charging behavior at the alumina–water interface and implications

Charging Behavior at the Alumina–Water Interface and Implicationsfor Ceramic Processing

George V. Franks*,w and Yang Ganz

Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia

The interaction of water and the alumina surface is comprehen-sively reviewed. Water can be incorporated in the alumina crys-tal structure resulting in the formation of aluminum hydroxidessuch as gibbsite. Alumina dissolves into water to an extent thatdepends primarily upon the solution pH and temperature. Thesoluble Al (III)aq species (hydrolysis products) likewise dependupon the solution pH, temperature, aluminum, and other saltconcentrations. The development of charge on the surface ofalumina is controlled by amphoteric surface ionization reactions.The charging behavior of both alumina powders and single crys-tal faces is compared. The differences can be explained by thereactivities of different types of surface hydroxyl groups. Thesubstantial difference in surface charging behavior of singlecrystal sapphire and alumina powders indicates that experi-ments and modeling conducted on single crystals is of limited usein predicting suspension behavior. The atomic scale structure ofthe hydroxylated sapphire (0001) basal plane is nearly identicalto the gibbsite (001) basal plane. The observed surface structuresare consistent with the charging behavior of the surfaces. Therole of surface charge on the adsorption of processing additivesis briefly discussed. How surface charge and processing additivesat the alumina aqueous solution interface influence surface forc-es between particles is reviewed. The influence of these forces onsuspension properties such as rheological behavior is outlined.The importance of controlling these behaviors to improve col-loidal ceramic powder processing is stressed.

I. Introduction

THE interface between alumina and water is important in awide number of applications. Alumina particles are fre-

quently prepared as aqueous suspensions for colloidal process-ing1,2 of slipcast,3 gelcast,4 and tape cast5 ceramic components.Aqueous sol–gel processing is useful for production of alumina

powders.6 Alumina is often used to coat titania pigment particlesthat are widely used in aqueous based latex paints.7 The coatingsare usually derived from aqueous solutions of aluminum sulfateor sodium aluminate. The transition aluminas are useful in ca-talysis8 and as supports for applications including automotivecatalytic convertors.9 Alumina also has important applications inepitaxial growth of films for example in blue LEDs.10 Naturalcorundum (a-Al2O3) has also been intensively studied for its im-portance in minerals processing11,12 and adsorption of heavymetal elements.13,14 The precipitation of gibbsite from supersat-urated aqueous solutions is an important step in Bayer process-ing of bauxite into alumina.15–22 Gibbsite is also abundant in soilas silicate minerals’ stably weathering product,23 its abundance innatural fresh water aquatic systems is important to ecology.24,25

A key parameter influencing the behavior of many of theseapplications relates to the interaction of the surface of the alu-minum oxide or hydroxide and solution species such as ions,polymers or surfactants. In some cases, the interaction of thesurface is of direct interest, such as in catalysis, adsorption forwater treatment and precipitation from pregnant Bayer solu-tions. In other applications, the important influence of waterand adsorbed additives at the interface is the effect on higher-level behavior such as viscosity or green body properties. Inthese applications, the surface of alumina plays the key role incontrolling the behavior. The primary factor that influences ad-sorption of a solution species to a solid surface is the charge onboth the surface and solution species. For this reason investi-gation of the charging behavior of alumina surfaces is the focusof the present paper.

Before discussing the charging behavior, we first review theincorporation of water into the crystal structure of alumina andthe resulting aluminum hydroxide phases. We then discuss thesolubility of alumina in water and the equilibrium solutionspeciation. After a thorough discussion of several aspects ofsurface charging behavior, we discuss the surface structure of thebasal plane of single crystal sapphire and compare it with sub-micron powder surface behavior. A brief description of someadsorption phenomena and interparticle surface forces follows.Finally, we discuss the influence of the interparticle forces on themacroscopic properties of alumina suspensions and the impli-cations for colloidal ceramic powder processing.

II. Aluminum Oxide and Hydroxide Phases

The first interaction between alumina and water which we con-sider is when water is incorporated into the crystal structure of

Feature

D. Green—contributing editor

Financial support for our research into alumina surfaces has been provided over anumber of years from the Australian Research Council, the University of Melbourne andthe University Newcastle.

*Member, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: gvfranks@unimelb.

edu.auzCurrent address: Department of Applied Chemistry, Harbin Institute of Technology,

[email protected].

Manuscript No. 23068. Received April 10, 2007; approved July 26, 2007.

Journal

J. Am. Ceram. Soc., 90 [11] 3373–3388 (2007)

DOI: 10.1111/j.1551-2916.2007.02013.x

r 2007 The American Ceramic Society

alumina. The various alumina and aluminum hydroxide phaseshave different crystal structures. In the present paper we use theterm alumina to refer to the anhydrous aluminum oxides of thechemical formula Al2O3, which include corundum (a), and anumber of transition aluminas (g, w, k, d, y, and Z). The alu-minum hydroxides include the trihydroxides (Al(OH)3) gibbsiteand bayerite, and monohydroxides (AlOOH) boehmite and dia-spore. The thermodynamically stable form of alumina is co-rundum while the transition aluminas are metastable phases,which occur during the thermal dehydration of the precursorhydroxides. Panel A describes the bulk crystal structures of theprimary aluminum oxide and hydroxides phases although otherless technologically important phases are also known to exist.

The hydroxides contain water within their structureprimarily as hydroxide ions. In effect, a water molecule is in-corporated into the crystal structure in place of an Al–O bondsuch that one hydrogen atom of the water molecule bonds to anoxygen anion while the oxygen and other hydrogen bond to analuminum ion resulting in the formation of two aluminumhydroxyls. This is not in fact how the hydroxide phases actual-ly form. Naturally occurring hydroxides form hydrothermallyand/or by weathering dissolution reactions of clay minerals.25

Synthetically, most hydroxide phases form by the condensationof soluble aluminum hydroxide species28 which will be discussedin more detail in Section III. The most technologically impor-tant example of synthesis of hydroxide by condensation ofsolution species is the precipitation of gibbsite during theBayer process.15–22

In order to produce alumina, hydroxides are produced fromsolution and thermally dehydrated. Panel B provides a summaryof the routes from the hydroxides to corundum.26,29 Typicallythe hydroxides with cubic close packing, boehmite and bayerite,form transition phases with face-centered cubic close packing(g, Z, d, y). Likewise, diaspore (with hexagonal close packing)and gibbsite form the transition aluminas with hexagonal closepacking (w and k). At high enough temperature the equilibriumphase corundum (a-alumina) will form in all cases. Upon cool-ing the a phase remains stable. One approach to making finegrained dense a-alumina is to use commercially available nano-sized g-alumina powders which can be sintered at lower tem-perature than corundum powders due to the phase transitionbetween g and a.30–32

III. Solubility and Solution Species

When alumina surfaces are immersed in water there is a finitesolubility of alumina into the water. The solubility limit and thetype of solution species depend upon the solution pH, temper-ature, other ionic species, and the solid phase. Solubility gener-ally increases with temperature as is common for most materials.The total Al (III)aq concentration in equilibrium with variousaluminas and aluminum hydroxides has a minimum pHabout 6–7 that ranges from about 10�6 to 10�7 M dependingon the solid phase.25,33,34 Figure 3 is an example of the solubilityof gibbsite as a function of pH at 251C. Solubility of gibbsiteincreases dramatically as pH is either decreased or increased. AtpH about 3.5 the concentration of Al (III)aq in solution in equi-librium with gibbsite is about 0.1 M and about 0.01 M at pH 12.Corundum and amorphous aluminum hydroxide solids are typ-ically slightly more soluble than gibbsite.25,34

The aluminum containing species present in solution gener-ally depend on the total Al (III)aq concentration, other ionicconcentrations, temperature and the pH. Only at low pH (belowabout pH 4 or 5) do the species Al31 exist as the majority so-lution species. As pH increases the metal cation reacts with hy-droxide anions in solution progressively to produce solublehydrolysis products according to the solution equilibria:33–35

Al3þ ! AlðOHÞ2þ ! AlðOHÞþ2 ! AlðOHÞ3 ! AlðOHÞ�4

The hydrolysis reactions of aluminum with hydroxide occurbetween about pH 5 and pH 8 at room temperature and low

Al(III) concentration so that the dominant solution species is thenegatively charged Al(OH)4

� above about pH 8. The mononu-clear (one Al ion) species form rapidly and reversibly. Thesespecies dominate at low ionic strength and aluminum concen-tration at room temperature (see Fig. 4(a)). At pHo3, the Al31

ion prevails in a state where it is coordinated by six water mol-ecules. At pH 47, the Al(OH)4

� ion has the aluminums coor-dinated by four hydroxide ions. Recent kinetic evidence hasbeen found for five-fold coordination (four water molecules andone hydroxide ion) of AlOH21 ions at pH between 4.3 and 7.36

A number of polynuclear solution species (ions that containmore than one Al ion) are known to exist under certain solutionconditions.33,37 The most studied of these include Al2(OH)2

41,Al3(OH)4

51 and Al13O4(OH)2471 (the so called keggin ion). These

polynuclear ions form slowly but are metastable for extendedperiods in supersaturated solutions. At low pH in room tem-perature solutions of low Al (III)aq and electrolyte concentra-tion, the polynuclear species occur only in low concentrations.When the concentration of Al (III)aq is increased, the occurrenceof Al31 is pushed to lower pH values and the Al(OH)4

� speciesdo not dominate until higher pH values because the cationicpolynuclear species dominate behavior when the pH is nearneutral (see Fig. 4(b)). Al (III)aq solution species at higher tem-peratures, pressures, and electrolyte concentrations are impor-tant in Bayer processing19 and in geothermal mineralformation.38,39 The concentration and type of background elec-trolyte ions (due to added salt, acid, or base) influence the typeand concentrations of solution species in a way that is still beinginvestigated. It is known that even the subtle difference betweenNa1, K1, and Cs1 in solution influences the solution speciesand solid precipitate in the Bayer process.17,18

When alumina solid phases are immersed in water at ambientconditions, the solution concentration of Al (III)aq will increaseslowly with time until the solution concentration reaches thesolubility limit. Because the dissolution process consumes H1 atlow pH or OH� at high pH, the pH of the suspension shiftstoward neutral as the Al (III)aq concentration increases. Even-tually equilibrium will be reached between the pH and the Al(III)aq concentration in solution. For example, submicronalumina powders dispersed in aqueous solution at pH around3 will increase in pH over a period of several hours to days to a pHabove pH 4 as dissolution slowly occurs. If the pH is maintainedat an extremely low or high pH value for a period of severaldays, the solution concentration can increase to near the equi-librium. If the pH is then adjusted to neutral where the solubilitylimit is lower, the dissolved aluminum species will either precip-itate out of solution usually as amorphous solids or as crystallinesolids, or remain in solution as metastable polynuclear speciessuch as keggin ions. The hydrolysis reaction of Al (III)aq con-taining salts (such as Al2(SO4)3) in neutral and slightly basicwater is a technologically important method for removing con-taminants from drinking water.34,35,40 In this application, theprecipitation of amorphous aluminum hydroxide species trapscontaminants within a floc or aggregate of the precipitate. Theflocs have much greater mass than the individual contaminantsso the flocs containing the contaminants can be removed fromwater by sedimentation.

IV. Surface Hydration, Charging, and SurfaceIonization Reactions

Clean surfaces of metal oxides such as alumina in vacuum arecomprised of atoms that have unsatisfied bonds. In vacuum,these unfulfilled bonds generally result in an equal number ofpositively charged metal ions and negatively charged oxygenions for many substances as shown schematically in Fig. 5(a).(Other substances can be polar, i.e., all cations on one face andall anions on the opposite face.) For a-alumina, the actual sur-face termination in vacuum41–46 is more complicated and will beconsidered in Section VII. When the metal oxide surface is im-mersed in water or exposed to ambient air (which usually has at

3374 Journal of the American Ceramic Society—Franks and Gan Vol. 90, No. 11

Panel A. Crystal Structure of Alumina Oxide and Hydroxide Phases. (After Levin and Brandon, 1998)26

a-Al2O3, corundum: The bulk structure of corundum has oxygen atoms arranged in approximately hexagonal close packed layers. Between any two layers ofoxygen (O) atoms, two-thirds of the octahedral sites are filled by aluminum (Al) atoms in an ordered array. The aluminum atoms do not lie on the mid-planebetween the oxygen layers. The aluminum atoms displace slightly toward the unoccupied octahedral site in the cation layer either above or below. The result is thatan ordered half of the aluminum ions lie just above and half of the aluminum ions lie just below the mid-plane between the oxygen layers. The hexagonal unit cell’ssize is a5 4.75 A, c5 13 A.26,27 The structure of a-Al2O3 is shown in Fig. 1.

g- and Z-Al2O3: The structures of g and Z alumina have oxygen in approximately face centered cubic packing. These phases exist as a defect spinel structure. In theideal spinel structure (AB2O4) the cations reside in one half of the octahedral and one eighth of the tetrahedral sites. In the g and Z alumina phases there isdistortion of the cubic lattice and there are a number of disordered cation vacancies in order to maintain the stoichometry Al2O3. Subtle differences in the oxygensub-lattice distortion and amount of cation ordering are the difference between g and Z alumina.

d-Al2O3: d-Alumina has oxygen ions in approximately face centered cubic packing. d-Alumina is a superstructure of the spinel structure with ordered cationvacancies. The unit cell is either orthorhombic or tetragonal.

y-Al2O3: y-Alumina has FCC packing of the oxygen anions and a monoclinic symmetry. The aluminum cations are equally distributed over octahedral andtetrahedral sites.

k- and w-Al2O3:Many researchers believe that k and w alumina have oxygen atoms in hexagonal close packing and that the aluminum cations reside in octahedralsites. Other researchers suggest that these phases may be either cubic or orthorhombic and some cations may exist in tetrahedral sites.

g-Al(OH)3, gibbsite: Gibbsite is a pillared structure where each pillar has a double layer of nearly close packed oxygen ions with aluminum ions filling 2/3 of theoctahedral sites between the two layers. The aluminum ions all lie on the midplanes between adjacent oxygen layers. Each oxygen atom is bound to a hydrogenatom. The stacking sequence of the double layers is AB BA. Because there are no aluminum ions between the adjacent hydroxyl layers, the basal plane is a weakcleavage plane. The resulting distortion results in a monoclinic structure. The gibbsite structure is shown in Fig. 2.

a-Al(OH)3, bayerite: Bayerite has the same pillared double layered structure as gibbsite but in this phase, the stacking sequence is AB AB. The basal plane is aneasy cleavage plane between the two hydroxide layers. The resulting distortion results in a monoclinic structure.

g-AlOOH, boehmite: The oxygen ions in boehmite are arranged in face centered cubic close packing with aluminum ions in between adjacent layers. Due to thearrangement of hydrogen ions, the structure is orthorhombic.

a-AlOOH, diaspore: In diaspore the oxygen layers are stacked in hexagonal close packing and the cations are in octahedral sites between layers. Diaspore has anorthorhombic structure.

Fig. 2. (a) View of a single layer of the gibbsite structure looking down upon the basal plane. (b) Side view of the gibbsite structure orthogonal to thebasal plane. The light gray spheres represent the hydrogen atoms.

Fig. 1. The a-alumina corundum crystal structure viewed along, (a) the /0001S direction (‘‘c’’ plane) and (b) the /10�10S direction. Blue spheresrepresent oxygen atoms and red spheres represent aluminum atoms.

November 2007 Charging Behavior at the Alumina–Water Interface and Implications 3375

least 15% relative humidity), the surface reacts with water toproduce surface hydroxyl groups13,25 (denoted Me–OH) asshown in Fig. 5(b). The reaction of a clean surface of aluminaand other metal oxides with water depends upon the partialpressure of water in contact with the surface. Most work hasbeen conducted on low-index crystallographic surfaces such as(0001) a-alumina, (0001) a-hematite, (100) and (110) rutile-tita-nia.13,25,47,48 In high vacuum the surface has not reacted withwater so it is not hydroxylated. At very low water pressures,only the defect sites of a surface react with water to form surfacehydroxyl groups. The defects include atoms at the edge of stepsbetween each atomic layer and vacancies in the structure. Thesedefects have higher energy and are more reactive than the ma-jority of surface atoms, which lie on the defect free crystallineterraces. At higher water partial pressures, the sites on the ter-races can also react with water resulting in a fully hydroxylatedsurface at high water partial pressures. For alumina the partialpressure of water required to initiate reaction of terrace sites onthe (0001) basal plane is about 1 Torr.13,25 The surface of sub-micron powder is somewhat different than single crystal samplesbecause the majority of the surface sites on the powders are de-fect sites.49,50 Also, the surface of alumina powders prepared bygrinding or milling (the majority of alumina is used after somemilling) can have surface hydroxyl groups that are the same asthose on gibbsite.51,52

It is also possible to dehydrate a fully hydroxylated surface bythermal treatment either in vacuum or air.50,51,53 The aluminasurface dehydrates in a step-wise fashion such that different typeof surface hydroxyl groups are removed sequentially. As de-scribed in more detail in Section VIII, there are several type of

surface hydroxyl groups. The primary difference between thetypes of surface hydroxyl groups is the number of aluminumions to which the hydroxide ion is coordinated. The hydrogenbonded water is removed at temperatures around 3001C, mul-tiply coordinated sites dehydrate next at temperatures betweenabout 5001 and 7001C and the singly coordinated sites only attemperatures above about 7001C. The last sites to dehydrate arethe defect sites. No hydroxyl groups are found at temperaturesabove about 9501C.50,51,53 The fully dehydroxylated aluminasurface has a structure with oxygen anions bridging two alumi-num cations.50

A single amphoteric site surface hydroxyl group reacts withacid and base at low and high pH, respectively, via surface ion-ization reactions as follows,54–56

Me2OHþ2 �!Ka1

Me2OHþHþ

Me2OH �!Ka2Me2O� þHþ

resulting in either a positively charged surface (Me–OH21) as in

Fig. 5(c), or a negatively charged surface (Me–O�) as inFig. 5(d). The values of the surface ionization reaction con-stants (Ka1 and Ka2) determine the point of zero charge (PZC)of the surface and depend upon the particular type of material(for example Al2O3, SiO2, TiO2, etc.). The PZC of different ox-ides can be estimated from the dielectric constant of the solid,the Pauling metal–oxygen bond strength and the metal–hydrox-ide bond length.57 These parameters influence the reactivity ofthe surface hydroxyl site with acid and base. In fact, as discussedin more detail in Section VIII, the surface ionization constantsdepend upon the particular coordination environment of themetal ion at the surface for any particular material.58–61 Some ofthe surface charge is compensated by bound ions which arepresent when there is salt in the solution. Site binding models areused to reconcile the apparent difference between surface chargemeasurements from titration data and z potential results. Fre-quently a large fraction of ions are bound. The methods of ac-counting for the surface charge, ion binding and z potentials arecalled surface complexation models and are discussed in detailby Hunter.55

For each type of material, there is a pH known as the PZC,where the majority of surface sites are neutral (Me–OH) and thenet charge on the surface is zero. At a pH below or above thePZC, the particle’s surfaces become positively (Me–OH2

1) ornegatively (Me–O�) charged due to the reaction with either acid(H1) or base (OH�), respectively. Figure 6 shows how the con-centration of surface sites changes with pH.

The surface charge at the solid–solution interface produces asurface potential. The surface charge is neutralized by counter-ions (ions with opposite charge to the surface). Due to thermalenergy and entropy, the majority of counterions are not boundto the charged surface sites. The counterions dissociate from thesurface sites for the same reason sodium chloride placed in waterdissociates into sodium cations and chloride anions. The count-erions form a diffuse cloud around the surface. The surface po-tential drops off as a function of distance from the surface asdetermined by the Poisson–Boltzmann equation. (See one of thecolloid texts55,56,62 for a detailed treatment.)

V. Electrical Double Layer (EDL) Repulsion andVan Der Waals Attraction

Because a layer of immobile ions and water molecules exists atthe surface, it is not straightforward to directly measure thesurface potential of particles. Instead, a closely related potentialknown as the z potential is usually measured. The z potential ismeasured through experiments where relative motion betweenthe surface and the solution is required. The z potential is thepotential at the plane of shear between the immobilized surfacelayer and the bulk solution. This plane is typically located only afew angstroms from the surface so the difference between the z

Panel B. Transformation Sequences of Hydroxides ThroughTransition Aluminas to Corundum Upon Heating. (After

Brandon and Levin, 1998)26

gibbsite

diaspore

boehmite

bayerite

0°C 300°C 600°C 900°C 1200°C

chi

heta

alpha

alpha

alpha

kappa

gamma

theta

delta theta

alpha

Fig. 3. Solubility of gibbsite in water at 251C (after Baes and Me-smer33). The maximum equilibrium solubility of Al (III)aq containingsolution species is indicated by the curved solid line.


potential and the surface potential is usually not much. The zpotential corresponds even more closely with the diffuse layerpotential, which is largely responsible for controlling the EDLinteraction force between surfaces. The pH where the z potentialis zero is known as the IEP. If the counterions are indifferent,i.e., they do not specifically adsorb, the IEP and the PZC of thesurface correspond.

As mentioned earlier, counterions form a diffuse cloud thatshrouds the surface in order to maintain electrical neutrality ofthe system. When two surfaces (or particles) are forced togethertheir counterion clouds begin to overlap and increase the con-centration of counterions in the gap between their surfaces. Ifboth surfaces have the same charge, this gives rise to a repulsivepotential due to the osmotic pressure of the counterions, which

Fig. 5. Schematic representation of the surface of metal oxides. The blue spheres represent oxygen, the red spheres a metal cation and the small grayspheres represent hydrogen. (a) In vacuum, unsatisfied bonds lead to positive and negative sites associated with metal and oxygen atoms, respectively.(b) The surface sites react with water or water vapor in the environment to form surface hydroxyl groups (Me–OH). At the isoelectric point (IEP) theneutral sites dominate, and the few positive and negative sites present exist in equal numbers. (c) At pH lower than the IEP, the surface hydroxyl groupsreact with H1 in solution to create a positively charged surface composed mainly of (Me–OH2

1) species. (d) At pH higher than the IEP, the surfacehydroxyl groups react with OH� in solution to create a negatively charged surface composed mainly of (Me–O�) species.

Fig. 4. (a) Fraction of solution species as a function of pH for 10�5 molal Al (III)aq at a total ionic strength of 1.0 in equilibrium with gibbsite at 251C.(b) Fraction of solution species as a function of pH for 10�1 molal Al (III)aq at a total ionic strength of 1.0 in equilibrium with gibbsite at 251C (after Baesand Mesmer33).


is known as the EDL repulsion. If the particles are of oppositecharge an EDL attraction will result.

The Debye length (k�1) is a measure of the thickness of thecounterion cloud (and thus the range of the repulsion). TheDebye screening parameter (k) is k ¼ 3:29

ffiffiffiffiffi½c�

p(nm�1) for

monovalent salts,62 where [c] is the molar concentration ofmonovalent electrolyte. An approximate expression for theEDL potential energy (VEDL) versus the surface to surface sep-aration distance (D) between two spherical particles of radius(R) with the same surface charge is VEDL ¼ 2peeoRC2

de�kD;

where Cd is the diffuse layer potential (typically equivalent tothe z potential), e the relative permittivity of water and eo thepermittivity of free space which is 8.854� 10�12C2 � (J �m)�1.This expression is valid when the surface potential is constantand below about 25mV and the separation distance between theparticles is small relative to their size.62

The van der Waals interaction is the other force that needs tobe considered to determine the overall interaction force betweentwo surfaces in aqueous solution. The van der Waals force isprimarily caused by the electrodynamic interactions betweencorrelated, fluctuating, instantaneous dipoles within the atomsthat comprise the material of interest. Pairwise summation ofthe interactions between two spherical particles of the same sizewhen the distance between the particles (D) is much less than theradius of the particles (R) results in the surprisingly simple re-lationship for the van der Waals potential energy, VvdW5�AR/12D. More complicated relationships arise if the particles arenot the same size or are small relative to the distance betweenthem.62 The sign and magnitude of the interaction for a partic-ular pair of particles interacting in a given medium is expressedas the numerical value of the Hamaker constant (A). When theHamaker constant is greater than zero the interaction is attrac-tive and when the Hamaker constant is less than zero the inter-action is repulsive. The Hamaker constant between two solidsurfaces depends upon the dielectric properties of the solids andintervening material. Several methods are available for estimat-ing the Hamaker constant depending on the level of approxi-mation desired and the electro-optical data available.62–67

Typical estimates for two alumina surfaces interacting acrosswater are that A5 5.270.5 depending on the model and dataused.

VI. f Potential and IEP Measurements

The IEP, z potential and surface charging of alumina have beeninvestigated by many researchers over the past 50 or more years.Kosmulski68–71 has produced a series of reviews in recent yearsthat summarize the results. Before Kosmulski’s compilations,68–71

the most comprehensive review of z potentials was published byParks in 1965.72 Many different brands of alumina powders and

many different techniques for measuring the z potentials andIEP have been investigated. The general conclusion of the bodyof work is that equiaxed alumina powders irrespective of theircrystal phase have IEP between about 8 and 10. There may besubtle differences between a-alumina and gibbsite particles al-though the variation in measurements between individual re-search publications is on the order of the possible observeddifferences. Table I below is a summary of some of the mea-surements. The IEP is defined as the pH where the z potential iszero, while the PZC is determined from titration measurements.The IEP and PZC are the same when the electrolyte is indiffer-ent, that is, the ions are not specifically adsorbing.

Investigation of the z potentials and IEPs of individual crys-tallographic planes of alumina surfaces is more recent, onlyhaving significantly commenced less than 20 years ago. The best-characterized surfaces are the basal ‘‘c’’ plane (0001) of sapphireand the basal plane (001) of gibbsite. See Table II for a summaryof the measurements. Streaming potential measurements andfitting diffuse layer potentials from surface force apparatus andatomic force microscopy (AFM) force data are the most fre-quently used methods for determining the z potentials and IEPsof macroscopic surfaces and single crystal planes. A newly de-veloped technique utilizing second harmonic generation has alsobeen used.92,93,96 These methods consistently produce IEP val-ues for sapphire (0001) and gibbsite (001) surfaces 3–5 pH unitsbelow that typically found for powder. The only exceptions arethe results of Veeramasuneni et al.78 and Tulpar et al.,97 whichpresent sapphire IEP values similar to powder for reasonsdescribed in Section IX.

The charging behavior of other crystallographic planes is lesswell characterized and there is less agreement between measure-ments from different laboratories and techniques.49,95,96,98,100,101

It is likely that differences in surface preparation and cleaningtechniques result in different surface structures and thus differ-ent charging behavior.98,102 One well documented example ofthe importance in surface preparation is the difference in surfacestructure of the sapphire (1�102) surface found by Trainoret al.100 (missing Al termination) and Catalano et al.101 (stoic-hiometric termination) which is believed to be due to two differ-ent surface preparation and cleaning methods.102

The difference between the IEP of a-alumina powder andc-plane sapphire has been a topic of debate since the differencewas first observed. Concerns were raised that the lower IEP ofsapphire crystals was due to contamination of the aluminasurface with soluble silicate species that can be present in thesolution that was in contact with glassware.71 If silicate speciesdeposited on the surface of alumina, the IEP of the aluminawould in fact be shifted to lower pH values.103 This was found tooccur when high surface area silica and alumina powders werestored together as suspension over periods of time greater thanweeks. This long time allowed the silica to dissolve and adsorbon to alumina powders lowering its IEP.103 There is nowsignificant evidence that the low IEP of (0001) a alumina (and(001) gibbsite) surfaces is not due to silica contamination. Sev-eral investigations referred to in Table II were conducted underconditions that the crystals were never in contact with solutionthat was contained in glassware. Several investigators examinedthe surface of the crystals with XPS49,91,95,96,98 and Auger elec-tron spectroscopy92,93 to prove that no silica was present on thesurface of the crystals. The IEPs of pure (0001) sapphire and(001) gibbsite clearly occur at pH values about 3 to 5 pH unitsbelow those of the micron sized powders of the same materials.The reason for this difference is explained in Section IX.

VII. Atomic Scale Surface Structure: Corundum and Gibbsite

The bulk structures of corundum and gibbsite are described inSection II. The surface of a-Al2O3 (0001) in vacuum has beenextensively modeled10,13,41,42 and characterized.10,13,43–46 The re-construction of the surface which occurs because the crystalstructure is incomplete leads to a variety of surface structures

Fig. 6. Number density per unit area of neutral (Me–OH), positive(Me–OH2

1) and negative (Me–O�) surface sites as a function of pH.


Table I. The pHIEP Values of a-Al2O3 and Gibbsite Particles

Name and size Measurement method Salt concentration and type pHIEP Reference

Linde ‘‘A’’ (0.1 mm) Electrophoresispotentiometric titration

1–100 mM KCl, KNO3,KClO4

9.170.1 Yopps andFuerstenau73

E. Merck AG (o20 mm) Electrophoresis 2 mM KCl 8.870.2 Dick et al.74

Alcoa A16 SG (0.2 mm) Electrophoretic masstransport

Water/unknown 8.0 Hashiba et al.75

Sumitomo AKP(0.5–1.3 mm)

Electrophoresis 10 mM NH4Cl 9.0 Velamakanni andLange76

Buehler LTD (1.0 mm) Titration 5 mM, 30 mM, 139 mMNaNO3

8.9(PZC)

Hayes et al.77

Alfa-Aesar (1 mm) Laser–Dopplerelectrophoresis

1 mM KNO3 9.1 Veeramasuneni et al.78

Alcoa A-16 (0.40 mm) Electrophoresis 10 mM KNO3 8.8 Ramakrishnan et al.79

Sumitomo AKP(0.23–0.7 mm)

Laser–Dopplerelectrophoresis

10 mM KNO3 9.070.3 Franks and Lange80

Alcoa A16 (0.36 mm) Electrophoresis Water/unknown 8.7 Yang andTroczynski81

Alcoa A-16SG (0.2–1 mm) Electro acoustic method Water/unknown 8.2–9.0 Costa et al.82

Sumitomo AKP-30(0.30 mm)

Electro acoustic method 0.01–1 M LiNO3, NaNO3,KNO3, CsNO3, KBr, KCl, KI

9.2 Johnson et al.83

Sumitomo AKP(0.31 mm)

Electro acoustic, streamingpotential

0.01 M NaNO3 9.0–9.3 Hackley et al.84

JFCC RP-1 (1.85 mm) Electro acoustic, streamingpotential

0.001 M NaNO3 7.6–7.8 Wasche et al.85

SumitomoAKP (0.23 mm) Electro acoustic method 10 mM KBr, KNO3 8.7 Franks and Meagher49

Gibbsite particles,hexagonal crystals,(0.5 mm)

Potentiometric titration 0.5 M NaCl 9.8(PZC)

Hiemstra et al.59

Synthetic gibbsiteparticles, (20–40 m2/g)

Potentiometric titration 0.005–0.1 M NaNO3 10.0(PZC)

Hiemstra et al.86

Synthetic gibbsiteparticles, (200 nmdiameter, 10 nm thickplates)

Potentiometric titration 0.02–0.1 M NaCl 9.0(PZC)

Rosenqvist et al.87

Synthetic gibbsiteparticles, (200 nmdiameter, 10 nm thickplates)

Electrophoresis 0.02–0.1 M NaCl 10.0(IEP)

Rosenqvist et al.87

Gibbsite particles,crystallized from Bayerliquor, (surface area,4 m2/g)

Potentiometric titration 0.001–0.1 M NaNO3 8.4–9.2(PZC)

Jodin et al.88

The particles are a alumina unless otherwise noted. PZC is the point of zero charge as determined by titration. IEP, isoelectric point.

Table II. Isoelectric Points of (0001) Basal Plane Sapphire and (001) Gibbsite

Surface Method Check for silica contamination IEP Reference

c-Plane sapphire SFA No o6.7 Horn et al.89

c-Plane sapphire SFA No 3.0 Ducker et al.90

Sapphire, orientation notspecified

AFM Plasma cleaned 9.3 Veeramasuneni et al.78

c-Plane sapphire Streaming potential andAFM

XPS 4.2 Larson et al.91

c-Plane sapphire Second harmonicgeneration and AFM

Auger electronSpectroscopy

5–6 Stack et al.92 and Stack et al.93

Sapphire, orientation notspecified

AFM Care was taken to avoidsilica contamination

5.0–5.9 Meagher et al.94

c-Plane sapphire Streaming potential andAFM

XPS 4.8–5.4 Franks and Meagher49

c-Plane sapphire Streaming potential XPS 6–7 Kershner et al.95

c-Plane sapphire Second harmonicgeneration

XPS 4.1 Fitts et al.96

c-Plane sapphire AFM Plasma cleaned 8.5 Tulpar et al.97

c-Plane sapphire AFM Not plasma cleaned 5–6 Tulpar et al.97

c-Plane sapphire Streaming potential XPS 4.2 Lutzenkirchen98

Gibbsite AFM Cleaved surface 5.9 Gan and Franks99

SFA, surface force apparatus; AFM, atomic force microscope; IEP, isoelectric point.


under differing conditions. Most work has been conducted onsapphire (0001) and gibbsite (001) surfaces. Low energy electrondiffraction (LEED)43 and X-ray methods44,45 have shown thatthe sapphire (0001) surface is terminated in vacuum with a singlelayer of aluminum atoms under most partial pressures. Thissingle layer of aluminum atoms contains half of those which liebetween two adjacent close-packed oxygen layers in the bulkstructure. Remember from Fig. 1 how the aluminum atoms lieon either side of the mid-plane between the close packed oxygenlayers. The surface structure in vacuum is illustrated in Fig. 7(a).Some earlier reports41,46 indicate that the (0001) sapphire sur-face can be terminated in a close-packed layer of oxygen atomsin some near vacuum conditions as shown in Fig. 7(b). Knowl-edge of the surface structure in vacuum is of limited use in un-derstanding the behavior of the surface of alumina in waterbecause the surface structure in water is significantly differentthan in vacuum.25

In a humid atmosphere or water, partially and fullyhydroxylated a-Al2O3 (0001) surfaces have been characterizedby LEED,43,104 X-ray methods,44,45 and dynamic-mode scan-ning force microscopy (SFM).105 The experimental results aresupported by calculations based on first-principles that indicatestrong surface relaxation occurs in the presence of watervapor due to interaction between water molecules and the sur-face.106–108 When exposed to water vapor, the unhydrated

alumina surface reacts to form a hydroxyl group (OH) termi-nated surface (the coverage of �OH depends on the partialpressure of water).13,25 Figure 7(b) shows the arrangement of theoxygen atoms on the (0001) sapphire surface in water (note, thehydrogen atoms are not shown). Eng et al.45 used Crystal Trun-cation Rod X-ray diffraction to determine the surface atom po-sitions. They found that significant surface relaxation occurssuch that the aluminum atoms between the top two oxygen lay-ers shift positions such that they lie closer to the mid plane be-tween the oxygen layers. Relaxation also occurs in the topmostoxygen layer. The result of the surface relaxations is that theoxygen and aluminum atoms in the surface layers reside innearly the same positions as the surface atoms in gibbsite. Thehydrogen atom positions were not determined by Eng et al.’s45

X-ray work but simulations106–108 have indicated that the hy-drogen atoms of the surface hydroxyl groups are gibbsite like aswell. At any particular instant, two of the three surface hydroxylgroups bound to any aluminum ion are nearly vertical to thesurface, while the remaining one lies down flat on the surface ina position just above the unoccupied octahedral site just below.These rearrangements produce a surface structure closer to thatof gibbsite than that of bulk sapphire. This viewpoint has alsogained support from infrared (IR) spectra studies of wateradsorption on a-Al2O3 (0001) single crystal surface.109 Figure8 is a schematic comparison of the hydrated a-alumina surface

Fig. 7. Schematics of two types of surface termination of a-Al2O3 (0001) plane. (a) (O3�O3) single Al termination found in vacuum; (b) (1� 1) Otermination found in water. Large blue spheres represent oxygen atoms and small red spheres represent aluminum atoms. Note that the hydrogen atomsof the surface hydroxyl groups of the oxygen-terminated surface in water are not shown.

Fig. 8. Comparison of the surface structure of a-Al2O3 and gibbsite in water (after Eng et al.45). Blue spheres represent oxygen atoms, red spheresrepresent aluminum atoms and the light gray spheres represent the hydrogen atoms. (a) The gibbsite surface has all the aluminum atoms along themidplane between the two uppermost oxygen layers. Two thirds of the surface hydroxyl groups are oriented orthogonal to the surface and one third areparallel to the surface. (b) Surface relaxation of the sapphire surface occurs such that the top layers of aluminum atoms move closer to the mid plane ofthe two uppermost oxygen layers and the oxygen atom positions relax from the close packed structure slightly to positions like on the gibbsite surface.Like gibbsite, two thirds of the surface hydroxyl groups are oriented orthogonal to the surface and one third are parallel to the surface.106–108


and the gibbsite surface. The surface of hydrated a-aluminapowders prepared by grinding were also found to contain sur-face hydroxyl groups that are gibbsite like after hydrothermalhydration.51,52

The advent of scanning probe microscopy (SPM) and AFMhas allowed for direct observation of the surface structure ofalumina in vacuum, low-pressure vapors,105 air,110,111 and wa-ter.111,112 Using a dynamic SFM technique Barth and Reich-ling105 investigated a reconstructured surface of sapphire (0001).They imaged a (O31�O31)R791 structure produced at 13001Cin ultra high vacuum with atomic resolution. After exposure tovery low (milliPascal) levels of water vapor they observed manyclusters of size 0.4–0.8 nm. They supposed these clusters weremost probably crystalline Al(OH)3. As early as 1993, Steinberget al.111 achieved unit cell resolution images of nanocrystalline a-alumina that was grown on mica by the van der Waals epitaxymethod. The 10–nm-thick alumina films were grown on freshlycleaved mica sheets by vacuum depositing Al2O3 from a pelletvaporized using an electron beam. They observed a hexagonallattice (period of 0.4770.02 nm) at various places on the film inboth force and height modes in air, water and salt solutions. Toour knowledge, there is only one paper that reports the surfacestructure of gibbsite obtained using AFM.113 However, few ex-perimental details are supplied. It is believed that the imagepublished was taken in dry air.

Recently we have examined sapphire samples cut and pol-ished parallel to the single crystal a-Al2O3 (0001) basal plane(miscutting angle between about 0.51 to 0.81) both in water112

and in air.110 In water, these surfaces have terraces about 20 nmwide112 as shown in Fig. 9. The height of the steps is about 2 A,which is equivalent to the distance between adjacent oxygenlayers.112 As shown in Fig. 10(a), closer inspection of the ter-races reveal hexagonal arrays of corrugations. A two dimen-sional fast Fourier transformation of the image data indicatesthe distance between the nearest corrugations is 4.7 A. The hex-agonal periodicity of the structure is clearly shown in Fig. 10(b).The hexagonal structure and the 4.7 A period is equivalent tothe known bulk unit cell size and structure for a-alumina,a5 4.75 A.26,27 Although we have not been able to resolve in-dividual atoms in our studies,110,112 our investigations reveal lowspots (dark spots in Fig. 10(a)) in the surface with hexagonalperiodicity of 4.7 A. Figure 10(c) helps to illustrate how thesefeatures correspond to the atomic arrangements on the surfaceof the sapphire basal plane. The hollows in the surface structureappear to correspond with the position where the surface hyd-roxyl groups are lying down (just above the unoccupied oct-ahedral site in the sublayer). The position where the surfacehydroxyl groups stand up from the surface perhaps correspondwith the high points (bright spots) in the surface structure.

VIII. Multisite Complexation Models of Alumina Surfaces

In Section IV the concept of surface hydroxyl groups (Me–OH)was introduced. In this section we discuss specifically the reac-tivity of the aluminum surface hydroxyl groups (Al–OH) foundon alumina surfaces. As early as the 1970s8 it was recognizedthat there are more than one type of aluminum surface hyd-roxyl. The difference between the different types of surfacehydroxyls primarily depends upon the number of aluminum at-oms to which the hydroxyl is bound8,50 as well as the coordina-tion of the aluminum ion (octahedral or tetrahedral). Ahydroxyl bound to one aluminum atom is known as a singlycoordinated surface hydroxyl (denoted Al–OH), likewise, whenbound to two or three aluminum ions the hydroxyl is referred toas doubly (Al2–OH) or triply (Al3–OH) coordinated, respective-ly. It has also been recognized that these different types of sur-face hydroxyl groups will have different pKa values controllingtheir charging behavior.8,58,59,114

In the late 1980s, Hiemstra and coworkers58,59 developed themultisite complexation (MUSIC) model to account for the pos-sibility of different type of surface hydroxyl groups with differ-ing reactivity (pKa values). They recognized that intrinsicequilibrium constants, where no effects of long-range electro-static effects exist, must be constrained before model fitting in

Fig. 9. The step-terrace surface structure (topographical image) of thesapphire basal (0001) plane in water with 0.51 off-cut angle. Image size:200� 200 nm2; scanning rate: 1 Hz. Each step is about 2 A in height,which corresponds to the distance between adjacent oxygen layers.

Fig. 10. (a) Atomic scale image of single crystal a-Al2O3 (0001) surface in water showing hexagonal arrangement of bright (high) and dark (low) pointswith a periodicity of 4.7 A. The image is an unfiltered deflection image. (b) Overlay of lines highlights the hexagonal structure of the surface. (c) Thecorrespondence between the dark regions and the position, where the alumina surface hydroxyls lie down above the octahedral vacancies in the sublayer,is highlighted by overlay of the top layer of a fully hydrated sapphire basal plane.


order to obtain realistic molecular-scale insights from surfacecomplexation models. The MUSIC model constrained many ofthe adjustable parameters used in earlier models.77 The modelparameters are pKa values, capacitance values and electrolytebinding constants as well as site densities. The pKa values areestimated based on the electrostatic approach, site densitiescome from the crystal structure and the rest is fitted to titrationdata. Additional improvements in the 1990s60 incorporatedequilibrium constants that are calculated based on Pauling’s va-lence rules and an empirical relationship (calibrated on somehydroxy-acid solution species). Using the improved MUSICmodel, Hiemstra et al.86 predicted the pKa values for the dep-rotonation reactions of singly and doubly coordinated gibbsitesurface hydroxyl groups.

For singly coordinated:

Al2OHþ1=2 �!pKa¼9:9Al2OH�1=2 þHþ

And for doubly coordinated:

Al22OHþ2 �!pKa1¼0

Al22OHþHþ �!pKa2¼11:9Al22O� þ 2Hþ

Careful examination of Figs. 8, 1 and 2 indicates that thebasal planes of sapphire and gibbsite are composed of doublycoordinated surface hydroxyl groups. Singly coordinated sur-face hydroxyl groups occur in greater proportion on prismaticand other high index planes. Singly coordinated surface hyd-roxyl groups also occur along the edge of steps on the basalplane. Gibbsite particles are typically hexagonal prismatic inshape. The overall charge of the particle results from the com-bination of contributions from the basal plane and the edgefaces.

Hiemstra et al.’s86 predictions of pKa values of aluminumsurface hydroxyl groups were successful in predicting the IEP ofsome gibbsite particles. Hiemstra’s results predict IEP around9.9 for gibbsite and alumina particles which is not a bad esti-mate. Moreover, for a number of gibbsite samples studied, ex-perimental surface charge measured from potentiometrictitrations correlate fairly well with samples’ edge surface area.But according to this model, all the surface charge at pH in theusual range of interest (pH 2–12) is due to the singly coordinatedsurface hydroxyls. According to Hiemstra’s model, the basalplanes which contain doubly coordinated surface hydroxylgroups are neutral over the usual pH range of interest. Morerecent experimental results presented in Table II indicate thatthe basal planes of sapphire and gibbsite are in fact at leastweakly charged.99

In addition, some researchers61,87,88 have found that the orig-inal MUSIC model can not explain some potentiometric titra-tion results on gibbsite. They have been particularly concernedwith the effect of titration time and the role of doubly coordi-nated sites in the overall charge and proton balance. Recently,Bickmore et al.61 emphasized the need to consider the limita-tions of theMUSICmodel. They argued that theMUSICmodeldoes not properly treat long-range electrostatic, short-range andsolvation contributions. Bickmore et al.61 also proposed an im-proved method of pKa prediction that combine bond-valence

theory and ab initio determined molecular structure of (hydr)ox-ide surfaces. The bond valance and metal–oxygen bond lengthsof the surface hydroxyl groups determined by ab initio surfacestructure calculations are used as input parameters for the MU-SIC model. Initially the ab initio calculations were performed forsurfaces terminated in vacuum; the inadequacy of this approachis discussed later.

Bickmore and co workers61,115 chose a surface unit cell withsix unique hydroxyl groups and used ab initio calculations tooptimize the vacuum terminated gibbsite (001) surface. Sixdifferent doubly coordinated surface hydroxyl sites resulteddue to subtle differences in bond length and ionicity whichemerge from the ab initio calculations. Table III gives the pKa

values estimated by Bickmore and coworkers61,115 for (001) bas-al and (100) edge surfaces of gibbsite. The six different doublycoordinated surface hydroxyl groups on the basal plane pro-duced six different pKa values over the range shown in Table III.The choice of six surface hydroxyl groups as the unit cell formodeling is somewhat arbitrary and if a larger unit cell wereused it is likely that more unique pKa values would result, butover a similar range as those presented in Table III. Althoughthe analysis has not been performed for the a-alumina (0001)surface, the similar surface structures of the basal gibbsite andcorundum structures suggest that similar pKa values are likely toresult. The prismatic planes of gibbsite contain both doubly co-ordinated and singly coordinated surface hydroxyl groups. Thedoubly coordinated groups are ‘‘set deeper in the surface’’ andare believed to be sterically hindered from reacting so that theydo not participate significantly in the charging behavior of theprismatic plane surfaces. (Note, that the similarity which wehave been drawing between the basal planes of sapphire andgibbsite cannot be made for the prismatic faces of the two ma-terials because of the different surface structures of these faces.)

Jodin et al.88 used a detailed IR spectroscopic characteriza-tion to determine the bond lengths and valences of the surfacehydroxyl groups on the basal and prismatic planes of gibbsite.This information was used to determine the pKa values of thesurface ionization reactions that dominate the basal and pris-matic planes of gibbsite. A range of pKa values were determinedwhich depended upon the amount of relaxation of the angle ofthe surface hydroxyl group. These researchers found for thedoubly coordinated basal sites:88

Al22OHþ2 �!pKa¼2 to 4Al22OHþHþ

And for singly coordinated edge sites on the prismatic planes:

Al2OHþ1=2 �!pKa¼7:9 to 9:9Al2OH�1=2 þHþ

Although there are some discrepancies between the variousmodels, one general feature of all the multisite surface chargingmodels is universal. The singly coordinated surface hydroxylgroups have higher pKa values than doubly coordinated surfacehydroxyls.8,61,86,88,114 In addition, the results of Bickmore61,115

and Jodin88 and coworkers reveal that there are some doublycoordinated surface hydroxyl groups on the basal surface thathave pKa values around 4–5.2. Experimental results from our

Table III. pKa Values Estimated by Applying the Bond-Valence Method to Gibbsite (001) and (100) Surfaces

Estimate of pKa for each type of surface hydroxyl61,115

Plane Reaction

Site number

1 2 3 4 5 6

Basal (001) Al2–OH21-Al2–OH1H1 �2.3 �1.6 �5.1 �0.4 5.2 10.8

Basal (001) Al2–OH-Al2–O�1H1 3.9 11.4 3.2 8.8 14.4 20

Prismatic (100) Al2–OH21-Al2–OH1H1 �2.2

Prismatic (100) Al–OH11/2-Al–OH�1/21H1 11.6


lab suggest that these sites are most likely dominating the be-havior of gibbsite basal surface producing the weak surfacecharging.99 In light of the significant similarity between the basalplanes of gibbsite and sapphire we believe that these results alsoare consistent with the experimental results on the basal plane ofsapphire (see Table II) which show the surface has IEP at pHaround 4–6.

However, there are still significant challenges for improvingMUSIC and bond valence models. The most recent work ofBickmore and coworkers116 explains that the methods can beimproved by better prediction of bond valance and length usingmolecular dynamics ab initiomodels that include the interactionof the surface with water. As the surface reconstructions in vac-uum are not the same as in water, accurate prediction of the pKa

values of different types of surface hydroxyl groups will rely onaccurate bond properties of the surface in water.

IX. Single Crystal Alumina Surfaces Versus Powder

The difference between the IEP of (0001) plane sapphire (TableII) and submicron a-alumina particles (Table I) can be explainedby the different types of surface hydroxyl groups found on thetwo different surfaces. The surface structure of the basal planesof hydrated sapphire and gibbsite are comprised primarily ofdoubly coordinated surface hydroxyl groups.43,45,50,106–108 Thesedoubly coordinated surface hydroxyl groups have pKa aroundpH 4–6 as discussed in the previous section. On the other hand,colloidal sized particles with radius of curvature on order of afew microns or less cannot express a large fraction of their totalarea in low index planes, such as (0001) that have surface hyd-roxyl groups coordinated to multiple aluminum ions. Instead,the surface of powder is composed of plane edges, steps, vacan-cies and other defects.49,50 This means that the surfaces of col-loidal powders are composed primarily of singly coordinatedsurface hydroxyl groups which have much higher pKa values,typically around pH 9–11. Figure 11 shows part of an a-aluminacrystal that has been cut into a spherical like shape. One can seethat there are likely to be many singly coordinated surface hyd-roxyl groups. Investigators have identified the surface hydroxylgroups found on several types of submicron a-alumina powdersto be similar to those found on gibbsite and/or other aluminumhydroxides.51,52

The concern that the sapphire surfaces may have low IEP dueto contamination by soluble silicate species can be dismissed.Several investigators whose results are presented in Table II,examined the surface of the crystals with XPS 49,91,92,95 and Au-ger electron spectroscopy92,93 to prove that no silica was presenton the surface of the crystals. Also the (0001) sapphire investi-gated by Franks and coworkers49,110,112 has been imaged byhigh-resolution AFM and has been found to have flat terracesabout 15–20 nm wide with steps of height 0.2 nm, equal to asingle layer of oxygen atoms as shown in Fig. 9. High-resolutionimages such as those shown in Fig. 10 demonstrate that the flatsurfaces are crystalline with few defects because the lattice scalestructure can be observed across the majority of the terrace.There is no evidence in these images for any contamination, in-organic (such as silica), or organic. The IEP of such surfaces arearound pH 5.49

Only two of the recent reports presented in Table II indicatethat the (0001) surfaces of sapphire have IEP similar to pow-der.78,97 These results only occur when the surfaces have beentreated with oxygen plasma. Although the authors of those pa-pers 78,97 recognised the influence of plasma treatment, they didnot give an explanation for this interesting fact. In otherwork,104 it was found that single crystal (0001) sapphire surfac-es can be roughened when exposed to a short (30 s) plasmatreatment. The impact of plasma treatment, according to Elamet al.,104 is that the surface contains an increased number of re-active sites such as edges and vacancies. The roughened surfaceproduced by the plasma treatment must contain a significantfraction of singly coordinated surface hydroxyl groups and in-crease the IEP toward that found for powders.

z Potential measurements on a-alumina platelets also help usto understand the role of low index planes such as (0001) in thecharging of submicron particles. Platelet-shaped a-alumina par-ticles have a lower IEP than the typical ‘‘potato’’-shaped parti-cles such as Sumitomo AKP particles. z Potential measurementswere preformed on Sumitomo AKP-50 ‘‘potato’’-shaped parti-cles and a-alumina platelets from ATO Chem (circa 1994) in0.01M NH4Cl using a Zeta Meter 3.0 microelectrophoresis ap-paratus. The results are shown in Fig. 12. The size of the plate-lets was about 5–20 mm across and about 1–3 mm thick as shownin Fig. 13. Quite clearly the z potential of the platelets is dom-inated by the contribution from the basal plane. There was sig-nificant scatter in the z potential measurements as indicated bythe wide red region in Fig. 12. This is because some to thesmaller equiaxed particles had only a small fraction of basalplane relative to total particle surface area and had higher IEPthan the high aspect ratio plate-like particles.

The charge on the alumina surface and the charge of the so-lution species are the primary factors that control adsorption ofthe dissolved processing additives at the alumina–water inter-face. Of course, dissolved species with charge opposite to that ofthe particle’s surface tend to adsorb while species with likecharge to the surface tend not to adsorb. Other factors suchas van der Waals interactions, hydrogen bonding and hydro-phobic interactions influence adsorption, but usually are secondorder effects.117 Because the IEP of single crystal sapphire anda-alumina powders are substantially different, the adsorptionbehavior of solution species will be significantly different on thetwo different types of surfaces. For example actinide cationswhich adsorb only weakly to alumina powders at pH belowabout six will readily adsorb on to the basal plane of sapphire atpH values as low as 4.5.118,119 Also, anionic polymers, which aretypically used to disperse alumina powders, adsorb to aluminaparticles over a wide range of pH while they would not beexpected to adsorb to single crystal sapphire except at pH belowabout 4 or 5.

Fig. 11. Schematic illustration of a submicron particle surface indicat-ing that a high fraction of the surface sites may be singly coordinatedsurface hydroxyls. Note this image is generated by ‘‘cutting’’ a co-rundum crystal and no accounting has been made for surface relaxationor interaction with water.


Experimental measurements and modeling of interactionforces, spectroscopy, and adsorption, performed on a-aluminasingle crystal samples is not likely to provide useful informationfor improving ceramic processing of alumina powders.Although studies on single crystal sapphire will aid in improv-ing fundamental knowledge about adsorption, surface chargingbehavior and interaction forces, these studies are of limited val-ue in predicting specific alumina suspension behavior due to thevastly different surface charging behavior. The authors believethat the development of macroscopic molecularly smooth al-umina surfaces with IEP near pH 9 will be beneficial to linkingsurface force and spectroscopy measurements for example withsuspension behavior. Preliminary results120 indicate that glassslides coated with aluminum nitrate solutions and heatedat moderate temperatures (3001–8001C) have IEP in the range7.5–8.0, but are not nearly as smooth as commercially availablesapphire samples.

X. Processing Additives at the Alumina–Water Interface andImplications for Ceramic Powder Processing

Most processing additives are used to improve suspension sta-bility and reduced viscosity through increasing the interparticlerepulsive forces. Dispersants (aka deflocculants) can be eitherorganic (polymers,2,121–127 surfactants2,121,128–132 or other small

molecules,133–137) or inorganic (silicate, phosphate, etc.).2,3,126

The adsorption of these additives to alumina and the resultinginterparticle interactions are primarily controlled by the surfacecharging behavior. The interaction between particles in suspen-sion may be repulsive or attractive. A combination of the surfaceproperties (including charging behavior), solution chemistry andadsorption of processing additives at the alumina water interfaceinfluences the interactions between individual particles.2,55,62

These interparticle interactions (forces) as described in SectionV control the behavior of suspensions of the particles such assuspension stability, settling rate, final sediment moisture, con-solidation behavior and shear flow properties such as shear yieldstress and viscosity.12,55,56

Particles that have repulsive forces between each other remainas individual particles in the liquid. When the particles are verysmall (less than about 5 mm) they have very little mass and willsettle very slowly under the influence of gravity because Brown-ian motion keeps them suspended. Because the settling rate ofparticles depends upon the particle (or aggregate) size and den-sity,138 attraction, which produces larger, more massive aggre-gates should be avoided in order to keep suspensions stableagainst sedimentation. Dispersed particle suspensions are alsorelatively easy to consolidate to relatively high volume fractionsof solids (low-moisture contents) even at low applied consoli-dation pressures compared with flocculated suspensions.139–141

The rheological behavior of aqueous suspensions depends uponthe volume fraction of solids, the size and shape of the particles,and the interparticle forces.2,142,143 Strong attraction betweenparticles results in high viscosities and yield stresses usually mak-ing processing difficult. Repulsion results in stable suspensionsthat have low viscosities, which typically facilitates processing.

It is important to have a well-dispersed suspension duringcolloidal processing because repulsive interactions produce lowviscosity suspensions and homogeneous, uniform and high-density green bodies. Low viscosity is important in suspensionhandling such as spraying ceramic glaze suspensions3,129,144–146

and in filling molds for complex shape forming such as in slipcasting,3 tape casting,5 and gelcasting.4,147,148 Repulsive interac-tions also allow for high and uniform green density in bodiesshaped by suspension pressure forming such as in slip castingand pressure filtration. These properties of the green body arecrucial to ensure that shrinkage during drying and sintering islow and, more important, uniform. If shrinkage is not uniform,distortion of the body can occur and stresses can develop whichlead to cracking of the component during drying or firing.

A dispersant will typically act to create either an EDL repul-sion as described in Section V by increasing the charge on theparticles’ surface or a steric repulsion that prevents particlesfrom being drawn into contact by van der Waals attraction.EDL repulsion 55,56,62 can be implemented by adjusting solutionconditions so that there is a similar surface charge on all theparticles. Surface charge can be created by adjusting the pH tovalues away from the IEP of the surface. One can readily rec-ognize that the different IEP values of sapphire single crystalsand alumina powders will produce significantly differing EDLbehavior for the two different types of alumina. EDL repulsioncan also be developed by adding a charged species (usually sur-factant or polymer) that adsorbs to the particles. The adsorptionof the charged species to the alumina surface will depend strong-ly on the charge of the surface so dramatically differing resultsare expected for single crystal and powdered alumina. Althoughadjustment of pH to control surface charge of alumina particlesto create EDL repulsion can be used, this method becomes lesseffective as particle size decreases.149

A more robust method, ‘‘steric repulsion’’150 which relies onthe adsorption of a soluble polymer on the particles surface isfrequently used to disperse alumina. The polymer must be ad-sorbed in sufficient quantity to completely cover the surface ofthe particles; typically about 0.5–2 wt% of polymer to weight ofsolid. Water must be a good solvent for the polymer so that thepolymer extends out from the surface and creates a cushion thatprevents particles from coming together (repulsion).55,56,62,150–152

Fig. 12. z Potential as a function of pH for a-alumina platelets and‘‘potato’’-shaped particles. The solid black line is for Sumitomo AKP-50‘‘potato’’-shaped particles. The red region indicates the range of mea-surements for a sample of ATO-chem a-alumina platelets. The plateletsare shown in Fig. 13. The larger platelets had z potentials near the lowerleft boundary of the region while the smaller particles had z potentialsnear the upper right boundary.

Fig. 13. Scanning electron micrograph of the ATO chem a-aluminaplatelets whose z potential is presented in Fig. 12. Note that both largeplatelets and small equiaxed particles can be found in the sample.


The polymers that work best in creating steric repulsion aretypically low to moderate molecular weight (about 2000–100 000MW). Electrosteric stabilization occurs when the polymer cre-ating the steric repulsion is also charged so as to create an EDLrepulsion.

The most commonly used polymeric dispersants for aluminain water are based on acrylic acid groups including sodium orammonium poly acrylates and methacrylates.3,122,123,127 Thesemolecules are negatively charged over much of the pH rangecommonly used for processing so they adsorb well on the al-umina particle’s surfaces which are positively charged at thetypical processing pH but are less likely to adsorb to singlecrystal sapphire surfaces. The resulting overall charge of theparticles surface with the adsorbed polymer becomes highlynegative when the optimum dose is used. The result is a strong,long-ranged electrosteric repulsion. The success of the poly acidsin dispersing alumina is due to the ability of these polymers tocreate both EDL and steric repulsion. When polyelectrolyteswith charge opposite to the surface are used as dispersants caremust be taken to avoid bridging flocculation which can occur ifthe dispersant is underdosed.151–155 Flocculation is usually to beavoided in ceramic processing1,2 although weak attraction maybe useful in preventing mass segregation for particles of greatlydiffering size or density.126,156

When surfactants are added to alumina suspensions, the ori-entation of the adsorbed molecules and the adsorbed amountaffect the type of forces between the particles. Typically, whenthe surfactant and particle surface have opposite charge, thehydrophilic head-group will adsorb to the particles’ surface dueto Coulombic attraction. When the solvent is water and thesurfactant adsorbs in the head-to-surface configuration with thehydrophobic tails forced to stick out into the aqueous solution,hydrophobic attraction is created between the surfactant-coatedparticles if the concentration of surfactant is low such that onlya monolayer or less is formed.128,129 Due to the resulting hy-drophobic attraction, this situation is usually to be avoided. Athigher levels of added surfactant, the molecules will adsorb assurface micelles or a bilayer, shielding the hydrophobic tailsfrom the water. This type of adsorption can create repulsion viatwo mechanisms: first, there is a short-range steric repulsion,and if the surfactant is ionic and sufficient surface charge de-velops an EDL repulsion can also exist.128,129

Several other small molecules can be used to disperse aluminain water. Most notably, citric acid and its sodium and ammo-nium salts134–136 have been used to produce highly negativelycharged alumina suspensions resulting in EDL repulsion. Thecitrate dispersed alumina suspensions are stable and have lowviscosity even at pH 9 the pristine IEP of alumina powders.Other physically adsorbed molecules such as sugars and lowmolecular weight polysaccharides have been used to induce ster-ic stability of alumina suspensions.137 Because the alumina sur-face is composed of surface hydroxyl groups, molecules such assilanes and alcohols can be chemically bound to the surface bycondensation reactions. Silanes with short chains can be reactedto the surface of alumina either in organic solvent with smalladditions of water or directly in aqueous solution.133 If the silanecontains a water soluble group such as an amine, carboxylate orethylene oxide, it can be used to stabilize alumina in water. Al-though both physically and chemically adsorbed small mole-cules can impart short range repulsion and improve stability ofsuspensions, the chemically adsorbed molecules are better thanphysically adsorbed molecules because they are harder to re-move from the surface during processing.133,157–159 Another fac-tor important, particularly in the dispersion of nanoaluminaparticles is that the length of the stabilizing molecule relative tothe particles size needs to be carefully controlled for optimumdispersion.160

XI. Conclusions

Although much research has shed light on the alumina–waterinterface and charging of alumina surfaces, there is still much

that we need to better understand. Solution speciation, surfacecharging behavior and surface interactions, particularly at ex-tremes of pH and at high electrolyte concentration, still need tobe explored. Better understanding of these phenomena may aidin improving productivity in Bayer processing of alumina. Ad-ditional research on the influence of terrace and edge sites on thecharging behavior of alumina, particularly investigations thatrule out organic contamination would be beneficial. In any case,because the charging behavior of the sapphire basal plane andsubmicron a-alumina particle surfaces are different, it should benoted that measurement and modeling on the sapphire (0001)surface such as of polymer adsorption, surface forces, etc., areonly of limited usefulness in predicting the behavior of concen-trated alumina powder suspensions. Suspension z potential mea-surements combined with knowledge of polymer adsorption andan understanding of surface forces provides a strong basis forunderstanding, predicting, and control of macroscopic suspen-sion behavior such as stability, flow, and consolidation. Controlof these behaviors is useful in developing cost effective ceramicpowder processing schemes to produce reliable high perfor-mance ceramics with minimal reject rates.

Note added in proof

Recently published ab initio calculations (E. M. Fernandez,R. I. Eglitis, G. Borstel, L. C. Balbas, ‘‘Ab initio calculations ofH2O and O2 adsorption on Al2O3 substrates,’’ Comput. Mater.Sci., 39, 587–592 (2007)) indicate that the basal plane of sap-phire and amorphous clusters have different surface hydroxylgroups with different dissociation energy, bond length andcharge distribution.

Acknowledgments

Y. G. thanks the University of Newcastle for his University of Newcastle Re-search Fellowship, 2004–2007. G. F. would like to acknowledge the informative ses-sions with Prof. Tom Healy and Dr. Victoria Bitter of Melbourne. Both authorswould like to thank A/Prof. Erica Wanless of Newcastle for assistance with and ac-cess to AFM facilities and Prof. Barry Bickmore of Brigham Young University andDr. Johannes Lutzenkirchen of Karlsruhe for providing comments on an early ver-sion of the manuscript. The photo of George Franks was taken by Grant Hobson.

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George VincentFranks, AssociateProfessor, com-pleted his under-graduate degreeat the Massachu-setts Institute ofTechnology(MIT) in Materi-als Science andEngineering in1985. He worked

for 7 years in the ceramic processing industry as a process de-velopment engineer for Norton Company and Ceramic ProcessSystems Incorporated. His industrial work focused mainly onnear net shape forming of ceramic green bodies and nonoxideceramic firing. He then completed a PhD at the University ofCalifornia at Santa Barbara in Materials in 1997 under theguidance of Prof. Fred. Lange. He then went to Australia as apost doctoral researcher at the University of Melbourne wherehis research concentrated on surface chemistry effects in sus-pension rheology. Between 1999 and 2005, George taught chem-ical engineering at the University of Newcastle, Australia.During this period he developed a novel gelcasting chemistry,investigated relationships between aggregate properties and sus-pension rheology and developed stimulant responsive floccul-ants for mineral tailing dewatering. He is now AssociateProfessor in the Department of Chemical and Biomolecular En-gineering at the University of Melbourne, Australia. He is amember of the Particulate Fluids Processing Centre and theAustralia Mineral Science Research Institute. In addition to alongstanding interest in the alumina water interface, his researchinterests include advanced ceramics powder processing and shapeforming, mineral processing (particularly flocculation), colloidand surface chemistry, ion specific effects and suspension rheol-ogy. He is Associate Editor of the Journal of the American Ce-ramic Society and referee for a number of other learned journals.He has 50 peer reviewed journal publications and three patents.

Prof. Yang Gan is an appointedprofessor in the Department ofApplied Chemistry at Harbin Insti-tute of Technology, China. He re-ceived his PhD in materials sciencein 2001 from the Institute of MetalResearch (China), where his thesisresearch was involved with thestudy of mechanical propertyof nanomaterials. From 2001 to2003, he was a postdoctoral re-search fellow at the University of

Science and Technology Beijing, studying the surface defectsusing AFM and STM. From 2004 to 2007, he was appointeda University Research Fellow at the Department of ChemicalEngineering and Chemistry at the University of Newcastle,Australia. Before returning to China, he spent several monthsat University of Melbourne as a visiting scholar. His currentresearch interests involve the surface structure and chemistry ofinorganic oxides at nano and atomic scale using AFM surfaceforce measurements and high-resolution imaging. He has beenwriting articles on AFM application for a microscopy magazine– Microscopy Today. He is a reviewer for journals Langmuir,Journal of Colloids and Interface Science and AIChE Journal.He is also on the book review panel for Physical SciencesEducational Reviews. He has been an active member of SPMdiscussion forum — SPM Digest since 2002.


charging behavior at the alumina–water interface and implications

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