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Measurement and Application of Zeta-Potential I Rudarsko-geoloiko-naftnizbornik Vol. 4 str. 147-151 Zagreb, 1992. UDC 537.39 St&ni Clanak MEASUREMENT AND APPLICATION OF ZETA-POTENTIAL Branko SALOPEK, Dragan KRASI~:and Suzana FILIPOVI~ Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 41000 Zagreb, Croatia Key-words. Electrokinetic phenomena, Zeta-Potential, Methods Kljube rue& ElektrokinetiEke pojave, Zeta-potencijal, Metode of measurement. mjerenja. The electrokinetic or zeta-potential is an important parameter ElektrokinetiEki ili zeta-potencijal vaZan je parametar dvojnog of the electrical double layer and represents a characteristic of elektrihog sloja i predstavlja jednoznatnu karakteristiku elektriC electrical properties of solid/liquid and liquidlgaseous interfaces. nih svojstava na kontaktu faza hrsto-tekuk i tekuk-plinovito. In the paper electrokinetic phenomena in this contact are described U radu su opisane elektrokinetitke pojave koje pri tom kontaktu as well as measuring methods. Changes of the values of zeta-poten- nastaju, kao i metode mjerenja potencijala. Prikazane su pro- tial depending on some properties of solidhiquid interface are mjene vrijednosti zeta-potencijala u ovisnosti o nekim svojstvima being considered. Evrste i tekuh faze. Introduction equal sign will be repulsed from the surface. By the added influence of mixing as a consequence of ther- In flotation (today the prevailing method in mine- mic movements and the influence of Brown's move- ral dressing) as well as in solid/liquid separation the ment, an electrical double layer is formed (S haw, surface properties of the minerals, i. e. the specific 1970). surface and surface charge, are of essential importan- This double layer consists of charged surface and ce. a neutralized surplus of opposite-and equally-charged In the mineral-water interface electrical forces are ions diffusely spreading through polar medium. The also acting. The ionized solid particle is surrounded double layer is characterized by the ion arrangement by liquid ions charged oppositely to the surface and the magnitude of electrical potential in the particle. During the movement of the solid particle vicinity of the charged particle surface. The existence through the liquid a liquid layer surrounding the of the electrical double layer is in close connection particle will move together with it. Thus, ionized with the electrostatic interaction between the partic- particle will carry a thin liquid shell making with it les of suspension and, consequently, with the suspen- an integral whole. The potential difference between sion stability. the external surface of that shell and the liquid as The electrical double layer consists of two parts, a whole is called electrokinetic potential or zeta- i.e.: an inner part which includes adsorbed ions and potential. an external or diffuse part in which the ions are The zeta-potential is used in colloid chemistry for arranged under the influence of electrical forces and observing the behaviour of dispersive systems in thermal movements. liquids. Besides, the zeta-potential characterizes the G o u y and C h a p m a n proposed a model of the electrical double layer on the solidmiquid interface, diffuse part of the double layer which assumes the a fact very important in flotation and flocculation following suppositions: processes (0c e p e k, 1989). In this paper the zeta-potential measuring methods a) the surface is plane, extending to infinity and (electrophoresis, electroosmosis, streaming potential uniformly charged; and sedimentation) are described as well as the b) ions in diffuse part bear a unit charge; measuring principles of the corresponding devices. c) the polar medium effects the double layer only Zeta-potentials of a few minerals of characteristic through its dielectric constant which is unchanged mineral groups are shown on specimens as well as in the whole diffuse part of the layer; the relation between potential magnitude and suspen- d) the polar medium has the characteristics of a sion stability. symmetrical electrolite. Thus, this model shows the relation between the Electrical double layer on the solidhiquid interface charge, potential and electrolite concentration, neglecting the ion size. J In the contact with a polar medium (water) the The magnitude of surface charge following thls majority of mineral particles show a definite surface model (S v a r o v s k y, 1977) is given by the equation: charge as the consequence of ionization, ionic adsor- ption and ionic dissolution. This surface charge influences the arrangement of neighbouring ions of the polar medium. Ions of the opposite sign will be attracted to the particle surface and the ions of-the Rudarsko-geoloSko-naftni zbornik, Vol. 4, Zagreb, 1992 where are: Zeta-potential and possibilities of its measurement o -' density of the.surface charge, The explanation of the phenomena connected with E - dielectric constant, k - Boltzman constant, the surface charge of a solid phase in contact with T - absolute temperature, a liquid one was made possible by the electrical e - charge of an electron, double layer theory. According to this theory the 9, - particle potential, electrokinetic or zeta-potential is a measurable pro- v - ion .valence of opposite charge. perty of an electrized interface. If an electrized interface between two phases is Stern noticed the drawback of neglecting the ion under the influence of an external electrical field size and therefore proposed a model which starts acting parallel with the interface surface, there arises from the following assumptions: a relative movement of one phase towards the other. a) ions have a finite size; The potential of the sliding plane, which can be b) ions cannot approach the surface at a distance experirnentaly measured, is the electrokinetic or zeta- smaller than the magnitude of ionic radius; potential (A d am s on, 1967). c) the possibility of a specific ion adsorption exists. From the above it is clear that the zeta-potential The ions which under the effects of electrostatic is changing according to the changes in the solution, and/or van der Waals forces adsorb on the particle what means that by zeta-potential measurements it surfaces, overcoming the effect of thermal move- is possible to follow the changes which take place ments, are colled specifically adsorbed ions (S haw, in the solution. Therefore, the measurement of the 1970). The centres of these ions are localized inside zeta-potential is an important controlling proceeding the Stem plane, that is between the particle surface in many technical processes. and the Stem plane (Fig. 1). The ions with centres For zeta-potential measuring electrophoresis is on the opposite side of the Stern plane form the mostly used because the measurements can be carried diffuse part of the electrical double layer (Gouy- out relatively simply and quickly, as there are avai- Chapman model). lable various measuring devices zetameters, on the market. A limitation of the method presents the particle size of the measured sample of approximately 10 nm . 10 pm. Electrophoresis appears when a fine (e. g., colloidal) dispersion of some dielectric Stern layer with matter in the electrolitic solution is exposed to the counter-ions effect of an electrical field. Single dispersed particles 3 are electrically charged according to the dispersion medium and an electrical double layer is formed bStern plane around each of them. If the liquid phase as a whole Distance - is prevented to flow in one direction, only charged Fig. 1 Schematic representation of electrical double layer particles will be kept in motion in the electrical field, and they will travel, depending on their charge, towards the catode or anode. The size of the charge according to the Stem model 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 the Stem layer. So, the definite expression of the Stern model is written as follows (S v a r ovsk y, 1977): Fig. 2 Schematic representation of the device for measuring elec- where are: trophoresis n - ion concentration, 1. cylindric tube containing portions of suspension; 2. om charge density of monolayer of the opposite electrodes for generating direct voltage: 3. power source; - 4. microscope for determing particles mobility speed; 5. charged ions, measuring insturment. A - frequency factor, Q, - van der Waals energy. Electroosmosis can be taken as a complement to The potential on the Stem plane which divides electrophoresis. Electroosmosis is the measurement the immovable part of the layer from its diffuse part of the flow of a liquid phase through a porous is called zeta-potential. This potential is a parameter membrane in a known electrical field, with constant representing electrical properties of an interface. temperature. The speed of electroosmotic flow is Salopek, B. et al.: Zeta-potential proportional to the strength by which an external solution through the membrane, a certain potential electrical field acts on the charge of liquid phase. difference, called the flow potential, can be provo- Streaming potential is a phenomenon opposite to ked. It is just proportional to the difference between electroosmosis. During the passing of the liquid pressures and the flow speed of the electrolite solu- phase through an electrized membrane due to the tion through the membrane pores. The flow potential potential difference of the two electrodes, electroo- as well as electroosmosis are phenomena depending smosis causes a certain difference of pressures, what on the colloidal membrane and its electrization. again causes a raising of the liquid level. Opposite The phenomenon opposite to the electrophoresis to this electroosmotic effect, by means of applying is the sedimentation potential. It realizes during the pressures in the same system and by forcing the movements of electrized particles in gravitational or sedimentation fields in centrifuges. It is rarely used in the study of electrokinetic phenomena because it is very difficult to be measured. The importance of zeta-potential in flotation and flocculation In the flotation process three phases are present: solid, liquid and gaseous.
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