofthe cation. This can also be seen in Figs. 1 and 2 in that NaCl gives a slightlylarger change than that obtained with KCl, plus the degree of hystersis is nonexistent for the suspensions prepared with double distilled water, but does occur forthe higher salt concentrations. There is normally a wide range of reported valuesfor the i.e.p. for any powder as the surface chemistry will vary according to how ithas been manufactured. This alumina powder will be electrostatically stabilised ifdispersed at pH values greater than 11 and less than 5.

3.2. Selecting a dispersant and its optimum amount

Traditionally in the ceramics industry, polyelectrolytes have been utilised toprevent flocculation of particles. Due to the charged nature of the polyelectrolytethey impart stability to the particles via an electrosteric mechanism. Hence,adsorption of these charged molecules onto a particle surface will alter the surfacecharge and hence the zeta potential. So using electroacoustics, it is possible tofollow the changes in the zeta potential with increasing amounts of polyelectrolyte.This is extremely useful in determining the optimum amount of polyelectrolyterequired to stabilise the particles under different conditions. If too little is addedthen some flocs will persist in suspension and if too much is added thendestabilisation may occur. A polyelectrolyte behaves in a similar manner to anordinary electrolyte, the effects of which are well known, i.e. addition of too muchsalt causes flocculation. Hence, it is crucial from a colloid stability argument tohave the correct amount of dispersant present in the system. Additionally, there isan economic argument to overdosing especially when working on the large scale,e.g. grinding of mineralsw112x.

As previously mentioned various methods exist for establishing the optimumamount of dispersants. These include rheology, adsorption isotherms and sedimen-tation. The latter two are time consuming and tedious. Electroacoustics is, however,a quick and simple technique to allow the study of numerous dispersants onto asuspension and then selection of the most suitable one for the system in question


Fig. 3. Adsorption of cationic and anionic dispersants onto a silicon nitride powder.

and to determine its optimum amount. A suitable dispersant is one that imparts alarge zeta potential to the particles when only a small amount is addedw113,114x.The optimum amount of dispersant as determined by electroacoustics, adsorptionisotherms and rheology experiments show excellent agreementw102x. Burke et al.w109x also demonstrated that electroacoustic results confirm optimum dispersantdosages as measured by sedimentation heights and particle sizing. However, asmentioned previously polyelectrolytes impart stability by electrosteric means sothere will be a steric contribution to the stabilisation, which will not be reflected inthe zeta potential measurements, but would be detected by atomic force microscopyor the surface force apparatus.

Fig. 3 shows the adsorption of a cationic polyelectrolyte(6569 Ciba Chemicals,Bradford, UK) and an anionic polyelectrolyte(Dispex N40, Ciba Chemicals,Bradford, UK) onto a silicon nitride powder. The volume fraction in both cases was0.12. The initial zeta potential of the suspension before addition of the anionicpolyelectrolyte wasy6.4 mV at a pH of 5.96 and a conductivity of 0.298 Sym.These zeta potentials are too low for electrostatic stabilisation to be effective;therefore, the particles are expected to be flocculated. The initial zeta potential ofthe suspension before addition of the cationic polyelectrolyte wasy4.3 mV at apH of 5.93 and a conductivity of 0.312 Sym, so in reasonable agreement with oneanother. Both curves show the same trends in that as the polyelectrolyte adsorbsonto the particle the zeta potential increases in magnitude. At a certain concentrationof dispersant the zeta potential no longer increases as the particles are nowcompletely covered in dispersant. This then is taken to be the optimum amount ofdispersant required to stabilise the powder. Any excess dispersant added after thispoint does not affect the zeta potential, as it remains unadsorbed in solution; hencethe zeta potential remains as a plateau. From Fig. 3, the plateau region begins at 3mgyg for the cationic polyelectrolyte and 1.8 mgyg for the anionic dispersant. Theplateau region is not exactly level because as more polyelectrolyte is added to thesystem it behaves like a simple salt, i.e. it reduces the zeta potential, so as more


Fig. 4. Effect of five commercial dispersants on Alcoa A16 powder. Background electrolyte strength 10mM KCl. ssDolapix CA,nsDolapix PC21,hsDolapix PC67, OpenesDolapix PC33 and closeddiamondssDolapix CE64.

polyelectrolyte is unadsorbed in solution the zeta potential will gradually decrease.Unadsorbed polyelectrolyte in solution may potentially cause destabilisation of theparticles via depletion flocculation.

Fig. 4 shows how the zeta potential of an alumina suspension(Alcoa A16,background electrolyte 10 mM KCl) can be altered by the addition of fivecommercially available polyelectrolytes. In this case, the dispersants utilised werefrom Zschimmer and Schwarz. Initially, the zeta potential of the suspension wasapproximatelyy29 mV. It can be clearly seen that addition of polyelectrolytesincreases the zeta potential of the suspension such that the suspension becomesmore stable. The trend for all five dispersants is very similar in that initially thezeta potential changes strongly with small amounts of dispersant and then after acertain concentration the zeta potential begins to plateau out as no more dispersantis adsorbed on the surface. However, each different dispersant imparts a differentfinal zeta potential and requires a different amount of dispersant to cover theparticles. The effect of dilution of the dispersants has been taken into account sothe dispersants can be compared. Fig. 4 shows that three dispersants plateau out ata dispersant concentration of 0.2 mgym , i.e. Dolapix CA, Dolapix PC21 and2

Dolapix PC67. Of these Dolapix CA imparts the greatest final zeta potential so thiswould be an excellent dispersant for the alumina. However, it must be noted thatthe likely stabilisation mechanism for polyelectrolyte dispersants is electrostericstabilisation, therefore, there may well be a steric contribution to the stabilisationmechanism depending on how the dispersant adsorbs. Dolapix CE 64 imparts afinal zeta potential ofy45 mV and requires approximately twice as much dispersantto do so, making it a poor candidate in comparision. The zeta potentials from thesuspension stabilised with Dolapix PC 33 do not appear to level out and furtherdata points would be required to determine the optimum amount.


Table 1Effect of volume fraction on the optimum adsorbed amount

Dispersant Volume Optimum amount of Final zetafraction dispersant(mgym )2 potential(mV)

Tiron 0.111 0.14 y53.00.214 0.13 y48.50.269 0.13 y52.00.360 0.14 y50.5

Duramax D3021 0.111 0.19 y53.00.219 0.22 y55.50.298 0.23 y59.50.359 0.22 y55.5

Aluminon 0.111 0.28 y51.00.221 0.28 y52.00.296 0.27 y51.00.358 0.29 y54.0

3.3. Effect of volume fraction on the zeta potential, the adsorbed amount and thei.e.p.

In the previous section, it was shown how electroacoustics could be used todetermine the optimum amount of dispersant for a given powder. However, it isimportant to know that these are independent of the volume fraction at which theexperiment is carried out. Table 1 shows the optimum amount of three commerciallyavailable dispersants required to cover an alumina powder at four different volumefractions. It can be seen that within error the adsorbed amounts are all similarindicating that the adsorbed amount was independent of the volume fraction atwhich the experiment was carried out. In the case of expensive powders, it is thenprudent to use the lowest volume fraction that generates a strong electroacousticsignal. In this case, the powder studied was Alcoa A16SG and the dispersants wereAluminon (Fluka) Tiron (Fluka) and Duramax D3021 and the background electro-lyte was 10 mM KCl. As all the final zeta potentials are large and negative thisindicates that all three dispersants are suitable to stabilise the powder againstflocculation. Fig. 5 shows another plot of zeta potential against pH for an AlcoaA16 powder suspended in 10 mM NaCl. The experiment is repeated at threedifferent volume fractions using 1 M HCl and 1 M NaOH to adjust the pH. Thei.e.p. for all three powders occurred at a pH of 8.2 and was independent of thevolume fraction.

The zeta potential of a suspension should be independent of the volume fraction(provided there is no substantial electrical double layer overlap) at which theexperiment is carried out. To investigate if this is true, a series of titania suspensionswas prepared ranging from 12.2 to 50% by weight in double distilled water. Thesuspensions were allowed to stand for 15 min before being placed into theAcoustosizer II. Five measurements were taken and the mean average and standarddeviation was calculated(see Table 2). The results are shown in Fig. 6a and b. Fig.


Fig. 5. Effect of volume fraction on the iso electric point of Alcoa A16 alumina. Background electrolytestrength 10 mM NaCl.es0.142,hs0.186 andss0.237.

Table 2Average mean values of pH, conductivity and zeta potential for different weight fractions of titania

Weight pH Conductivity Zeta potentialfraction Sym mV

12.2 7.15"0.05 0.0141"0.0003 10.9"1.018.2 7.30"0.02 0.0197"0.0005 10.0"0.225.0 7.30"0.01 0.0190"0.0004 6.9"0.132.0 7.43"0.01 0.0321"0.0008 6.2"0.138.5 7.46"0.02 0.0387"0.0007 5.2"0.144.8 7.49"0.02 0.0479"0.0010 4.0"0.150.0 7.48"0.03 0.0547"0.0018 3.0"0.1

6a clearly shows a decrease in the zeta potential with increasing weight fraction,however, this can be easily explained by the corresponding increase in conductivityand increase in pH seen in Fig. 6b. Therefore, in order to truly test the effect ofvolume fraction the pH and ionic conductivity must be the same for all samples.This could be obtained by centrifuging down the most concentrated sample andusing the supernatant to disperse the remaining powder.

3.4. Effect of dispersant on the iso electric point

The degree to which polyelectrolytes are dissociated depends strongly on the pH.For example, polyacrylic acid is fully dissociated at approximately pH 9, so atlower pH values there are fewer dissociated groups. The degree of dissociationcontrols the configuration of the polyelectrolyte in solution and hence, how itadsorbs onto a particle surface. In this experiment a titanium dioxide(Tioxide)suspension at 20% by weight was prepared in double distilled water and theoptimum amount of Dolapix PC33 added as a dispersant. The zeta potential of thesuspension was then recorded as a function of the amount of the pH. This wascompared with the iso electric point of the titanium dioxide suspension without thedispersant present. The results in Fig. 7 shows how the addition of the dispersant


Fig. 6. (a) Effect of weight fraction on the zeta potential of a titanium dioxide suspension.(b) Corre-sponding conductivities and pH values as a function of weight fraction.

Fig. 7. Effect of adding a dispersant on the iso electric point of a titanium dioxide suspension.

moves the effective i.e.p. from approximately 7.8 to 5.9 when the particles arecompletely covered in dispersant.

3.5. Effect of powder surface area on the optimum adsorbed amount

Another factor that may effect the adsorbed amount of polyelectrolyte is the sizeof the particles. Unfortunately in the processing of ceramic powders, it is extremely


Fig. 8. Effect of salt concentration on the addition of a cationic dispersant. Reprinted from PowderTechnology, 113, Greenwood and Kendall, Effect of ionic strength on the adsorption of cationic poly-electrolytes onto alumina studied using electroacoustic measurements.

difficult to obtain monodisperse particles, however it is straight-forward to obtainpowders of the same purity, but with different surface areas. In a previouslypublished paper Greenwood and Kendallw110x investigated the optimum amount ofDarvan 821A required to cover a series of Sumitomo alumina powders of varyingsurface area. All the powders were 99.9% pure alumina. Within experimental error,it was seen that the adsorbed amount of dispersant was independent of the surfacearea of the powders i.e. 0.59 mgym , in other words the adsorbed amount was2

independent of the powder surface area. This agrees with the results of Kayes andRawlins w115x, Baker et al.w116x, Luckham and Faers,w117x who all investigatedthe adsorption of block copolymers onto different sized polystyrene lattices. It isknown that the conformation of polymers changes as the particle size changes atconstant adsorbed amount, i.e. thicker layers are found on larger particlesw118–121x.

3.6. Effect of ionic strength on adsorption of dispersants

Another factor that affects the adsorption of polyelectrolytes from solution ontoceramic particles is the ionic strength of the suspension, see Greenwood and Kendall,w111x. Fig. 8 shows the adsorption of a cationic polyelectrolyte called polyDADMAC(PC20 HV) onto an alumina powder as a function of different background electrolytestrengths. The three curves follow the same trend in that the particles are initiallynegatively charged and the addition of the polyelectrolyte reduces the magnitude ofthe zeta potential. At a certain concentration zero zeta potential is achieved, butflocculation does not occur due to the steric component of the stabilisation. Additionof further polyelectrolyte now causes charge reversal until a point is reached whereno more polyelectrolyte adsorbs and the plateau region is reached.

In double distilled water, the polyelectrolyte has an extended configuration andwhen it adsorbs onto a particle it does so with the conformation of a train, i.e. it


Fig. 9. Dissolution of material from two zirconia powders. Reprinted from two papers in(a) Journal ofthe European Ceramic Society, 19, Greenwood and Kendall, Selection of suitable dispersants for aqueoussuspensions of zirconia and titania powders using electroacoustics.(b) Journal of the European CeramicSociety, 20, Greenwood and Kendall, Acoustophoretic studies of aqueous suspensions of alumina and 8mol.% yttria stabilised zirconia powders.

runs along the surface. Hence, very little adsorbs onto the particles and completecoverage is obtained at 0.10 mgym . However, increasing the ionic strength to 102

mM such that the charges on the polyelectrolyte are screened slightly(as well asthe particle charge). Thus, the polyelectrolytes configuration in solution is nowmore coiled and it adsorbs on a particle with more loops and tails giving a thickerlayer and a four-fold increase in the adsorption per unit surface area, i.e. approxi-mately 0.40 mgym . Increasing the salt concentration further to 50 mM now causes2

the coil to collapse further and the adsorbed amount becomes lower(0.20 mgym ),2

until eventually a critical salt concentration will be reached at which no adsorptionoccurs. The magnitude of the zeta potential on complete dispersant adsorption isconstant with screening by the salt, i.e. the zeta potential falls fromq50 mV toq25 mV to q20 mV as the salt concentration increases. A trend that is alsoreflected in the initial zeta potentials. The thickness of the layer is important interms of stabilisation as it determines how close two particles can approach oneanother. The effect of ionic strength on anionic dispersants will be discussed inSection 3.8.

3.7. Dissolution effects

Insoluble powders such as alumina and titania when dispersed in water show aconstant zeta potential with time. However, other powders may demonstrate avariation in zeta potential with time. Thus, it is important, as one of the initial testson a powder, to check whether there is some dissolution of material from thepowder that may effect the surface chemistry of the particles and hence the zetapotential. This involves measuring the zeta potential every 15 min or so over asuitable period of 4–6 h. Fig. 9 shows the increase in zeta potential with time fortwo yttria doped zirconia powdersw100,103x. Both powders reveal the same trend;


Table 3Physical properties of the dispersants

Code Description Molecular weight Activity(Daltons) (%)

PAA 1 Darvan 821A* 3500 40PAA 2 Alcosperse EN 4000 26.1PAA 3 Dispex A40* 10 000 40PAA 4 ASCN I 21 000 24.4PAA 5 Polyscience I 60 000 35PAA 6 DP2 7621 99 200 26.4PAA 7 Polyscience II 225 000 20PMAA 1 Aldrich I 6500 30PMAA 2 Aldrich II 9500 30PMAA 3 Darvan 7 10–16 000 25PMAA 4 Darvan C* 10–16 000 25

Ammonium salt. Rest sodium salts.*

they are initially positively charged and the zeta potential becomes more positiveover time. This increase with time may be attributed to the dissolution of yttriafrom the powder readsorbing onto the powder surface. The rate of increase for the8 mol.% yttria doped powder was greater than the 3 mol.% one, because there ismore yttria present to dissolve out. Shojai et al.w123x also reported on the dissolutionof yttria from an yttria doped zirconia under acidic conditions. They concluded thatthe major species leached out in acidic aqueous media was Y . However, it was3q

not clear if the equilibrium limit of the dissolved ions was reached or if the ionswere readsorbed onto the surface. Similar results have been noted for spinelw122xpowders, but the increase in zeta potential was this time attributed to sodium ionsadsorbing onto the powder surface.

3.8. Molecular weight of the dispersant

Another factor that can be investigated by electroacoustics is the molecular weightof the polyelectrolyte. In this work a series of polyacrylic acids and polymethacrylicacids were adsorbed onto an alumina powder(SDK160) at three different electrolyteconcentrations namely no added salt, 1 mM and 5 mM KCl. The characteristics ofthe polyelectrolytes are given in Table 3. As can be seen from the table thedispersants are supplied as diluted down aqueous solutions of polyelectrolytes, so itis vital to allow for this dilution factor when comparing results. So all results arequoted in terms of mg of active dispersant per unit surface area of powder. Allwater used was double distilled and the experiments were carried out at 258C. 400ml of suspension was prepared at a volume fraction of 0.086(150 g in 400 ml ofwater). Due to the viscosity of some of the higher molecular weight dispersantsthese were diluted down further with a known amount of distilled water. The otherdispersants were added as supplied using the automatic titration software. Theoptimum adsorbed amount was recorded from the usual shape of the curve and


Fig. 10. Optimum amount of adsorbed polyacrylic acid as a function of molecular weight. Diamondssdouble distilled water, Squaress1 mM KCl and triangless5 mM KCl.

Table 4Initial characteristics of SDK 160 alumina powder suspensions in double distilled water

Dispersant Initial Initial Initial zetatype pH conductivity potential(mV)

(Sym)

PAA 1 8.51 0.042 y4.4PAA 2 8.63 0.032 y6.0PAA 3 8.56 0.047 y7.2PAA 4 8.52 0.037 y5.3PAA 5 8.71 0.039 y4.5PAA 6 8.80 0.054 y7.7PAA 7 8.50 0.042 y5.0PMAA 1 8.53 0.039 y3.3PMAA 2 8.54 0.044 y6.1PMMA 3 8.56 0.040 y4.9PMAA 4 8.57 0.045 y4.6

plotted against the molecular weight of the polyacrylic acid for the three differentsituations. The results are shown in Fig. 10 for the polyacrylic acids and in Table 7for the polymethacrylic acid.

Table 4 to Table 6 show the initial characteristics of the suspensions beforeaddition of the dispersants. With no dispersant present the initial zeta potential ofthe suspension in double distilled water wasy5.4"1.2 mV at a pH of 8.58"0.09and a conductivity of 0.042 Sym (Table 4). Increasing the salt concentration slightlyto 1 mM KCl (Table 5) reduced the initial zeta potential by a small amount toy5.2"1.6 mV, whereas the conductivity rose slightly(as would be expected) to0.050 Sym and the pH fell slightly to 8.49"0.14. Increasing the salt concentrationfurther to 5 mM(Table 6), again reduced the zeta potential slightly toy4.4"0.7mV, increased the conductivity to 0.096 Sym, but increased the pH slightly to


Table 5Initial characteristics of SDK 160 alumina powder suspensions in 1 mM KCl


(Sym)


Table 6Initial characteristics of SDK 160 alumina powder suspensions in 5 mM KCl


(Sym)


8.61"0.16. Given the error bars it can be seen that the pH stays constant withincreasing salt concentration, whilst the zeta potential decreases slightly due toscreening by the salt ions.

With no salt present the optimum adsorbed amount was 0.42 mgym independent2

of the molecular weight. Adding a small amount of salt(1 mM KCl) gives anincreased adsorbed amount of 0.60 mgym again independent of the molecular2

weight. Increasing the salt concentration further to 5 mM KCl reduces the adsorbedamount to 0.31 mgym , once again independent of the molecular weight. Once2

again the results can be explained by considering the configuration of the polyelec-trolye and how it adsorbs on the surface. With no salt present the polyacrylic acidadsorbs onto the surface as a train and due to the highly charged nature of thepolyelectrolyte the chains arrange themselves as far away from one another as


Table 7Optimum amount of PMAA dispersant against background electrolyte strength

Dispersant Optimum amount in Optimum amount Optimum amountcode pure water(mgym )2 in 1 mM KCl (mgym )2 in 5 mM KCl (mgym )2

PMAA 1 0.57 0.39 0.39PMAA 2 0.62 0.44 0.44PMAA3 0.58 0.46 0.42PMAA4 0.56 0.42 0.47

possible. Increasing the salt concentration allows the trains to pack closer togetherdue to charge screening, such that the chains can now pack closer together andhence the adsorbed amount is increased. The independence from molecular weightis due the dispersant adsorbing as a train. If the polyelectrolyte adsorbed with loopsand tails instead then a strong dependence on molecular weight would be expected.Increasing the salt concentration to 5 mM has a dramatic difference as it affects thepolyelectrolyte configuration such that the adsorbed amount is much less than theprevious two cases, but without further experiments it is not possible to suggestwhat exactly is occurring at this point.

The results from the polymethacrylic dispersants are not as clear cut due to thelimited number of samples and molecular weights. The highest adsorbed amountoccurs with the double distilled water(0.60 mgym ) and adding salt reduces the2

adsorbed amount to approximately 0.42 mgym in both cases. All three sets of2

results show an independence of molecular weight within error, so again suggestingthat the polymethacrylic acid also adsorbs in the train configuration due to strongelectrostatic attraction.

Similar work has been performed by Santhiya and co workersw69x studying theadsorption of polyacrylic acid onto alumina using electroacoustics and rheology.The adsorption density decreased with increasing pH but increased with increasingmolecular weight. This work differs to the current work in that they used low ionicstrength and low pH suspensions. Close examination of their Fig. 2 data revealsthat at high pH there is no molecular weight dependence as noted here.

4. Conclusions

Electroacoustics is a powerful technique for investigating the zeta potential ofconcentrated aqueous suspensions. The technique is extremely useful for investigat-ing the colloidal processing of ceramic powders. By optimising the processingconditions the interparticle forces can be manipulated to prevent flocculation andflaws eliminated in the final sintered body. Variables that can be altered include thepH and powder surface area along with the molecular weight of the dispersant. Thetechnique is also suitable for detecting dissolution of material out of powders.

The background electrolyte strength has been high lighted as being extremelyimportant as it determines the conformation of the polyelectrolyte in solution, hencecontrolling the manner in which it adsorbs at the interface. How the polyelectrolyte


adsorbs then determines how much is adsorbed and whether this is sufficient toimpart stability to the particles. The nature of the acids and bases used to adjust thepH of the suspension can also play an important role.

References

w1x R.J. Hunter, Review recent developments in the electroacoustic characterisation of colloidalsuspensions and emulsions, Colloids Surf. A 141(1998) 37–65.

w2x R.W. O’Brien, The electroacoustic equations for a colloidal suspension, J. Fluid Mech. 212(1990) 81–93.

w3x R.W. O’Brien, B.R. Midmore, A. Lamb, R.J. Hunter, Electroacoustic studies of moderatelyconcentrated colloidal suspensions, Faraday Disc. 90(1990) 301–312.

w4x R.W. O’Brien, J. Colloid Interface Sci. 171(1995) 495–504.w5x A.A. Griffiths, Philos. Trans. R. Soc. Lond. A221(1920) 163.w6x K. Kendall, N. Alford, J.D. Birchall, Naure 330(1987) 51–53.w7x P. Debye, J. Chem. Phys. 1(1933) 13.w8x E. Yaeger, J. Bugosh, F. Hovoraka, J. McCarthy, J. Chem. Phys. 17(1949) 411.w9x J. Hermans, Phil. Mag. 25(1938) 426.

w10x J. Hermans, Phil. Mag. 26(1938) 674.w11x A. Rutgers, Nature 157(1946) 74.w12x J.A. Enderby, Proc. R. Soc.(Lond) A 207 (1951) 329–342.w13x F. Booth, J.A. Enderby, Proc. Phys. Soc. 45(1952) 321–324.w14x R. Zana, E.B. Yaeger, J. Phys. Chem. 71(1967) 3502.w15x R. Zana, E.B. Yaeger Ultrasonic vibration potentials in: J.O’M Bockris, B.E. Conway, R.E. White

(Eds.), Modern Aspects of Electrochemistry, vol. 14, 1982, 1–61.w16x B.J. Marlow, D. Fairhurst, H.P. Pendse, Langmuir 4(1988) 611–626.w17x T. Oja, D. Cannon, G.L. Petersen, US Patent 4497, 208.w18x A. Babchin, R.S. Chow, R.P. Sawatsky, Adv. Coll. Inter. Sci. 30, 1989.w19x A.S. Dukhin, P.J. Goetz, Langmuir 12(1996) 4335–4344.w20x J.L. Valdez, in: S.G. Malghan(Ed.), Electroacoustics for characterisation of particulates and

suspensions, US Department of Commerce, Washington, DC, 1993, pp. 111–128, NIST Specialpublication 856.

w21x R.W. O’Brien, J. Fluid Mech. 190(1988) 71–86.w22x P. Mazur, J.Th.G. Overbeek, Rec. Trav. Chim(Pays-Bas) 70 (1951) 83.w23x R.W. O’Brien, P. Garside, R.J. Hunter, Langmuir 10(1994) 931–935.w24x R.J. Hunter, Zeta Potential in Colloid Science, Academic Press, New York, 1981.w25x R.W. O’Brien, D.W. Cannon, W.N. Rowlands, Electroacoustic determination of particle size and

zeta potential, J. Coll. Inter. Sci. 173(1995) 406–418, Chapter 3.w26x A.S. Dukhin, H. Ohshima, V.N. Shilov, P.J. Goetz, Langmuir 15(1999) 3445–3451.w27x A.S. Dukhin, V.N. Shilov, H. Ohshima, P.J. Goetz, Langmuir 15(1999) 6692–6706.w28x A.S. Dukhin, V.N. Shilov, H. Ohshima, P.J. Goetz, Langmuir 16(2000) 2615–2620.w29x A.S. Dukhin, P.J. Goetz, S. Truesdail, Langmuir 17(2001) 964–968.w30x A.S. Dukhin, P.J. Goetz, Coll. Surf. A 144(1998) 49–58.w31x R.O. James, J. Texter, P.J. Scales, Langmuir 7(1991) 1993–1997.w32x R.O. James, R.J. Hunter, R.W. O’Brien, Langmuir 8(1992) 420–423.w33x P.J. Scales, E. Jones, Langmuir 8(2) (1992) 385–389.w34x W.N. Rowlands, R.J. Hunter, Clays Clay Miner. 40(1992) 287–291.w35x R.J. Hunter, M. James, Clays Clay Miner. 40(1992) 644–649.


w36x R.O. James, T.W. Healy, J. Colloid Interface Sci. 40(1972) 42–81.w37x S.G. Malghan, in: S.G. Malghan(Ed.), Electroacoustics for Characterisation of Particulates and

Suspensions, US Department of Commerce, Washington DC, 1993, pp. 111–128, NIST Specialpublication 856.

w38x C. Feldmann, J. Merikhi, J. Coll. Inter. Sci. 223(2000) 229–234.w39x N. Ushifusa, M.J. Cima, J. Am. Cer. Soc. 74(10) (1991) 2443–2447.w40x P.T. Pei, J.F. Kelly, S.G. Malghan, Coll. Surf. A 70(3) (1993) 277–287.w41x T.S.B. Sayers, Coll. Surf. 77(10) (1993) 39–47.w42x N.D. Sanders, K.J. Roth, Tappi J. 76(11) (1993) 209–214.w43x N.D. Sanders, K.J. Roth, Nord. Pulp Pap. Res. 8(1) (1993) 148–152.w44x J. Gron, P. Dahlvik, Ind. Carta 33(8) (1995) 395–398,(in Italian).w45x U. Paik, V.A. Hackley, J. Am. Cer. Soc. 83(10) 2381–2384.w46x J. Texter, Langmuir 8(1) (1992) 291–298.w47x V.A. Hackley, S.G. Malghan, J. Mater. Sci. 29(17) (1994) 4420–4430.w48x H.M. Wang, R.A. McCauley, Ceram. Trans. 62(1996) 149–156.w49x (a) V.A. Hackley, R. Premachandran, S.G. Malghan, Key Eng. Mater. 89–91(Silicon nitride 93)

(1994) 679–68294(b) V.A. Hackley, J. Am. Cer. Soc. 80(9) (1997) 2315–2325.

w50x V.A. Hackley, U. Paik, B.H. Kim, S.G. Malghan, J. Am. Cer. Soc. 80(7) (1997) 1781–1788.w51x S.G. Malghan, P.S. Wang, A. Sivakumar, P. Somasundaran, Compos. Inter. 1(3) (1993) 193–210.w52x S.G. Malghan, R. Premachandran, P.T. Pei, Powder Tech. 79(1) (1994) 43–52.w53x U. Paik, V.A. Hackley, H.W. Lee, J. Am. Cer. Soc. 82(4) (1999) 833–840.w54x C. Walldal, J. Colloid Interface Sci. 217(1) (1999) 49–59.w55x C.A.M. Baltar, J.F. Oliveira, Miner. Eng. 11(5) (1998) 463–467.w56x P.A. Smith, R.A Haber, J. Am. Cer. Soc. 78(7) (1995) 1737–1744.w57x V.E. Subhin, R.J. Hunter, R.W. O’Brien, J. Colloid Interface Sci. 159(1993) 174–183.w58x R.T. Klingbiel, H. Coll, R.O. James, J. Texter, Coll. Surf. 68(1–2) (1992) 103–109.w59x R.W. O’Brien, W.N. Rowlands, R.J. Hunter, in: S.B. Malghan(Ed.), NIST Special publication

856 Electroacoustics for charcterisation of particulates and suspensions. Washington DC,(1993),pp 1–22.

w60x R.W. O’Brien, W.N. Rowlands, R.J. Hunter, Characterization of ceramic materials using electro-acoustics, in: J.H. Adair, J.A. Casey, C.A. Randall, S. Venigall(Eds.), Scientific and TechnologicalApplications of Colloidal Suspensions. Ceramic Transactions, 54, American Ceramics Society,Westerville, OH, 1994, pp. 53–66.

w61x R.W. O’Brien, J. Colloid Interface Sci. 171(1995) 495–504.w62x A. Pettersson, G. Marino, A. Pursiheimo, J.B. Rosenholm, J. Colloid Interface Sci. 228(2000)

73–81.w63x J.K. Beattie, A. Djerdjev, J. Am. Cer. Soc. 83(10) (2000) 2360–2364.w64x C. Knosche, H. Friedrich, M. Stintz, Part. Part. Syst. Characterisation 14(4) (1997) 175–180.w65x M. Rasmusson, S. Wall, Coll. Surf. A 122(1–3) (1997) 169–181.w66x C.J. Shih, B.H. Lung, M.H. Hon, Mater. Chem. Phys 60(2) (1999) 150–157.w67x J. Cessarano III, I.A. Aksay, A. Bleier, J. Am. Cer. Soc. 71(4) (1988) 250–255.w68x J. Cessarano III, I.A. Aksay, J. Am. Cer. Soc. 71(12) (1988) 1062–1067.w69x D. Santhiya, G. Nandini, S. Subramanian, K.A. Natarajan, S.G. Malghan, Coll. Surf. A 133

(1988) 157–163.w70x M. Rasmusson, W. Rowlands, R.W. O’Brien, R.J. Hunter, J. Colloid Interface Sci. 189(1997)

92–100.w71x C.A. Prestidge, W.N. Rowlands, Miner. Eng. 10(10) (1997) 1107–1118.w72x R.J. Hunter, R.W. O’Brien, Coll. Surf A 126(1997) 123–128.w73x R.W. O’Brien, W.N. Rowlands, J. Colloid Interface Sci 159(1993) 471–476.w74x W.N. Rowlands, R.W. O’Brien, J. Colloid Interface Sci. 175(1995) 190–200.


w75x W.N. Rowlands, R.W. O’Brien, R.J. Hunter, V. Patrick, J. Colloid Interface Sci. 188(1997)325–335.

w76x M. Kosmulski, J.B. Rosenholm, J. Phys. Chem. 100(28) (1996) 11 681–11 687.w77x M. Kosmulski, J.B. Rosenholm, Langmuir 15(1999) 8934.w78x S.B. Johnson, P.J. Scales, T.W. Healy, Langmuir 15(1999) 8935–8936.w79x S.B. Johnson, P.J. Scales, T.W. Healy, Langmuir 15(1999) 2836–2843.w80x S.B. Johnson, A.S. Russel, P.J. Scales, Coll. Surf. 141(1998) 119–130.w81x Y.G. Berube, P.L. de Bruyn, J. Colloid Interface Sci. 28(1968) 92.w82x L. Gierst, L. Vandenberghen, E. Nicholas, A. Fraboni, J. Electrochem. Soc. 113(1996) 1025.w83x G.V. Franks, S.B. Johnson, P.J. Scales, D.V. Boger, T.W. Healy, Langmuir 15(1999) 4411–4420.w84x S.B. Johnson, G.V. Franks, P.J. Scales, T.W. Healy, Langmuir 15(1999) 2836–2843.w85x M. DeBoer, R.G. Leliveld, J.W. Geus, H.G. Bruil, Appl. Catalysis A 102(1) (1993) 35–51.w86x F.N. Desai, H.R. Hammad, K.F. Hayes, Langmuir 9(11) (1993) 2888–2894.w87x A. Djerdjev, Coagulation of alpha alumina suspensions observed by electroacoustics. B.Sc Thesis

University of Sydney 1996.w88x V.A. Hackley, S.G. Malghan, Ceram Trans. 62(1996) 117–124.w89x S.B. Johnson, D.R. Dixon, P.J. Scales, Coll. Surf. 146(1999) 281–291.w90x T. Ukigai, H. Sugawara, N. Tobori, Chem. Lett. 5(1995) 371–372.w91x A.L. Costa, C. Galassi, R. Greenwood, J. Colloid Interface Sci. 212(1999) 350–356.w92x R. Pradip, R.S. Premachandran, S.G. Malghan, Bull. Mater. Sci. 17(6) (1994) 53–60.w93x R.S. Premachandran, S.G. Malghan, Powder Technol. 79(1) (1994) 53–60.w94x V.A. Hackley, J. Am. Cer. Soc. 80(9) (1997) 2315–2325.w95x V.A. Hackley, P.S. Wang, S.G. Malghan, Mater. Chem. Phys. 36(1–2) (1993) 112–118.w96x C. Galassi, F. Bertoni, S. Ardizzone, C.L. Bianchi, J. Mater. Res 15(1) (2000) 155–163.w97x C. Galassi, F. Bertoni, S. Ardizzone, C.L. Bianchi, J. Mater. Res 15(1) (2000) 164–169.w98x L. Bergstrom, E. Laarz, R. Greenwood Proceedings of Ultrasonic and Dielectric Characterisation

Techniques for Suspended Particles. NIST(Aug 1997).w99x E. Laarz, L. Bergstrom, J. Eur. Cer. Soc. 20(4) (2000) 431–440.

w100x R. Greenwood, K. Kendall, J. Eur. Cer. Soc. 19(1999) 479–488.w101x E. Roncari, C. Galassi, C. Bassarello, J. Am. Cer. Soc. 83(12) (2000) 2993–2998.w102x R. Greenwood, L. Bergstrom, J. Eur. Cer. Soc. 17(2) (1997) 537–548.w103x R. Greenwood, K Kendall, J. Eur. Cer. Soc. 20(2000) 77–84.w104x H. Maier, J.A. Baker, J.C. Berg, J. Colloid Interface Sci. 119(1987) 512–517.w105x L.K. Koopal, J. Lyklema, Disc. Faraday Soc. 59(1975) 230.w106x M.L. Carasso, W.N. Rowlands, R.W. O’Brien, J. Colloid Interface Sci. 193(1997) 200–214.w107x M. Carasso, The effect of polymer adsorption on the electroacoustic signals of colloidal silica,.

Ph.D. Thesis, University of Sydney, 1996.w108x N.P. Miller, J.C. Berg, Coll. Surf. A 59(1991) 119–128.w109x H. Hodne, A. Saasen, Cem. Concr. Res. 30(2000) 1767–1772.w110x M. Burke, R. Greenwood, K. Kendall, J. Mater. Sci. 33(1998) 5149–5156.w111x R. Greenwood, K. Kendall, Powder Technol. 113(2000) 148–157.w112x R. Greenwood, S.W. Kingman, N.A. Rowson, G. Brown, Powder Technol. 123(2002) 199–207.w113x C. Galassi, E. Roncari, R. Greenwood, A. Piancastelli, 5th EuroCeramic Conference, Key

Engineering Materials, Vol. 132–136, Trans Tech Publications Switzerland,(1997), p. 329–332.w114x R. Greenwood, K. Kendall. Paper Number 5. World Congress on Particle Technology, Brighton

1998. CD Rom I. Chem. E.w115x J.B. Kayes, D.A. Rawlins, Coll. Polymer Sci. 257(1979) 622.w116x J.A. Baker, J.C. Berg, R.A. Pearson, Langmuir 5(1989) 339.w117x M. Faers, P.F. Luckam, Coll. Surf. A 86(1994) 317.w118x M.S. Ahmed, M.S.El - Aasser, J.W. Vanderhoff in: Goddard, B. Vincent(Eds.), Polymer

adsorption and dispersion stability A.C.S Symposium Series(1984), p. 77.


w119x M.J. Garvey, Th.F. Tadros, B. Vincent, J. Colloid Interface Sci. 49(1974) 57.w120x M.J. Garvey, Th.F. Tadros, B. Vincent, J. Colloid Interface Sci. 55(1976) 440.w121x J.A. Baker, H. Maier, E. Killman, Coll. Surf. A 3(1988) (1988) 51.w122x R. Greenwood, K. Kendall, Brit. Ceram. Trans. 97(4) (1998) 174–179.w123x F. Shojai, A.B.A. Pettersson, T. Mantyla, J.B. Rosenholm, J. Eur. Cer. Soc. 20(3) (2000)

277–283.

review of the measurement of zeta potentials in concentrated aqueous suspensions using...

Documents