Zeta Potential An Introduction In 30 Minutes

Zeta PotentialAn Introduction in 30 MinutesIntroductionZeta potential is a physical propertywhich is exhibited by any particle insuspension. It can be used tooptimize the formulations ofsuspensions and emulsions.Knowledge of the zeta potential canreduce the time needed to producetrial formulations. It is also an aid inpredicting long-term stability.deflocculation. Figure 1 schematicallyrepresents some of these processes.There are various examples ofcolloidal systems that includeaerosols, emulsions, colloidalsuspensions and association colloids.In certain circumstances, theparticles in a dispersion may adhereto one another and form aggregatesof successively increasing size, whichmay settle out under the influence ofgravity. An initially formed aggregateis called a floc and the process of itsformation flocculation. The floc may ormay not sediment or phase separate.If the aggregate changes to a muchdenser form, it is said to undergocoagulation. An aggregate usuallyseparates out either by sedimentation(if it is more dense than the medium)or by creaming (if it less dense thanthe medium). The terms flocculationand coagulation have often been usedinterchangeably. Usually coagulationis irreversible whereas flocculationcan be reversed by the process of1where A is the Hamaker constant andD is the particle separation. Therepulsive potential VR is a far morecomplex function.VR 2 π ε a ζ2 exp(-κD)where a is the particle radius, π is thesolvent permeability, κ is a function ofthe ionic composition and ζ is the zetapotential.Colloid ScienceThree of the fundamental states ofmatter are solids, liquids and gases. Ifone of these states is finely dispersedin another then we have a ‘colloidalsystem’. These materials have specialproperties that are of great practicalimportance.2VA -A/(12 π D )Figure 1: Schematic diagramshowing various mechanisms wherestability may be lost in a colloidaldispersionColloidal Stability andDVLO TheoryThe scientists Derjaguin, Verwey,Landau and Overbeek developed atheory in the 1940s which dealt withthe stability of colloidal systems.DVLO theory suggests that thestability of a particle in solution isdependent upon its total potentialenergy function VT. This theoryrecognizes that VT is the balance ofseveral competing contributions:VT VA VR VSVS is the potential energy due to thesolvent, it usually only makes amarginal contribution to the totalpotential energy over the last fewnanometers of separation. Much moreimportant is the balance between VAand VR, these are the attractive andrepulsive contributions. Theypotentially are much larger andoperate over a much larger distanceZetasizer Nano series technical noteFigure 2(a): Schematic diagram of thevariation of free energy with particleseparation according to DVLO theory.DVLO theory suggests that thestability of a colloidal system isdetermined by the sum of these vander Waals attractive (VA) andelectrical double layer repulsive (VR)forces that exist between particles asthey approach each other due to theBrownian motion they are undergoing.This theory proposes that an energybarrier resulting from the repulsiveforce prevents two particlesapproaching one another andadhering together (figure 2 (a)). But ifthe particles collide with sufficientenergy to overcome that barrier, theMRK654-01

adsorbs, the thickness of thecoating is sufficient to keepparticles separated by stericrepulsions between the polymerlayers, and at those separationsthe van der Waals forces are tooweak to cause the particles toadhere.attractive force will pull them intocontact where they adhere stronglyand irreversibly together.Therefore if the particles have asufficiently high repulsion, thedispersion will resist flocculation andthe colloidal system will be stable.However if a repulsion mechanismdoes not exist then flocculation orcoagulation will eventually take place. Electrostatic or chargestabilization - this is the effect onparticle interaction due to thedistribution of charged species inthe system.Each mechanism has its benefits forparticular systems. Steric stabilizationis simple, requiring just the addition ofa suitable polymer. However it can bedifficult to subsequently flocculate thesystem if this is required, the polymercan be expensive and in some casesthe polymer is undesirable e.g. whenFigure 2(b): Schematic diagram of thevariation of free energy with particleseparation at higher salt concentrationsshowing the possibility of a secondaryminimum.If the zeta potential is reduced (e.g. inhigh salt concentrations), there is apossibility of a “secondary minimum”being created, where a much weakerand potentially reversible adhesionbetween particles exists (figure 2 (b)).These weak flocs are sufficientlystable not to be broken up byBrownian motion, but may disperseunder an externally applied force suchas vigorous agitation.Therefore to maintain the stability ofthe colloidal system, the repulsiveforces must be dominant. How cancolloidal stability be achieved? Thereare two fundamental mechanisms thataffect dispersion stability (figure 3): Steric repulsion - this involvespolymers added to the systemadsorbing onto the particlesurface and preventing theparticle surfaces coming intoclose contact. If enough polymer2Origins of Surface ChargeMost colloidal dispersions in aqueousmedia carry an electric charge. Thereare many origins of this surfacecharge depending upon the nature ofthe particle and it’s surroundingmedium but we will consider the moreimportant mechanisms.Ionisation of Surface GroupsDissociation of acidic groups on thesurface of a particle will give rise to anegatively charged surface.Conversely, a basic surface will takeon a positive charge (figure 4). In bothcases, the magnitude of the surfacecharge depends on the acidic or basicstrengths of the surface groups andon the pH of the solution. The surfacecharge can be reduced to zero bysuppressing the surface ionisation bydecreasing the pH in case ofnegatively charged particles (figure4(a)) or by increasing the pH in thecase of positively charged particles(figure 4(b)).Figure 3: Steric and electrostaticstabilization mechanisms ofcolloidal dispersionsa ceramic slip is cast and sintered, thepolymer has to be ‘burnt out’. Thiscauses shrinkage and can lead todefects.Electrostatic or charge stabilizationhas the benefits of stabilizing orflocculating a system by simplyaltering the concentration of ions inthe system. This is a reversibleprocess and is potentiallyinexpensive.It has long been recognised that thezeta potential is a very good index ofthe magnitude of the interactionbetween colloidal particles andmeasurements of zeta potential arecommonly used to assess the stabilityof colloidal systems.Zetasizer Nano series technical noteFigure 4(a): Origin of surfacecharge by ionisation of acidicgroups to give a negativelycharged surfaceFigure 4(b): Origin of surfacecharge by ionisation of basicgroups to give a positively chargedsurfaceMRK654-01

Differential loss of ions fromthe crystal latticeAs an example, consider a crystal ofsilver iodide placed in water. Solutionof ions occurs. If equal amounts ofAg and I- ions were to dissolve, thesurface would be uncharged. In factsilver ions dissolve preferentially,leaving a negatively charged surface(figure 5). If Ag ions are now addedthe charge falls to zero. Furtheraddition leads to a positively chargedsurface.Figure 5: Origin of surface chargeby differential solution of silverions from a AgI surfaceAdsorption of charged species(ions and ionic surfactants)Surfactant ions may be specificallyadsorbed on the surface of a particle,leading, in the case of cationicsurfactants, to a positively chargedsurface (figure 6(a)) and, in the caseof anionic surfactants, to a negativelycharged surface (figure 6(b)).Figure 6(a): Origin of surfacecharge by specific adsorptionof a cationic surfactant. R hydrocarbon chain3Figure 6(b): Origin of surfacecharge by specific adsorptionof an anonic surfactant. R hydrocarbon chainThe ElectricalDouble LayerThe development of a nett charge atthe particle surface affects thedistribution of ions in the surroundinginterfacial region, resulting in anincreased concentration of counterions, ions of opposite charge to that ofthe particle, close to the surface. Thusan electrical double layer exists roundeach particle.Zeta PotentialThe liquid layer surrounding theparticle exists as two parts; an innerregion (Stern layer) where the ionsare strongly bound and an outer(diffuse) region where they are lessfirmly associated. Within the diffuselayer there is a notional boundaryinside which the ions and particlesform a stable entity. When a particlemoves (e.g. due to gravity), ionswithin the boundary move it. Thoseions beyond the boundary stay withthe bulk dispersant. The potential atthis boundary (surface ofhydrodynamic shear) is the zetapotential (figure 7).The magnitude of the zeta potentialgives an indication of the potentialstability of the colloidal system. If allthe particles in suspension have alarge negative or positive zetapotential then they will tend to repeleach other and there will be notendency for the particles to cometogether. However, if the particleshave low zeta potential values thenthere will be no force to prevent theZetasizer Nano series technical noteFigure 7: Schematic representation ofzeta potentialparticles coming together andflocculating.The general dividing line betweenstable and unstable suspensions isgenerally taken at either 30 or -30mV. Particles with zeta potentialsmore positive than 30 mV or morenegative than -30 mV are normallyconsidered stable. However, if theparticles have a density different formthe dispersant, they will eventuallysediment forming a close packed bed(i.e. a hard cake).Factors Affecting Zeta Potential(1) pHIn aqueous media, the pH of thesample is one of the most importantfactors that affects its zeta potential. Azeta potential value on its own withoutdefining the solution conditions is avirtually meaningless number.Imagine a particle in suspension witha negative zeta potential. If morealkali is added to this suspension thenthe particles tend to acquire morenegative charge. If acid is added tothis suspension then a point will bereached where the charge will beMRK654-01