Measurement and Application of Zeta Potential

7/27/2019 Measurement and Application of Zeta Potential

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UD C 537.39 St&ni Clanak

I

MEASUREMENT AND APPLICATION OF ZETA-POTENTIAL

Rudarsko-geoloiko-naftnizbornik

Branko SALOPEK, Dragan KRASI~: nd Suzana FILIPOVI~Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 41000 Zagreb, Croatia

Key-words.Electrokinetic phenomena, Zeta-Potential, Methodsof measurement.

Vol. 4

The electrokinetic or zeta-potential is an important parameterof the electrical double layer and represents a characteristic ofelectrical properties of solid/liquid and liquidlgaseous interfaces.In the paper electrokinetic phenomena in this contact are describedas well as measuring methods. Changes of the values of zeta-poten-tial depending on some properties of solidhiquid interface arebeing considered.

Introduction

str. 147-151

In flotation (today the prevailing method in mine-ral dressing) as well as in solid/liquid separation thesurface properties of the minerals, i. e. the specificsurface and surface charge, are of essential importan-ce.In the mineral-water interface electrical forces arealso acting. The ionized solid particle is surroundedby liquid ions charged oppositely to the surfaceparticle. During the movement of the solid particlethrough the liquid a liquid layer surrounding theparticle will move together with it. Thus, ionizedparticle will carry a thin liquid shell making with itan integral whole. The potential difference betweenthe external surface of that shell and the liquid asa whole is called electrokinetic potential or zeta-potential.The zeta-potential is used in colloid chemistry forobserving the behaviour of dispersive systems inliquids. Besides, the zeta-potential characterizes theelectrical double layer on the solidmiquid interface,a fact very important in flotation and flocculationprocesses ( 0 c e p e k, 1989).In this paper the zeta-potential measuring methods(electrophoresis, electroosmosis, streaming potentialand sedimentation) are described as well as themeasuring principles of the corresponding devices.Zeta-potentials of a few minerals of characteristicmineral groups are shown on specimens as well asthe relation between potential magnitude and suspen-sion stability.

Zagreb, 1992.

Electrical double layer on the solidhiquid interfaceIn the contact with a polar medium (water) themajority of mineral particles show a definite surfacecharge as the consequence of ionization, ionic adsor-ption and ionic dissolution. This surface charge

influences the arrangement of neighbouring ions ofthe polar medium. Ions of the opposite sign will beattracted to the particle surface and the ions of-the

Kljube rue& ElektrokinetiEke pojave, Zeta-potencijal, Metodemjerenja.ElektrokinetiEki ili zeta-potencijal vaZan je parametar dvojnogelektrihog sloja i predstavlja jednoznatnu karakteristiku elektriCnih svojstava na kontaktu faza hrsto-tekuk i tekuk-plinovito.U radu su opisane elektrokinetitke pojave koje pri tom kontaktunastaju, kao i metode mjerenja potencijala. Prikazane su pro-mjene vrijednosti zeta-potencijala u ovisnosti o nekim svojstvimaEvrste i tekuh faze.

equal sign will be repulsed from the surface. By theadded influence of mixing as a consequence of ther-mic movements and the influence of Brown's move-ment, an electrical double layer is formed (S haw,1970).This double layer consists of charged surface anda neutralized surplus of opposite-and equally-chargedions diffusely spreading through polar medium. Thedouble layer is characterized by the ion arrangementand the magnitude of electrical potential in thevicinity of the charged particle surface. The existenceof the electrical double layer is in close connectionwith the electrostatic interaction between the partic-les of suspension and, consequently, with the suspen-sion stability.The electrical double layer consists of two parts,i.e.: an inner part which includes adsorbed ions andan external or diffuse part in which the ions arearranged under the influence of electrical forces andthermal movements.G o u y and Ch a p m a n proposed a model of thediffuse part of the double layer which assumes thefollowing suppositions:a) the surface is plane, extending to infinity and

uniformly charged;b) ions in diffuse part bear a unit charge;c) the polar medium effects the double layer onlythrough its dielectric constant which is unchangedin the whole diffuse part of the layer;d) the polar medium has the characteristics of asymmetrical electrolite.Thus, this model shows the relation between thecharge, potential and electrolite concentration,neglecting the ion size. JThe magnitude of surface charge following thlsmodel (S v a r o v sk y, 1977) is given by the equation:


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Rudarsko-geoloSko-naftni zbornik, Vol. 4, Zagreb, 1992where are:o -' density of the.surface charge,E - dielectric constant,k - Boltzman constant,T - absolute temperature,e - charge of an electron,9, - particle potential,v - ion .valence of opposite charge.

Stern noticed the drawback of neglecting the ionsize and therefore proposed a model which startsfrom the following assumptions:a) ions have a finite size;b) ions cannot approach the surface at a distancesmaller than the magnitude of ionic radius;c) the possibility of a specific ion adsorption exists.The ions which under the effects of electrostaticand/or van der Waals forces adsorb on the particlesurfaces, overcoming the effect of thermal move-ments, are colled specifically adsorbed ions (S haw,1970). The centres of these ions are localized insidethe Stem plane, that is between the particle surfaceand the Stem plane (Fig. 1). The ions with centreson the opposite side of the Stern plane form thediffuse part of the electrical double layer (Gouy-Chapman model).

Stern layer withcounter-ions3

b S t e r n p l an e Distance -Fig. 1 Schematic representation of e lectrical double layerThe size of the charge according to the Stemmodel is given by the equation:

where ug is the charge given by Gouy's model, 9,is replaced with q,, while a, is the charge of theStem layer. So, the definite expression of the Sternmodel is written as follows (S v a r ov sk y, 1977):

where are:n - on concentration,om - charge density of monolayer of the oppositecharged ions,A - frequency factor,Q, - van der Waals energy.

The potential on the Stem plane which dividesthe immovable part of the layer from its diffuse partis called zeta-potential. This potential is a parameterrepresenting electrical properties of an interface.

Zeta-potential and possibilities of its measurementThe explanation of the phenomena connected withthe surface charge of a solid phase in contact witha liquid one was made possible by the electricaldouble layer theory. According to this theory the

electrokinetic or zeta-potential is a measurable pro-perty of an electrized interface.If an electrized interface between two phases isunder the influence of an external electrical fieldacting parallel with the interface surface, there arisesa relative movement of one phase towards the other.The potential of the sliding plane, which can beexperirnentaly measured, is the electrokinetic or zeta-potential (A d am s on , 1967).From the above it is clear that the zeta-potentialis changing according to the changes in the solution,what means that by zeta-potential measurements itis possible to follow the changes which take placein the solution. Therefore, the measurement of thezeta-potential is an important controlling proceedingin many technical processes.For zeta-potential measuring electrophoresis ismostly used because the measurements can be carriedout relatively simply and quickly, as there are avai-lable various measuring devices zetameters, on themarket. A limitation of the method presents theparticle size of the measured sample of approximately10 nm . . .10 pm. Electrophoresis appears when afine (e. g., colloidal) dispersion of some dielectricmatter in the electrolitic solution is exposed to theeffect of an electrical field. Single dispersed particlesare electrically charged according to the dispersionmedium and an electrical double layer is formedaround each of them. If the liquid phase as a wholeis prevented to flow in one direction, only chargedparticles will be kept in motion in the electricalfield, and they will travel, depending on their charge,towards the catode or anode.

Fig. 2 Schematic representation of the device for measuring elec-trophoresis1. cylindric tube containing portions of suspension; 2.electrodes for generating direct voltage: 3. power source;4. microscope for determing particles mobility speed; 5.measuring insturment.Electroosmosis can be taken as a complement toelectrophoresis. Electroosmosis is the measurement

of the flow of a liquid phase through a porousmembrane in a known electrical field, with constanttemperature. The speed of electroosmotic flow is


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Salopek, B. et al.: Zeta-potentialproportional to the strength by which an externalelectrical field acts on the charge of liquid phase.Streaming potential is a phenomenon opposite toelectroosmosis. During the passing of the liquidphase through an electrized membrane due to thepotential difference of the two electrodes, electroo-smosis causes a certain difference of pressures, whatagain causes a raising of the liquid level. Oppositeto this electroosmotic effect, by means of applyingpressures in the same system and by forcing the

solution through the membrane, a certain potentialdifference, called the flow potential, can be provo-ked. It is just proportional to the difference betweenpressures and the flow speed of the electrolite solu-tion through the membrane pores. The flow potentialas well as electroosmosis are phenomena dependingon the colloidal membrane and its electrization.The phenomenon opposite to the electrophoresisis the sedimentation potential. It realizes during themovements of electrized particles in gravitational orsedimentation fields in centrifuges. It is rarely usedin the study of electrokinetic phenomena because itis very difficult to be measured.The importance ofzeta-potential in flotation and flocculation

Fig. 3 Schematic representation of the device for measuring elec-troosmosis1. porous m ass through which solution flow s (membrane);2. measuring electrodes; 3. operating electrodes; 4. mea-suring instrument; 5. parallel capillary; 6. power sour?

Fig. 4 Schemat ic repr&ntation of the device for measuring stra-ming potential1. porous m ass through which solution flows (membrane);2. measuring electrodes; 3. measuring instrument; 4. pro-voking and measuring pressure

Fig. 5 Schematic representation of the dev ice ofr measuring sedi-mentation potential1. container-collector with emptying possibilty for the par-ticles illloving towards the electrode; 2. operating electro-des; 3. measuring electrodes; 4. suspension container; 5.power source; 6. measuring instrument

In the flotation process three phases are present:solid, liquid and gaseous. By their interaction itcomes to oxidation, hydratation and hydrolysis; thatcauses lesser or bigger changes in the properties ofthe.mineral surface. Here, two factors are important(F ue rs te na u, 1982):- interaction of water molecules with mineral'surfa-ce, either in liquid or gaseous surroundings;- electrical double layer in the solid/liquid interface.Oriented water layers in mineral surface have asignificant effect on wetting the solids, as well ason the nature of adsorption in an interface. On theother hand, electrical double layer can control theprocesses of flotation in many ways (Fu e r s t e n a u,1982); the following must be mentioned:- the symbol and size of the surface charge controlsthe physical adsorption of the flotation reagents;- a high surface charge can inhibit the chemicaladsorption of the collectors;- the flocculation and dispersion of mineral suspen-sions is controlled by the electrical double layer;- slime coatings are determined by electrical doublelayer interaction;- the double layer on air bubbles has a significanteffect on naturally floating mineral systems;- flotation kinetics relate directly to the effect ofdouble layers on the kinetics of film thinning.To be able to separate by means of flotation fromsome paragenesis only a certain mineral, it is indi-spensable to know the following about the presentminerals (N e y, 1973):- the sort and compostition (main parts, at least);- solubility and, if possible, the speed of solubilityin clear water, in acids and bases;- zeta-potential depending on pH values.On the basis of mineral properties which are mani-fested during flotation, i. e. which influence theirfloatability minerals may be divided in a few groups(N e y, 1973):Group 1: Naturally hydrophobic minerals;Group 2: Minerals floating with sulfhydrilic collec-tors;Group 3: Minerals soluble in acids or sensitive toacids;Group 4: Oxide and silicate At u re s generally insen-sitive to acids;Group 5: Silicates which form monomineral rocks.


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Salopek, 3. t al.: Zeta-potentialabrupt change of concentration of potential determi-ning H+, i. e. OH- ions in the electrical doublelayer takes place. In this narrow pH region thepotential abruptly increases from 0 to 45 (mV),approx.The curve >>be hows the zeta-potential of redmud with an addition of kationic flocculant Praestol184 K of molecular mass 5 x 16.This flocculantshows a positive zeta-potential which from the valueof 55 (mV) in acid regions gradually drops to 25(mV) in alkali regions. In a sedimentation experi-ment a very pure overflow has been obtained.The curves PC

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Measurement and Application of Zeta Potential