3.3 Electrokinetic phenomena 3.3.1 Introduction see wide application in the selective separation of different components present in a colloidal dispersion, as well as be In 1808, the Russian chemist Ferdinand Fiodorovich Reuss, studied in all its different aspects. a colloid scientist, investigated the behaviour of wet clay. He Based on the consideration that a flux of water is usually observed that the application of a potential difference not produced by a hydrostatic head, Reuss also performed an only caused a flow of electric current, but also a remarkable ingenious experiment, ‘opposite’ of the first of the two movement of water towards the negative pole. The transport previously mentioned experiments. As illustrated in Fig. 1 A, of a liquid through a porous medium soaked with the liquid he measured the electrical potential difference displayed at itself, with a potential difference applied to the boundaries the boundary of a porous bed through which a fluid was was subsequently called electroosmosis. In general, this term flowing. In this way, he discovered that a flux of water indicates the movement of a liquid, with respect to a though a porous membrane or a capillary generated a stationary surface, that takes place inside porous media or potential difference called the streaming potential. within capillaries as an effect of an applied electrical field. A fourth phenomenon, the opposite of electrophoresis, The pressure necessary to counterbalance the osmotic flux is was later discovered by Friedrich Ernst Dorn. If quartz referred to as electroosmotic. particles are permitted to fall in water, as shown in Fig. 1 B, it In a second series of experiments, Reuss dipped two is observed that an electrical potential difference called tubes filled with water in a layer of wet clay and then sedimentation potential is located between two electrodes at introduced two electrodes in the tubes. After having applied different heights. a constant potential difference (and therefore an electrical The phenomena previously described are collectively field), he observed a transport of clay particles towards the called electrokinetic phenomena, classified according to positive pole in addition to electroosmosis. With this Table 1. experiment, transport phenomena of systems dispersed in a A peculiar aspect of this phenomena is represented by fluid triggered by an electrical field were shown for the first the fact that they emphasize coupled phenomena, where a time. The phenomenon, called electrophoresis, would later certain effect can be caused not only by the force directly Fig. 1. A, schematic illustration of an experiment AB in which a streaming potential is generated as an effect of the flux of a liquid through a porous sect; B, schematic illustration of Dorn’s experiment demonstrating the existence of a sedimentation potential. II VOLUME V / INSTRUMENTS 197 SURFACES AND DISPERSE SYSTEMS Table 1. Classification of electrokinetic phenomena Electrical forces Mechanical forces solid at rest solid in motion solid at rest solid in motion electroosmosis electrophoresis streaming potential sedimentation potential applied to it (for instance, the current generated by an solid and a liquid phase. As such, they apply to systems electrical field), but also by forces associated with different where the ratio is high between the interphase area and the effects (for instance, an electric current caused by a pressure volume, such as capillaries and porous materials soaked in difference which typically generates a fluid flow). An liquids or dispersions of solid particles in a liquid. In fact, important confirmation of this aspect emerged 20 years after electrokinetic processes are due to the opposite charges on Reuss experiments when another group of phenomena was the solid particles and in the liquid. On the solid, the charge discovered presenting the same type of coupling: is generated by the presence of ions on the surface due to thermoelectric phenomena. In particular, Thomas Johann either their selective adsorption from the solution or the Seebek observed that by heating the ends of a bimetallic ionization of molecules present on the surface itself. These couple at different temperatures, an electrical potential phenomena do not arise in liquids characterized by small difference was generated, whereas Jean-Charles-Athanase values of the dielectric constant, such as chloroform, Peltier noticed that, inversely, a current transport through the diethylether and carbon disulphide. On the other hand, these couple caused a heat transfer from one junction to the other. phenomena are observed in polar liquids such as acetone, This group of phenomena found a descriptive frame alcohols and especially water. within the context of thermodynamics of irreversible At the boundary, there is a segregation of positive or processes. In order to illustrate this aspect, we will refer to negative electric charges perpendicular to the surface itself. the motion of a fluid (for instance, water) through a porous For example, on a silica surface in contact with an aqueous sect or a membrane, expressing the fluxes of electrical solution, there are some hydroxylic groups, derived from charges and of water molecules through the current intensity SiO2 hydration, that forms silic acid. This compound causes I and the water volumetric flux JV . Their coupling can be the following ionic dissociation: described by the following relationships: ᭤ 2−+ Ϫ᭣ + HSiO23Ϫ SiO 3 2H [1] IL=+∆∆ϕ LP 11 12 producing negative charges on the surface that exert an [2] =+∆∆ϕ JLV 21 LP 22 attractive action on hydrogen ions in the solution, forming an where Df indicates the electrical potential difference and electric double layer (Fig. 2). Another example is silver DP the hydrostatic potential difference, whereas parameters L , L , L , L are the phenomenological parameters. The ϩ 11 22 12 21 H ϩ ϩ Onsager reciprocity relationship is applied to the mixed H H Hϩ ϩ terms L describing coupling by assuming L ϭL . From H ϩ ij 12 21 2Ϫ 2Ϫ H 2Ϫ these expressions, it is possible to notice that even if no SiO3 SiO3 SiO3 potential difference is applied (i.e. if Dfϭ0), then simply the presence of a pressure difference can produce an electric SiO2 current. On the other hand, if no pressure difference is applied, the presence of an electromotive force can still generate a water flux by electroosmosis. Moreover the A following relationships are valid: I J ϩ [3] ==L V = L H ϩ ∆∆P 12 ϕ 21 H ∆ϕ =0 ∆P=0 ϩ ϩ ϩ H H 2Ϫ H ϩ Actually, when other information is unavailable, the SiO3 H Ϫ Ϫ previous equations are also unable to provide the amount of SiO2 SiO2 3 3 ϩ electric or volumetric flux, as thermodynamics alone is not Hϩ H sufficient to calculate the values of the L coefficients. In Ϫ ij 2Ϫ SiO2 order to deal with this problem, it is necessary to extensively SiO3 SiO2 3 investigate the influence that electric charges on the surfaces ϩ ϩ H H have on the behaviour of fluids in contact with them, as 2Ϫ 2Ϫ SiO3 SiO3 illustrated below. 2Ϫ ϩ Hϩ Hϩ SiO3 H Hϩ ϩ Hϩ H 3.3.2 Formation and structure of the electrical double layer B Electrokinetic phenomena arise from the polarization Fig. 2. Formation of an electric double layer on a silica surface: process that takes place at the contact surface between a A, plane surface; B, spherical surface. 198 ENCYCLOPAEDIA OF HYDROCARBONS ELECTROKINETIC PHENOMENA iodide particles suspended in a solution of potassium iodide, infinite distance from the surface, the solution itself must be the molecules of which are adsorbed on the surface. electrically neutral, there is The study of the characteristics of electrical double 0 = [8] ∑CZii 0 layers was conducted by various authors who investigated i this problem at different levels. The first and most and it is possible to derive − χ significant studies, credited to G. Gouy and D.L. Chapman, [9] yy()z = 0e z date back to the beginning of the Twentieth century. These two authors described the surface as an infinite surface on where y0 is the value of the potential at the surface. which a continuous electric charge is distributed in contact Parameter c is expressed by with a solution containing point-like ions having opposite 8πe2 charges. At an infinite distance from the surface, the [10] χ 2 = ∑ CZ02 εkT ii electrical potential identifies with that of the solution, B i whereas when close to the surface, the potential gradually Therefore, the potential decreases exponentially; the 1/c varies until it assumes the values corresponding to the term has the dimensions of a length and represents the width surface itself. In this zone, two regions can be identified. where the surface double layer is basically located. One region includes the ions adsorbed on the surface and the By applying [5] and using the Debye-Hückel other, called the diffuse region, encompasses the ions present approximation, the following expression for the charge in the solution, whose distribution is determined by the density as a function of the coordinate z is obtained: conflict between electrostatic interactions to which they are 2 ε d y ε − χ subjected as well as random thermal movements. In general, [11] r =− =− χ 20y e z π 2 π ion adsorption produces an electrostatic energy barrier that 44dz hinders particle coagulation with the formation of a An important parameter in this analysis is the surface precipitated phase which is more stable from a electric charge density, referring to the unit area s0. Using thermodynamic point of view. In conclusion, at every the previous equation, s0 is expressed by: interphase there is an electrical double layer present, ∞ ∞ ε d 2y originating from the asymmetry of the force field involved. [12] σ =−r()zdz =dz = 0 ∫ ∫ π 2 In order to describe the characteristics of the double 0 0 4 dz layer in quantitative terms, it is appropriate to refer to a ε dy εχy0 =− = plane surface by simulating the layer of adsorbed ions with a 4π dz 4π continuous charge distribution.