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Zeta Potential     An Introduction in 30 Minutes                                                                          ...
attractive force will pull them into            adsorbs, the thickness of the           Origins of Surface Chargecontact w...
Differential loss of ions fromthe crystal latticeAs an example, consider a crystal ofsilver iodide placed in water. Soluti...
3+neutralised. Further addition of acid      A trivalent ion such as Al will               Streaming potential: the electr...
from the mobility is straightforward for   electrophoresis in combination with          passed to a digital signal process...
Shaw, D.J. (1992) Introduction ToColloid And Surface Chemistry,Butterworth Heinemann, UK.Everett, D.H. (1994) Basic Princi...
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Potencial zeta em 30 minutos


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Uma introdução ao conceito de "potencial zeta", em uma nota técnica produzida pela Malvern, que é a fabricante do equipamento Zetasizer Nano Series.

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Potencial zeta em 30 minutos

  1. 1. Zeta Potential An Introduction in 30 Minutes 2Introduction deflocculation. Figure 1 schematically VA = -A/(12 π D ) represents some of these processes.Zeta potential is a physical property where A is the Hamaker constant andwhich is exhibited by any particle in D is the particle separation. Thesuspension. It can be used to repulsive potential VR is a far moreoptimize the formulations of complex function.suspensions and emulsions.Knowledge of the zeta potential can VR = 2 π ε a ζ2 exp(-κD)reduce the time needed to produce where a is the particle radius, π is thetrial formulations. It is also an aid in solvent permeability, κ is a function ofpredicting long-term stability. the ionic composition and ζ is the zeta potential.Colloid ScienceThree of the fundamental states ofmatter are solids, liquids and gases. Ifone of these states is finely dispersed Figure 1: Schematic diagramin another then we have a ‘colloidal showing various mechanisms wheresystem’. These materials have special stability may be lost in a colloidalproperties that are of great practical dispersionimportance. There are various examples of Colloidal Stability andcolloidal systems that include DVLO Theoryaerosols, emulsions, colloidalsuspensions and association colloids. The scientists Derjaguin, Verwey, Landau and Overbeek developed a In certain circumstances, the theory in the 1940s which dealt withparticles in a dispersion may adhere Figure 2(a): Schematic diagram of the the stability of colloidal one another and form aggregates variation of free energy with particle DVLO theory suggests that theof successively increasing size, which separation according to DVLO theory. stability of a particle in solution ismay settle out under the influence of dependent upon its total potentialgravity. An initially formed aggregate energy function VT. This theoryis called a floc and the process of its recognizes that VT is the balance of DVLO theory suggests that theformation flocculation. The floc may or several competing contributions: stability of a colloidal system ismay not sediment or phase separate. determined by the sum of these vanIf the aggregate changes to a much VT = VA + VR + VS der Waals attractive (VA) anddenser form, it is said to undergo electrical double layer repulsive (VR) VS is the potential energy due to thecoagulation. An aggregate usually forces that exist between particles as solvent, it usually only makes aseparates out either by sedimentation they approach each other due to the marginal contribution to the total(if it is more dense than the medium) Brownian motion they are undergoing. potential energy over the last fewor by creaming (if it less dense than This theory proposes that an energy nanometers of separation. Much morethe medium). The terms flocculation barrier resulting from the repulsive important is the balance between VAand coagulation have often been used force prevents two particles and VR, these are the attractive andinterchangeably. Usually coagulation approaching one another and repulsive contributions. Theyis irreversible whereas flocculation adhering together (figure 2 (a)). But if potentially are much larger andcan be reversed by the process of the particles collide with sufficient operate over a much larger distance energy to overcome that barrier, the 1 Zetasizer Nano series technical note MRK654-01
  2. 2. attractive force will pull them into adsorbs, the thickness of the Origins of Surface Chargecontact where they adhere strongly coating is sufficient to keepand irreversibly together. particles separated by steric Most colloidal dispersions in aqueous repulsions between the polymer media carry an electric charge. ThereTherefore if the particles have a layers, and at those separations are many origins of this surfacesufficiently high repulsion, the the van der Waals forces are too charge depending upon the nature ofdispersion will resist flocculation and weak to cause the particles to the particle and it’s surroundingthe colloidal system will be stable. adhere. medium but we will consider the moreHowever if a repulsion mechanism important mechanisms.does not exist then flocculation or • Electrostatic or chargecoagulation will eventually take place. stabilization - this is the effect on Ionisation of Surface Groups particle interaction due to the Dissociation of acidic groups on the distribution of charged species in surface of a particle will give rise to a the system. negatively charged surface. Each mechanism has its benefits for Conversely, a basic surface will take particular systems. Steric stabilization on a positive charge (figure 4). In both is simple, requiring just the addition of cases, the magnitude of the surface a suitable polymer. However it can be charge depends on the acidic or basic difficult to subsequently flocculate the strengths of the surface groups and system if this is required, the polymer on the pH of the solution. The surface can be expensive and in some cases charge can be reduced to zero by the polymer is undesirable e.g. when suppressing the surface ionisation by decreasing the pH in case of negatively charged particles (figureFigure 2(b): Schematic diagram of the 4(a)) or by increasing the pH in thevariation of free energy with particle case of positively charged particlesseparation at higher salt concentrations (figure 4(b)).showing the possibility of a secondaryminimum.If the zeta potential is reduced (e.g. inhigh salt concentrations), there is a Figure 3: Steric and electrostaticpossibility of a “secondary minimum” stabilization mechanisms ofbeing created, where a much weaker colloidal dispersionsand potentially reversible adhesionbetween particles exists (figure 2 (b)).These weak flocs are sufficiently a ceramic slip is cast and sintered, thestable not to be broken up by polymer has to be ‘burnt out’. This Figure 4(a): Origin of surfaceBrownian motion, but may disperse causes shrinkage and can lead to charge by ionisation of acidicunder an externally applied force such defects. groups to give a negativelyas vigorous agitation. charged surface Electrostatic or charge stabilizationTherefore to maintain the stability of has the benefits of stabilizing orthe colloidal system, the repulsive flocculating a system by simplyforces must be dominant. How can altering the concentration of ions incolloidal stability be achieved? There the system. This is a reversibleare two fundamental mechanisms that process and is potentiallyaffect dispersion stability (figure 3): inexpensive.• Steric repulsion - this involves It has long been recognised that the polymers added to the system zeta potential is a very good index of Figure 4(b): Origin of surface adsorbing onto the particle the magnitude of the interaction charge by ionisation of basic surface and preventing the between colloidal particles and groups to give a positively charged particle surfaces coming into measurements of zeta potential are surface close contact. If enough polymer commonly used to assess the stability of colloidal systems. 2 Zetasizer Nano series technical note MRK654-01
  3. 3. 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, Figure 6(b): Origin of surfaceleaving a negatively charged surface charge by specific adsorption(figure 5). If Ag+ ions are now added of an anonic surfactant. R =the charge falls to zero. Further hydrocarbon chainaddition leads to a positively chargedsurface. The Electrical Double Layer The development of a nett charge at the particle surface affects the distribution of ions in the surrounding interfacial region, resulting in an increased concentration of counter Figure 7: Schematic representation of ions, ions of opposite charge to that of zeta potential the particle, close to the surface. Thus an electrical double layer exists round particles coming together andFigure 5: Origin of surface charge each particle. differential solution of silverions from a AgI surface Zeta Potential The general dividing line between stable and unstable suspensions isAdsorption of charged species The liquid layer surrounding the generally taken at either +30 or -30(ions and ionic surfactants) particle exists as two parts; an inner mV. Particles with zeta potentials region (Stern layer) where the ionsSurfactant ions may be specifically more positive than +30 mV or more are strongly bound and an outeradsorbed on the surface of a particle, negative than -30 mV are normally (diffuse) region where they are lessleading, in the case of cationic considered stable. However, if the firmly associated. Within the diffusesurfactants, to a positively charged particles have a density different form layer there is a notional boundarysurface (figure 6(a)) and, in the case the dispersant, they will eventually inside which the ions and particlesof anionic surfactants, to a negatively sediment forming a close packed bed form a stable entity. When a particlecharged surface (figure 6(b)). (i.e. a hard cake). moves (e.g. due to gravity), ions within the boundary move it. Those ions beyond the boundary stay with Factors Affecting Zeta Potential the bulk dispersant. The potential at this boundary (surface of (1) pH hydrodynamic shear) is the zeta In aqueous media, the pH of the potential (figure 7). sample is one of the most important The magnitude of the zeta potential factors that affects its zeta potential. A gives an indication of the potential zeta potential value on its own withoutFigure 6(a): Origin of surface defining the solution conditions is acharge by specific adsorption stability of the colloidal system. If all the particles in suspension have a virtually meaningless number.of a cationic surfactant. R = Imagine a particle in suspension withhydrocarbon chain large negative or positive zeta potential then they will tend to repel a negative zeta potential. If more each other and there will be no alkali is added to this suspension then tendency for the particles to come the particles tend to acquire more together. However, if the particles negative charge. If acid is added to have low zeta potential values then this suspension then a point will be there will be no force to prevent the reached where the charge will be 3 Zetasizer Nano series technical note MRK654-01
  4. 4. 3+neutralised. Further addition of acid A trivalent ion such as Al will Streaming potential: the electric fieldwill cause a build up of positive compress the double layer to a generated when a liquid is forced tocharge. Therefore a zeta potential greater extent in comparison with a flow past a stationary charged surfaceversus pH curve will be positive at low monovalent ion such as Na+.pH and lower or negative at high pH. Sedimentation potential: the electricThere may be a point where the plot Inorganic ions can interact with field generated when chargedpasses through zero zeta potential. charged surfaces in one of two particles sedimentThis point is called the isoelectric distinct ways (i) non-specific ionpoint and is very important from a adsorption where they have no effect Electrophoresispractical consideration. It is normally on the isoelectric point. (ii) specific ion When an electric field is appliedthe point where the colloidal system is adsorption, which will lead to a across an electrolyte, chargedleast stable. change in the value of the isoelectric particles suspended in the electrolyte point. The specific adsorption of ions are attracted towards the electrode ofA typical plot of zeta potential versus onto a particle surface, even at low opposite charge. Viscous forcespH is shown in figure 8. In this concentrations, can have a dramatic acting on the particles tend to opposeexample, the isoelectric point of the effect on the zeta potential of the this movement. When equilibrium issample is at approximately pH 5.5. In particle dispersion. In some cases, reached between these two opposingaddition, the plot can be used to specific ion adsorption can lead to forces, the particles move withpredict that the sample should be charge reversal of the surface. constant velocity.stable at pH values less than 4(sufficient positive charge is present) 3. Concentration of a formulation The velocity is dependent on theand greater than pH 7.5 (sufficient component strength of electric field or voltagenegative charge is present). Problems gradient, the dielectric constant of the The effect of the concentration of awith dispersion stability would be medium, the viscosity of the medium formulation component on the zetaexpected at pH values between 4 and and the zeta potential. potential can give information to assist7.5 as the zeta potential values are in formulating a product to give The velocity of a particle in a unitbetween +30 and -30mV. maximum stability. The influence of electric field is referred to as its known contaminants on the zeta electrophoretic mobility. Zeta potential potential of a sample can be a is related to the electrophoretic powerful tool in formulating the mobility by the Henry equation:- product to resist flocculation for example. UE = 2 ε z f(κa) 3η Electrokinetic Effects where UE = electrophoretic mobility, z An important consequence of the = zeta potential, ε = dielectric existence of electrical charges on the constant, η = viscosity and f(κa) = surface of particles is that they Henry’s function.Figure 8: Typical plot of zeta potential interact with an applied electric field.versus pH showing the position of the These effects are collectively defined The units of κ, termed the Debyeisoelectric point and the pH values as electrokinetic effects. There are length, are reciprocal length and κ-1 iswhere the dispersion would be four distinct effects depending on the often taken as a measure of theexpected to be stable way in which the motion is induced. “thickness” of the electrical double These are: layer. The parameter ‘a’ refers to the Electrophoresis: the movement of a radius of the particle and therefore κa charged particle relative to the liquid it measures the ratio of the particle2. Conductivity radius to electrical double layer is suspended in under the influence ofThe thickness of the double layer (κ-1) an applied electric field thickness (figure 9). Electrophoreticdepends upon the concentration of determinations of zeta potential areions in solution and can be calculated Electroosmosis: the movement of a most commonly made in aqueousfrom the ionic strength of the medium. liquid relative to a stationary charged media and moderate electrolyteThe higher the ionic strength, the surface under the influence of an concentration. F(κa) in this case ismore compressed the double layer electric field 1.5, and this is referred to as thebecomes. The valency of the ions will Smoluchowski approximation.also influence double layer thickness. Therefore calculation of zeta potential 4 Zetasizer Nano series technical note MRK654-01
  5. 5. from the mobility is straightforward for electrophoresis in combination with passed to a digital signal processor 4systems that fit the Smoluchowski M3-PALS. and then to a computer 5. Themodel, i.e. particles larger than about Zetasizer Nano software produces a0.2 microns dispersed in electrolytes The M3-PALS Technique frequency spectrum from which thecontaining more that 10-3 molar salt. electrophoretic mobility and hence The Zetasizer Nano Series uses a zeta potential is calculated. TheFor small particles in low dielectric combination of laser Doppler intensity of the detected, scatteredconstant media (eg non-aqueous velocimetry and phase analysis light light must be within a specific rangemedia), f(κa) becomes 1.0 and allows scattering (PALS) in a patented for the detector to successfullyan equally simple calculation. This is technique called M3-PALS to measure it. This is achieved using anreferred to as the Huckel measure particle electrophoretic attenuator 6, which adjusts theapproximation. mobility. Implementation of M3-PALS intensity of the light reaching the enables even samples of very low sample and hence the intensity of the mobility to be analysed and their scattering. To correct for any mobility distributions calculated. differences in the cell wall thickness PALS can give an increase in and dispersant refraction, performance of greater than 100 compensation optics 7 are installed times that associated with standard to maintain optimum alignment. measurement techniques. This allows the measurement of high conductivity samples, plus the ability to accurately measure samples that have low particle mobilities, such as samplesFigure 9: Schematic illustrating dispersed in non-aqueous solvents.Huckel and Smoluchowskis Low applied voltages can now beapproximations used for the used to avoid any risk of sampleconversion of electrophoretic mobility effects due to Joule heating.into zeta potential Further information discussing the techniques of laser Doppler electrophoresis and M3-PALS can beMeasuring Electrophoretic found in various articles available onMobility the Malvern Instruments web-siteThe essence of a classical micro-electrophoresis system is a capillary Optical Configuration of acell with electrodes at either end to Zeta Potential Instrumentwhich a potential is applied. Particles Figure 10: Optical configuration ofmove towards the electrode, their A zeta potential measurement system the Zetasizer Nano series for zetavelocity is measured and expressed in comprises of six main components potential measurementsunit field strength as their mobility. (figure 10). Firstly, a laser 1 is used to provides a light source to illuminateEarly methods involved the process of the particles within the sample. Fordirectly observing individual particles zeta potential measurements, this Referencesusing ultra-microscope techniques light source is split to provide an Derjaguin, B.V. and Landau, L. (1941)and manually tracking their progress incident and reference beam. The Acta Physiochim. URSS, 14, 633.over a measured distance. This incident laser beam passes throughprocedure, although still being used the centre of the sample cell 2, and Verway, E.J.W. and Overbeek, J. many groups world wide, suffers the scattered light at an angle of G. (1948) Theory of the Stability offrom several disadvantages, not least about 13o is detected 3. When an Lyophobic Colloids, Elsevier,that of the strenuous effort required to electric field is applied to the cell, any Amsterdam.make a measurement, particularly particles moving through thewith small or poorly scattering measurement volume will cause the Hunter, R.J. (1988) Zeta Potential Inparticles. The technique used in intensity of light detected to fluctuate Colloid Science: Principles AndMalvern’s Zetasizer Nano range of with a frequency proportional to the Applications, Academic Press, UK.instruments is laser Doppler particle speed and this information is 5 Zetasizer Nano series technical note MRK654-01
  6. 6. 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 Zeta Potential UsingPhase Analysis Light Scattering(PALS), Technical Note available Zeta Potential: A NewTechnique, Technical Note availablefrom the Measurement of ZetaPotential Using M3-PALS, TechnicalNote available Malvern Instruments Ltd Enigma Business Park • Grovewood Road • Malvern • Worcestershire • UK • WR14 1XZ Tel: +44 (0)1684 892456 • Fax: +44 (0)1684 892789 Malvern Instruments Worldwide Sales and service centers in over 50 countries for details visit more information at 6 Zetasizer Nano series technical note MRK654-01