Double layer, electric measurements

The presence of a net charge at the particle surface produces an asymmetric distribution of ions in the surrounding region. This means that the concentration of counterions close to the surface are higher than the ions with the same charge as the particle. Thus, an electrical double layer is measured around such a particle placed in water. 

In addition, there are techniques developed in other fields of colloid science, which are not directly related to classical electrochemistry. In surface force experiments, for instance, the distance dependence of the electric double layer is measured precisely. This will be discussed later. 

More information about the ubiquitous presence in seawater of natural organic surfactants has been obtained by the use of seawater/solid interfaces than from studies of the interface between seawater and air. For practical reasons, it is usually very much simpler to study the adsorption of organic matter from solution onto solid surfaces, where a variety of powerful techniques such as electrocapillarity, electrical double-layer capacitance measurements, electrophoresis and ellipsometry can be used to study the progress of adsorption and the nature of the adsorbed layer. Neihof and Loeb (1972, 1974) and Loeb and Neihof (1975, 1977) have demonstrated by electrophoresis and ellipsometry that a wide variety of solid surfaces become covered by a strongly adsorbed film of polymeric acids upon exposure to seawater. Hunter (1977) found the same type of effect and has shown by electrophoretic studies at different pH and metal ion concentrations that phenolic and carboxylic groups are probably responsible. This adsorbed organic material seems hkely to represent an important part of the natural surfactants in seawater and, as such, will adsorb at the air/sea interface as well. 

The factor (1 in Eq. (2) measures the tangential electric field at the particle siuface. It is this component which generates the electrophoretic or electroacoustic motion. For a fixed frequency, it can be seen from Eq. (4) that (1 +J) depends on the permittivity of the particles and on die function X - Kg/K a, where Ks is the surface conductance of the double layer X measures the enhanced conductivity due to the charge at the particle surface. It is usually small unless the zeta potential is very high, so for most emulsions with large ka, X has a negligible effect. The ratio fp/f is also small for oil-in-water emulsions. Equation (4) can then be reduced to/= 0.5 and hence the dynamic mobility becomes ... 

Electrokinetic phenomena involve the combined effects of motion and an electric field. When an electric field is applied to a colloidal suspension, the particles move with a velocity that is proportional to the applied field strength. The motion is called electrophoresis. It is a valuable source of information on the sign and magnitude of the charge and on the potential associated with the double layer. The measured potential, called the I potential, is an important guide to the stability of lyophobic colloids (25). The most widely used method for measuring the potential is the microelectrophoretic technique, in which the motion of individual particles is followed in a microscope. The technique is used with very dilute suspensions. Modern instrumentation provides for automated, rapid measurements and for the use of concentrated suspensions. 

Ishino, T., Hieda, H., Tanaka, K. and Gemma, N. (1994) Electrical double-layer forces measured with an atomic-force microscope while electrochemically controlling surface-potential of the cantilever. Japanese Journal of Applied Physics Part 2, 33, L1552-L1554. 

The interface between mercury and an aqueous solution is a very favourable case for studyir the properties of the electrical double layer By measuring the surface tension the free energy of the double layer is directly accessible The potential difference between the two phases can be altered within wide limits by applying an external potential difference From the relation between surface tension and potential the charge and the capacity of the double layer can be derived by differentiation (see 3, eq (20), 4, p 131) Moreover, these two magnitudes can also be directly determined by experiment So it is not to be wondered at that our best and most extensive data on the double layer are coming from the mercury water interface. 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

Electrically, the electrical double layer may be viewed as a capacitor with the charges separated by a distance of the order of molecular dimensions. The measured capacitance ranges from about two to several hundred microfarads per square centimeter depending on the stmcture of the double layer, the potential, and the composition of the electrode materials. Figure 4 illustrates the behavior of the capacitance and potential for a mercury electrode where the double layer capacitance is about 16 p.F/cm when cations occupy the OHP and about 38 p.F/cm when anions occupy the IHP. The behavior of other electrode materials is judged to be similar. 

Electrode surfaces in elec trolytes generally possess a surface charge that is balanced by an ion accumulation in the adjacent solution, thus making the system electrically neutral. The first component is a double layer created by a charge difference between the electrode surface and the adjacent molecular layer in the flmd. Electrode surfaces may behave at any given frequency as a network of resistive and capacitive elements from which an elec trical impedance may be measured and analyzed. 

The capacitance. The electrical double layer may be regarded as a resistance and capacitance in parallel see Section 20.1), and measurements of the electrical impedance by the imposition of an alternating potential of known frequency can provide information on the nature of a surface. Electrochemical impedance spectroscopy is now well established as a powerful technique for investigating electrochemical and corrosion systems. 

For an ideally polarizable electrode, q has a unique value for a given set of conditions.1 For a nonpolarizable electrode, q does not have a unique value. It depends on the choice of the set of chemical potentials as independent variables1 and does not coincide with the physical charge residing at the interface. This can be easily understood if one considers that q measures the electric charge that must be supplied to the electrode as its surface area is increased by a unit at a constant potential." Clearly, with a nonpolarizable interface, only part of the charge exchanged between the phases remains localized at the interface to form the electrical double layer. 

The interaction between two double layers was first considered by Voropaeva et a/.145 These concepts were used to measure the friction between two solids in solution. Friction is proportional to the downward thrust of the upper body upon the lower. However, if their contact is mediated by the electrical double layer associated with each interface, an electric repulsion term diminishes the downward thrust and therefore the net friction. The latter will thus depend on the charge in the diffuse layer. Since this effect is minimum at Eam0, friction will be maximum, and the potential at which this occurs marks the minimum charge on the electrode. 

If an electrode is brought into contact with an electrolyte solution or a molten electrolyte, the establishment of the electrochemical double layer will be accompanied by a transfer of electrical charge. In a suitable arrangement this charge can be measured as an external current. If the contact is made in a way which adjusts the electrode potential upon immersion exactly to the value of Epzc, the current will be nil. Various methods briefly described below have been devised to detect exactly this situation. 

Surface forces measurement is a unique tool for surface characterization. It can directly monitor the distance (D) dependence of surface properties, which is difficult to obtain by other techniques. One of the simplest examples is the case of the electric double-layer force. The repulsion observed between charged surfaces describes the counterion distribution in the vicinity of surfaces and is known as the electric double-layer force (repulsion). In a similar manner, we should be able to study various, more complex surface phenomena and obtain new insight into them. Indeed, based on observation by surface forces measurement and Fourier transform infrared (FTIR) spectroscopy, we have found the formation of a novel molecular architecture, an alcohol macrocluster, at the solid-liquid interface. 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

Photoemission phenomena are of great value for a number of areas in electrochemistry. In particnlar, they can be used to study the kinetics and mechanism of electrochemical processes involving free radicals as intermediates. Photoemission measurements can be also used to study electric double-layer structure at electrode surfaces. For instance by measuring the photoemission current in dilute solution and under identical conditions in concentrated solutions (where we know that / = 0), we can find the value of / in the dilute solution by simple calculations using Eq. (29.9). 

The charges present on the insulator surface in contact with the solution give rise to an accumulation of ions of opposite sign in the solution layer next to the surface, and thus formation of an electric double layer. Since straightforward electrochemical measurements are not possible at insulator surfaces, the only way in which this EDL can be characterized quantitatively is by measuring the values of the zeta potential in electrokinetic experiments (see Section 31.2). 

Benderskii VA, Velichko GI. 1982. Temperature jump in electric double-layer study. Part I. Method of measurements. J Electroanal Chem 140 1-22. 

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