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Zeta-Meter Inc. 1
Electrokinetics andColloid Behavior
Zeta potential can help youunderstand and control colloidalsuspensions. Examples includecomplex biological systems such asblood and functional ones like paint.Colloidal suspensions can be asthick as paste (like cement) or asdilute as the turbidity particles ina lake. Water, milk, wine, clay,dyes, inks, paper and pharmaceu-ticals are good examples of usefulcolloidal systems. Water is a com-mon suspending liquid, althoughnon-aqueous liquids are used aswell. In many cases, the perform-ance of a suspension can be im-proved by understanding the effectof colloidal behavior on such prop-erties as viscosity, settling andeffective particle size.
We can often tailor the charac-teristics of a suspension by under-standing how individual colloidsinteract with one another. At timeswe may want to maximize the
behavior of concentrated suspen-sions. On standing, a weak matrixof flocculant particles is formed.This prevents the closely packedsediment that would form andcause “caking” problems if the par-ticles were highly stabilized andremained completely discrete.
Surface forces at the interface ofthe particle and the liquid are veryimportant because of the micro-scopic size of the colloids. One ofthe major surface effects is elec-trokinetic. Each colloid carries a“like” electrical charge which pro-duces a force of mutual electro-static repulsion between adjacentparticles. If the charge is highenough, the colloids will remaindiscrete, disperse and in suspen-sion. Reducing or eliminating thecharge has the opposite effect —the colloids will steadily agglomer-ate and settle out of suspension orform an interconnected matrix.This agglomeration causes thecharacteristics of the suspensionto change.
Particle charge can be controlledby modifying the suspending liq-uid. Modifications include chang-ing the liquid’s pH or changing theionic species in solution. Another,more direct technique is to use sur-face active agents which directlyadsorb to the surface of the colloidand change its characteristics.
Uncharged Particles are free tocollide and aggregate.
repulsive forces between them inorder to keep each particle discreteand prevent them from gatheringinto larger, faster settling agglom-erates. Examples include pharma-ceuticals and pastes. Sometimeswe have the opposite goal and wantto separate the colloid from theliquid. Removing the repulsiveforces allows them to form largeflocs that settle fast and filter eas-ily. Viscosity is another propertythat can be modified by varying thebalance between repulsion and at-traction.
In some cases, an in-betweenstate of weak aggregation can bethe best solution. Paints are a goodexample. Here, the weak aggrega-tion causes viscosity to be a func-tion of shear rate. Stirring willproduce enough shear to reducethe viscosity and promote blend-ing.
Reversible aggregation is alsouseful in controlling the settling
The Interaction of Colloids
Zeta Potential:A Complete Course in 5 Minutes
Charged Particles repel each other.
2
The diffuse layer can be visualized as a chargedatmosphere surrounding the colloid.
The Double LayerThe double layer model is used
to visualize the ionic environmentin the vicinity of a charged colloidand explains how electrical repul-sive forces occur. It is easier tounderstand this model as a se-quence of steps that would takeplace around a single negative col-loid if its neutralizing ions weresuddenly stripped away.
We first look at the effect of thecolloid on the positive ions (oftencalled counter-ions) in solution.Initially, attraction from the nega-tive colloid causes some of the posi-tive ions to form a firmly attachedlayer around the surface of the
colloid; this layer of counter-ions isknown as the Stern layer.
Additional positive ions are stillattracted by the negative colloid,but now they are repelled by theStern layer as well as by otherpositive ions that are also trying toapproach the colloid. This dynamicequilibrium results in the forma-tion of a diffuse layer of counter-ions. They have a high concentra-tion near the surface which gradu-ally decreases with distance, untilit reaches equilibrium with thecounter-ion concentration in thesolution.
In a similar, but opposite, fash-ion there is a lack of negative ionsin the neighborhood of the surface,
because they are repelled by thenegative colloid. Negative ions arecalled co-ions because they havethe same charge as the colloid.Their concentration will graduallyincrease with distance, as the re-pulsive forces of the colloid arescreened out by the positive ions,until equilibrium is again reached.
The diffuse layer can be visual-ized as a charged atmosphere sur-rounding the colloid. The chargedensity at any distance from thesurface is equal to the difference inconcentration of positive and nega-tive ions at that point. Chargedensity is greatest near the colloidand gradually diminishes towardzero as the concentration of posi-tive and negative ions merge to-gether.
The attached counter-ions in theStern layer and the charged at-mosphere in the diffuse layer arewhat we refer to as the doublelayer. The thickness of this layerdepends upon the type and concen-tration of ions in solution.
Stern Layer
Diffuse Layer
Highly NegativeColloid
Ions In EquilibriumWith Solution
Negative Co-IonPositive Counter-Ion
Two Ways to Visualize the DoubleLayerThe left view shows the change incharge density around the colloid. Theright shows the distribution of positiveand negative ions around the chargedcolloid.
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Zeta PotentialThe double layer is formed in
order to neutralize the charged col-loid and, in turn, causes an elec-trokinetic potential between thesurface of the colloid and any pointin the mass of the suspending liq-uid. This voltage difference is onthe order of millivolts and is re-ferred to as the surface potential.
The magnitude of the surfacepotential is related to the surfacecharge and the thickness of thedouble layer. As we leave the sur-face, the potential drops off roughlylinearly in the Stern layer and thenexponentially through the diffuselayer, approaching zero at theimaginary boundary of the doublelayer. The potential curve is usefulbecause it indicates the strength ofthe electrical force between par-ticles and the distance at whichthis force comes into play.
A charged particle will move witha fixed velocity in a voltage field.This phenomenon is called electro-phoresis. The particle’s mobility isrelated to the dielectric constantand viscosity of the suspending liq-uid and to the electrical potentialat the boundary between the mov-ing particle and the liquid. Thisboundary is called the slip planeand is usually defined as the pointwhere the Stern layer and the dif-fuse layer meet. The Stern layer is
The electrokinetic potential between thesurface of the colloid and any point in themass of the suspending liquid is referredto as the surface potential.
Zeta Potential vs. Surface PotentialThe relationship between zetapotential and surface potentialdepends on the level of ions in thesolution.
Variation of Ion Density in theDiffuse LayerThe figures above are two representa-tions of the change in charge densitythrough the diffuse layer. One showsthe variation in positive and negativeion concentration with distance from anegative colloid. The second showsthe net effect — the difference inpositive and negative charge density.
considered to be rigidly attached tothe colloid, while the diffuse layeris not. As a result, the electricalpotential at this junction is relatedto the mobility of the particle and iscalled the zeta potential.
Although zeta potential is anintermediate value, it is sometimesconsidered to be more significantthan surface potential as far aselectrostatic repusion is concerned.
Zeta potential can be quantifiedby tracking the colloidal particlesthrough a microscope as they mi-grate in a voltage field.
Distance From Colloid
Ion
Co
nce
ntr
atio
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Variation In Ion Concentration
Diffuse Layer
Distance From Colloid
Ch
arg
e D
ensi
ty
Charge DensityVariation
Diffuse Layer
Distance From Colloid
Zeta Potential
Surface Potential
Po
ten
tial
Stern Layer
Diffuse Layer
Zeta Potential (Low Concentration)
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The Balance ofRepulsion & Attraction
The DLVO Theory (named afterDerjaguin, Landau, Verwey andOverbeek) is the classical explana-tion of the stability of colloids insuspension. It looks at the balancebetween two opposing forces — elec-trostatic repulsion and van derWaals attraction — to explain whysome colloidal systems agglomer-ate while others do not.
Electrostatic repulsion becomessignificant when two colloids ap-proach each other and their doublelayers begin to interfere. Energy isrequired to overcome this repul-sion. An electrostatic repulsioncurve is used to indicate the energythat must be overcome if the par-ticles are to be forced together. Ithas a maximum value when theyare almost touching and decreasesto zero outside the double layer.The maximum energy is related tothe surface potential and the zetapotential.
The DLVO theory explains the tendency of colloidsto agglomerate or remain discrete.
The height of the barrier indicateshow stable the system is. In orderto agglomerate, two particles on acollision course must have suffi-cient kinetic energy due to theirvelocity and mass, to “jump over”this barrier. If the barrier iscleared, then the net interaction isall attractive, and as a result theparticles agglomerate. This innerregion is after referred to as anenergy trap since the colloids canbe considered to be trapped togetherby van der Waals forces.
In many cases we can alter theenvironment to either increase ordecrease the energy barrier, de-pending upon our goals. Variousmethods can be used to achievethis, such as changing the ionic
Elecrostatic Repulsion is alwaysshown as a positive curve.
Van der Waals attraction is ac-tually the result of forces betweenindividual molecules in each col-loid. The effect is additive; that is,one molecule of the first colloid hasa van der Waals attraction to eachmolecule in the second colloid. Thisis repeated for each molecule in thefirst colloid, and the total force isthe sum of all of these. An attrac-tive energy curve is used to indi-cate the variation in van der Waalsforce with distance between theparticles.
The DLVO theory explains thetendency of colloids to agglomerateor remain discrete by combiningthe van der Waals attraction curvewith the electrostatic repulsioncurve to form the net interactionenergy curve. At each distance,the smaller value is subtracted fromthe larger to get the net energy.The net value is then plotted —above if repulsive and below if at-tractive — and a curve is formed. Ifthere is a repulsive section, thenthe point of maximum repulsiveenergy is called the energy barrier.
Van der Waals Attraction is shown asa negative curve.
Att
ract
ive
En
erg
yR
epu
lsiv
e E
ner
gy
Electrical Repulsion
Distance Between Colloids
Net Interaction Energy
Energy Barrier
Energy Trap
Van der Waals Attraction
InteractionThe net interaction curve is formed bysubtracting the attraction curve fromthe repulsion curve.
Distance Between Colloids
Att
ract
ive
En
erg
yVan der Waals Attraction
Distance Between Colloids
Rep
uls
ive
En
erg
y
Electrical Repulsion
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In many cases we can alter the environment toeither increase or decrease the energy barrier,depending upon our goals.
Zet
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1 10 100 1,000 10,000 100,000Concentration of Electrolyte, mg/L
2:11:11:21:4
3:1
KClCaCl2
K 2SO4
AlCl3
Na4 2 O7P
Effect of Type and Concentration ofElectrolytesSimple inorganic electrolytes can havea significant impact on zeta potential.The effect depends on the relativevalence of the ions and on theirconcentration. Relative valence canalso be thought of as the type ofelectrolyte, with type being the ratiobetween the valences of the cationand the anion.
In this example, the zeta potential of adilute suspension of colloidal silica wasmodified by adding different electro-lytes. Aluminum chloride is a 3:1electrolyte and its trivalent cationseasily push the zeta potential towardzero. Contrast this with the effect ofpotassium sulfate, a 1:2 electrolyte.First the zeta potential becomesincreasingly negative until a plateau isreached at about 50 mg/L. At about500 mg/L, the zeta potential begins todecrease because the ions arecompressing the double layer.
environment, or pH, or addingsurface active materials to directlyaffect the charge of the colloid. Ineach case, zeta potential measure-ments can indicate the impact ofthe alteration on overall stability.
Nothing is ever as simple as itfirst seems. There are other effectsthat must be considered wheneveryou work with particle stability.Steric stabilization is the most sig-nificant one. Usually this involvesthe adsorption of polymers on par-ticle surfaces. You can visualizethe adsorbed layer as a barrieraround each particle, preventingthem from coming close enough forvan der Waals attraction to causeflocculation. Unlike electrostaticstabilization, there are no longrange repulsive forces and the par-ticles are subject to attractive forcesuntil the outer portions of the stericmolecules contact each other.
Mechanical bridging by poly-mers can be an effective floccu-lating technique. Some long chainpolymers are large enough to ad-sorb to the surface of several par-ticles at the same time, bindingthem together in spite of the elec-trostatic forces that would normallymake them repel each other.
In practice, a combination of ef-fects can be used to create stablesystems or to flocculate them. Forinstance, stable dispersions can becreated by a combination of electro-static repulsion and steric hin-drance. Electronegative disper-sions can be flocculated using longchain cationic polymers which si-multaneously neutralize chargeand bridge between adjacent par-ticles.
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CeramicsSlip casting is used in volume
production of ceramic ware. A sus-pension of clay is prepared andpoured into porous molds, whichdraw off the water from the clayparticles by capillary action. A fil-ter cake of clay forms as the wateris drawn off. The structure of theclay layer depends on the degree ofdispersion of the clay suspension.
Control of Slip CastingClay suspensions for slip casting musthave their viscosity minimized so thatthey pour readily and release trappedair bubbles easily. The above figureshows the effect of pH on the apparentviscosity and zeta potential of thoria(ThO3). Note that a maximum zetapotential corresponds to a low appar-ent viscosity.
Clays & Drilling FluidsClays are an essential part of
paper, adhesives, ointments, rub-ber and synthetic plastics. In eachof these systems, we have to dealwith dispersions of clay in water orother fluids. Clay colloid chemis-try helps us to tailor their charac-teristics to fit the task.
Clays are also used as drillingfluids in water well and petroleumwell production. They are calleddrilling muds and are chemicallyconditioned to vary their proper-ties during drilling. A highlycharged suspension is desirable forthe initial drilling operation. Thiskeeps the clay colloids discrete, al-lowing them to penetrate into theporous wall of the drilled hole andclog the soil pores, forming a thin,impermeable cake which preventsthe loss of drilling fluid. Later, theclay charge may be reduced to forma flocculated suspension in orderto keep it from clogging the lower,pumping zone of the well.
Minerals & OresMany raw mineral ores such as
those for copper, lead, zinc andtungsten are separated by firstgrinding the ore, mixing it with acollector and suspending it in wa-ter. The next step is flotation. Airis bubbled through the mixture andthe collector causes the mineralparticles to adhere to the bubblesso that they can be recovered at thesurface. The efficiency of this proc-ess depends upon the degree ofadsorption between the collectorand the mineral and can be con-trolled by the zeta potential of theparticles. In another interestingapplication, zeta potential studieshave been used to minimize theviscosity of coal slurries.
Determining Point of Zero ChargeThese experiments with alpha-aluminashow good correlation between thepoint of zero charge as determined byzeta potential and the point of maxi-mum subsidence rate. Subsidencerate is a measure of the degree ofcoagulation.
Zeta Potential Applications
AdhesivesAgricultural ChemicalsAsbestosAtomic Energy
BeveragesBiochemistryBiomedicine
1
Zeta Potential
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Apparent Viscosity
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Subsidence Time
Zero Point of Charge
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PharmaceuticalsThe physical properties of a
pharmaceutical suspension affectthe user’s response to the product.A successful suspension will notcake and will, therefore enjoy along shelf life. With fine colloidsthis can be achieved by adding asuspending agent to increase thezeta potential, and produce maxi-mum repulsion between adjacentparticles. This highly dispersedsystem will settle very slowly, butany that do settle will pack tightlyand aggravate caking.
Another, and sometimes moreeffective, approach is to formulatea weakly flocculated suspension.The suspended particles form light,fluffy agglomerates held togetherby van der Waals forces. The floc-culated particles settle rapidlyforming a loosely adhering masswith a large sediment height in-stead of a cake. Gentle agitationwill easily resuspend the particles.Weak flocculation requires a zetapotential of almost zero.
Fluidization of an AntacidSuspensionFluidization is an alternative toflocculation. A negatively chargedcolloidal polyelectrolyte is used as a“fluidizing” agent. The polyelectrolyteadsorbs onto the surfaces of insolubleparticles and deflocculates them oncethe zeta potential exceeds the criticalvalue.
This graph illustrates the fluidization ofan aluminum hydroxide suspensionusing carrageenan sodium as the“fluidizing” agent. The drops in zetapotential and viscosity of the suspen-sion correlate quite well with eachother and are produced by an increasein the concentration of carrageenan.
PaintsThe pigments in paint must be
well dispersed in order for the paintto perform successfully. If the pig-ment agglomerates, then the paintwill seem to have larger pigmentparticles and may fail color qual-ity. Gloss and texture are also af-fected by the degree of dispersionbetween the particles in the paint.Zeta potential measurements canbe used in this application to con-trol the composition of the paintand the dosage of additive requiredfor an optimum dispersion.
CoalDairy ProductsDetergentsDry Powder Technology
DyestuffsEmulsionsFibers
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eta
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Carrageenan, %
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Optimum ZP
Relative Viscosity
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FoodsLatex ProductionPetrochemicals
PetroleumPhotographic EmulsionsPigments
Water and WastewaterCoagulation
Zeta potential is a convenientway to optimize coagulant dosagein water and wastewater treat-ment. The most difficult suspendedsolids to remove are the colloids.Due to their small size, they easilyescape both sedimentation and fil-tration. The key to effective colloidremoval is reduction of their zetapotential with coagulants, such asalum, ferric chloride and/or cati-onic polymers. Once the charge isreduced or eliminated, then no re-pulsive forces exist and gentle agi-tation in a flocculation basin causesnumerous successful colloid colli-sions. Microflocs form and growinto visible floc particles that settlerapidly and filter easily.
PapermakingRetention of fines and fibers can
be increased through zeta poten-tial control. This reduces theamount of sludge produced by thewastewater treatment facility aswell as the load on white waterrecycle systems. Zeta potentialmeasurements also assist the pa-permaker in understanding theeffect of various paper ingredientsas well as the physical characteris-tics of the paper particles them-selves.
Zeta Potential Control of Alum DoseThere is no single zeta potential thatwill guarantee good coagulation forevery treatment plant. It will usually bebetween 0 and -10 mV but the targetvalue is best set by test, using pilotplant or actual operating experience.
Once the target ZP is established, thenthese correlations are no longernecessary, except for infrequentchecks. Just take a sample from therapid mix basin and measure the zetapotential. If the measured value ismore negative than the target ZP, thenincrease the coagulant dose (and vice-versa).
In this example a zeta potential of -3mV corresponds to the lowest filteredwater turbidity and would be used asthe target ZP.
Synthetic Size Retention in Paper-makingThe point of maximum size retentioncorresponds to a zeta potential of +4,which can be considered the optimumZP. More positive or more negativevalues of the zeta potential cause adrop in the percent of size retained.Operating at the optimum value resultsin titanium oxide savings, improvedsheet formation, increased wire life,improved sizing, pitch control, andbiocide reduction.
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Optimum ZP
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Turbidity Ze
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Order a CatalogOur catalog describes our instru-ments and accessories in depth,and will help you select theappropriate configuration.
Zeta-Meter, Inc.765 Middlebrook AvenuePO Box 3008Staunton, VA 24402, USA
Telephone ............... 540-886-3503Toll-Free (USA) ....... 800-333-0229Fax .......................... 540-886-3728
