Potencial Zeta Malvern

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1 Zetasizer Nano series technical note MRK654-01

IntroductionZeta 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.

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.

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 of

deflocculation. Figure 1 schematicallyrepresents some of these processes.

Colloidal 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 + VS

VS is the potential energy due to thesolvent, it usually only makes amarginal contribution to the total

potential 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 distance

VA = -A/(12 D2)

where A is the Hamaker constant andD is the particle separation. Therepulsive potential VR is a far morecomplex function.

VR = 2 a 2

exp(-D)

where a is the particle radius, is thesolvent permeability, is a function of

the ionic composition and is the zetapotential.

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, the

Zeta Potential

An Introduction in 30 Minutes

Figure 1: Schematic diagramshowing various mechanisms wherestability may be lost in a colloidaldispersion

Figure 2(a): Schematic diagram of thevariation of free energy with particleseparation according to DVLO theory.


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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.

If the zeta potential is reduced (e.g. inhigh salt concentrations), there is apossibility of a secondary minimumbeing 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 that

affect dispersion stability (figure 3): Steric repulsion - this involves

polymers added to the systemadsorbing onto the particlesurface and preventing theparticle surfaces coming intoclose contact. If enough polymer

adsorbs, the thickness of thecoating is sufficient to keepparticles separated by stericrepulsions between the polymer

layers, and at those separationsthe van der Waals forces are tooweak to cause the particles toadhere.

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 be

difficult to subsequently flocculate thesystem if this is required, the polymercan be expensive and in some casesthe polymer is undesirable e.g. when

a 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.

Origins of Surface ChargeMost colloidal dispersions in aqueousmedia carry an electric charge. There

are many origins of this surfacecharge depending upon the nature ofthe particle and its surroundingmedium but we will consider the moreimportant mechanisms.

Ionisation of Surface Groups

Dissociation 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 basic

strengths 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 2(b): Schematic diagram of thevariation of free energy with particleseparation at higher salt concentrationsshowing the possibility of a secondaryminimum.

Figure 3: Steric and electrostaticstabilization mechanisms of

colloidal dispersions

Figure 4(a): Origin of surfacecharge by ionisation of acidicgroups to give a negativelycharged surface

Figure 4(b): Origin of surfacecharge by ionisation of basicgroups to give a positively chargedsurface


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Differential loss of ions fromthe crystal lattice

As an example, consider a crystal of

silver 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.

Adsorption of charged species(ions and ionic surfactants)

Surfactant ions may be specificallyadsorbed on the surface of a particle,leading, in the case of cationic

surfactants, to a positively chargedsurface (figure 6(a)) and, in the caseof anionic surfactants, to a negativelycharged surface (figure 6(b)).

The ElectricalDouble LayerThe development of a nett charge atthe particle surface affects thedistribution of ions in the surrounding

interfacial 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 Potential

The 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 all

the 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 the

particles 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 the

particles have a density different formthe dispersant, they will eventuallysediment forming a close packed bed(i.e. a hard cake).

Factors Affecting Zeta Potential

(1) pH

In 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 be

Figure 6(b): Origin of surfacecharge by specific adsorptionof an anonic surfactant. R =hydrocarbon chain

Figure 5: Origin of surface chargeby differential solution of silverions from a AgI surface

Figure 6(a): Origin of surfacecharge by specific adsorption

of a cationic surfactant. R =hydrocarbon chain

Figure 7: Schematic representation ofzeta potential


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neutralised. Further addition of acidwill cause a build up of positivecharge. Therefore a zeta potentialversus pH curve will be positive at low

pH and lower or negative at high pH.There may be a point where the plotpasses through zero zeta potential.This point is called the isoelectricpoint and is very important from apractical consideration. It is normallythe point where the colloidal system isleast stable.

A typical plot of zeta potential versuspH is shown in figure 8. In thisexample, the isoelectric point of thesample is at approximately pH 5.5. Inaddition, the plot can be used topredict that the sample should be

stable at pH values less than 4(sufficient positive charge is present)and greater than pH 7.5 (sufficientnegative charge is present). Problemswith dispersion stability would beexpected at pH values between 4 and7.5 as the zeta potential values arebetween +30 and -30mV.

2. Conductivity

The thickness of the double layer (-1

)depends upon the concentration ofions in solution and can be calculatedfrom the ionic strength of the medium.The higher the ionic strength, themore compressed the double layerbecomes. The valency of the ions willalso influence double layer thickness.

A trivalent ion such as Al3+

willcompress the double layer to agreater extent in comparison with amonovalent ion such as Na

+.

Inorganic ions can interact withcharged surfaces in one of twodistinct ways (i) non-specific ionadsorption where they have no effecton the isoelectric point. (ii) specific ionadsorption, which will lead to achange in the value of the isoelectricpoint. The specific adsorption of ionsonto a particle surface, even at lowconcentrations, can have a dramaticeffect on the zeta potential of theparticle dispersion. In some cases,specific ion adsorption can lead tocharge reversal of the surface.

3. Concentration of a formulationcomponent

The effect of the concentration of aformulation component on the zetapotential can give information to assistin formulating a product to givemaximum stability. The influence ofknown contaminants on the zetapotential of a sample can be apowerful tool in formulating theproduct to resist flocculation forexample.

Electrokinetic EffectsAn important consequence of theexistence of electrical charges on thesurface of particles is that theyinteract with an applied electric field.These effects are collectively definedas electrokinetic effects. There arefour distinct effects depending on theway in which the motion is induced.These are:

Electrophoresis: the movement of acharged particle relative to the liquid itis suspended in under the influence ofan applied electric field

Electroosmosis: the movement of aliquid relative to a stationary chargedsurface under the influence of anelectric field

Streaming potential: the electric fieldgenerated when a liquid is forced toflow past a stationary charged surface

Sedimentation potential: the electricfield generated when chargedparticles sediment

Electrophoresis

When an electric field is appliedacross an electrolyte, chargedparticles suspended in the electrolyteare attracted towards the electrode ofopposite charge. Viscous forcesacting on the particles tend to opposethis movement. When equilibrium isreached between these two opposingforces, the particles move with

constant velocity.The velocity is dependent on thestrength of electric field or voltagegradient, the dielectric constant of themedium, the viscosity of the mediumand the zeta potential.

The velocity of a particle in a unitelectric field is referred to as itselectrophoretic mobility. Zeta potentialis related to the electrophoreticmobility by the Henry equation:-

UE = 2 z f(a)

3

where UE = electrophoretic mobility, z

= zeta potential, = dielectricconstant, = viscosity and f(a) =Henrys function.

The units of, termed the Debyelength, are reciprocal length and

-1is

often taken as a measure of thethickness of the electrical doublelayer. The parameter a refers to the

radius of the particle and therefore ameasures the ratio of the particleradius to electrical double layer

thickness (figure 9). Electrophoreticdeterminations of zeta potential aremost commonly made in aqueousmedia and moderate electrolyte

concentration. F(a) in this case is1.5, and this is referred to as theSmoluchowski approximation.Therefore calculation of zeta potential

Figure 8: Typical plot of zeta potentialversus pH showing the position of theisoelectric point and the pH valueswhere the dispersion would beexpected to be stable


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from the mobility is straightforward forsystems that fit the Smoluchowskimodel, i.e. particles larger than about0.2 microns dispersed in electrolytes

containing more that 10-3

molar salt.

For small particles in low dielectricconstant media (eg non-aqueous

media), f(a) becomes 1.0 and allowsan equally simple calculation. This isreferred to as the Huckelapproximation.

Measuring ElectrophoreticMobilityThe essence of a classical micro-electrophoresis system is a capillarycell with electrodes at either end towhich a potential is applied. Particlesmove towards the electrode, theirvelocity is measured and expressed inunit field strength as their mobility.

Early methods involved the process ofdirectly observing individual particlesusing ultra-microscope techniquesand manually tracking their progressover a measured distance. Thisprocedure, although still being used

by many groups world wide, suffersfrom several disadvantages, not leastthat of the strenuous effort required tomake a measurement, particularlywith small or poorly scatteringparticles. The technique used inMalverns Zetasizer Nano range ofinstruments is laser Doppler

electrophoresis in combination withM3-PALS.

The M3-PALS TechniqueThe Zetasizer Nano Series uses acombination of laser Dopplervelocimetry and phase analysis lightscattering (PALS) in a patentedtechnique called M3-PALS tomeasure particle electrophoreticmobility. Implementation of M3-PALSenables even samples of very lowmobility to be analysed and theirmobility distributions calculated.

PALS can give an increase inperformance of greater than 100times that associated with standard

measurement techniques. This allowsthe measurement of high conductivitysamples, plus the ability to accuratelymeasure samples that have lowparticle mobilities, such as samplesdispersed in non-aqueous solvents.Low applied voltages can now beused to avoid any risk of sampleeffects due to Joule heating.

Further information discussing thetechniques of laser Dopplerelectrophoresis and M3-PALS can befound in various articles available onthe Malvern Instruments web-site

Optical Configuration of aZeta Potential InstrumentA zeta potential measurement systemcomprises of six main components(figure 10). Firstly, a laser1 is usedto provides a light source to illuminatethe particles within the sample. Forzeta potential measurements, thislight source is split to provide anincident and reference beam. Theincident laser beam passes throughthe centre of the sample cell 2, and

the scattered light at an angle ofabout 13

ois detected 3. When an

electric field is applied to the cell, anyparticles moving through themeasurement volume will cause theintensity of light detected to fluctuatewith a frequency proportional to theparticle speed and this information is

passed to a digital signal processor4and then to a computer5. TheZetasizer Nano software produces afrequency spectrum from which the

electrophoretic mobility and hencezeta potential is calculated. Theintensity of the detected, scatteredlight must be within a specific rangefor the detector to successfullymeasure it. This is achieved using anattenuator6, which adjusts theintensity of the light reaching thesample and hence the intensity of thescattering. To correct for anydifferences in the cell wall thicknessand dispersant refraction,compensation optics7 are installedto maintain optimum alignment.

ReferencesDerjaguin, B.V. and Landau, L. (1941)Acta Physiochim. URSS, 14, 633.

Verway, E.J.W. and Overbeek, J. Th.

G. (1948) Theory of the Stability ofLyophobic Colloids, Elsevier,Amsterdam.

Hunter, R.J. (1988) Zeta Potential InColloid Science: Principles AndApplications, Academic Press, UK.

Figure 9: Schematic illustratingHuckel and Smoluchowski'sapproximations used for theconversion of electrophoretic mobilityinto zeta potential

Figure 10: Optical configuration ofthe Zetasizer Nano series for zeta

potential measurements


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Shaw, D.J. (1992) Introduction ToColloid And Surface Chemistry,Butterworth Heinemann, UK.

Everett, D.H. (1994) Basic PrinciplesOf Colloid Science, The Royal Societyof Chemistry, UK.

Ross, S. and Morrison, I.D. (1988)Colloidal Systems and Interfaces,John Wiley and Sons, USA.

Lyklema, J. (2000) Fundamentals ofInterface and Colloid Science: Volume1 (Fundamentals), Academic Press,UK.

Measuring Zeta Potential: LaserDoppler Electrophoresis, TechnicalNote available from

www.malvern.co.uk

Measuring Zeta Potential UsingPhase Analysis Light Scattering(PALS), Technical Note available fromwww.malvern.co.uk

Measuring Zeta Potential: A NewTechnique, Technical Note availablefrom www.malvern.co.uk

Simplifying the Measurement of ZetaPotential Using M3-PALS, TechnicalNote available from

www.malvern.co.uk

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