Dispersion systems electric double layer

Cd + Bi alloy electrodes (1 to 99.5% Bi) have been prepared by Shuganova etal. by remelting alloy surfaces in a vacuum chamber (10-6 torr) evacuated many times and thereafter filled with very pure H2. C dispersion in H20 + KF has been reported to be no more than 5 to 7%. C at Emin has been found to be independent of alloy composition and time. The Emin, independent of the Bi content, is close to that ofpc-Cd. Only at a Bi content 95% has a remarkable shift of toward less negative E (i.e., toward o ) been observed. This has been explained by the existence of very large crystallites (10-4 to 10-3 cm) at the alloy surface. Each component has been assumed to have its own electrical double layer (independent electrode model262,263). The behavior of Cd + Bi alloys has been explained by the eutectic nature of this system and by the surface segregation of Cd.826,827... 

Source J. Lyklema, Fundamentals of Electrical Double Layers in Colloidal Systems. In Colloidal Dispersions (J. Goodwin, Ed.), Royal Society of Chemistry, London, 1982, pp. 47-70. aThe signs in the table indicate the sign of the acquired charges. 

The influence of the electric potential of the surface of the drops was shown by Watanabe and Gotoh (W3) for the case of mercury droplets in aqueous solutions. In the case of oil drops in water the electric double layer is in the water phase, which makes possible a real interaction between the double layers of the two drops that approach each other. In the case of water drops in an oil phase, however, the electric double layers are on the inside of the drops, so that the interaction of these layers when two drops approach each other is much smaller [see Sonntag and Klare (S5)], which means that the potential barrier is much smaller or may even be absent, and the attraction by London-van der Waals forces predominates. This at least is a first explanation of why systems in which water is the dispersed phase show much higher interaction rates than systems in which oil is the dispersed phase. 

Valuable information about the properties of electrical double layers can be obtained from electrocapillary experiments. In an electrocapillary experiment the surface tension of a metal surface versus the electrical potential is measured. The capacitance and the point of zero charge are obtained. Surface charge densities for disperse systems can be determined by potentiometric and conductometric titration. 

A characteristic feature of colloidal dispersions is the large area-to-volume ratio for the particles involved. At the interfaces between the dispersed phase and the dispersion medium characteristic surface properties, such as adsorption and electric double layer effects, are evident and play a very important part in determining the physical properties of the system as a whole. It is the material within a molecular layer or so of the interface which exerts by far the greatest influence on particle-particle and particle-dispersion medium interactions. 

An electrical double layer (edl) existing on the solid-solution interface is essentially connected with the surface properties of the system. The amount of accumulated charge influences the adsorption of ions and molecules. In the latter case it also influences the configuration of the adsorbed species. On the other hand, the adsorption of the ions and molecules varies surface properties of the interface (functional groups) and thus, the distribution of the charge in the interfacial region. The existence of the electric charge at the interface influences the dispersed system stability. 

At the beginning, the electric double layer at the solid-aqueous electrolyte solution interface was characterized by the measurements of the electrokinetic potential and stability of dispersed systems. Later, the investigations were supported by potentiometric titration of the suspension, adsorption and calorimetric measurements [2]. Now, much valuable information on the mechanism of the ion adsorption can be obtained by advanced spectroscopic methods (especially infrared ATR and diffuse spectroscopy) [3], Mosbauer spectroscopy [4] and X-ray spectroscopy [5]. Some data concerning the interface potential were obtained with MOSFET [6], and AFM [7]. An enthalpy of the reaction of the metal oxide-solution systems can be obtained by... 

The structure electrical double layer at the silica-aqueous electrolyte interface was one of the earlier examined of the oxide systems. At the beginning the investigations were performed with application of electrokinetic methods next, with potentiometric titrations. The properties of this system were very important for flotation in mineral processing. Measurements proved that pHpZC and pHiep are equal to 3, but presence of some alkaline or acidic contaminants may change the position of these points on pH scale. Few examples, concerning edl parameters are shown in Table 3. Presented data concern a group of systems of different composition of the liquid phase and solid of a different origin. The latest measurements of this system takes into account the kinetics of the silica dissolution [152], and at zeta measurements, also the porosity of dispersed solid [155]. 

The earlier concepts of microemulsion stability stressed a negative interfacial tension and the ratio of interfacial tensions towards the water and oil part of the system, but these are insuflBcient to explain stability (13). The interfacial free energy, the repulsive energy from the compression of the diffuse electric double layer, and the rise of entropy in the dispersion process give contributions comparable with the free energy, and hence, a positive interfacial free energy is permitted. 

For monodisperse or unimodal dispersion systems (emulsions or suspensions), some literature (28-30) indicates that the relative viscosity is independent of the particle size. These results are applicable as long as the hydrodynamic forces are dominant. In other words, forces due to the presence of an electrical double layer or a steric barrier (due to the adsorption of macromolecules onto the surface of the particles) are negligible. In general the hydrodynamic forces are dominant (hard-sphere interaction) when the solid particles are relatively large (diameter >10 (xm). For particles with diameters less than 1 (xm, the colloidal surface forces and Brownian motion can be dominant, and the viscosity of a unimodal dispersion is no longer a unique function of the solids volume fraction (30). 

Another method of employing surfactants to achieve flocculation is to first treat the particles with an ionic surfactant to disperse them. A readily soluble electrolyte is then added which has the effect of compressing the electrical double layer surrounding each particle, allowing flocculation to occur. Subsequent dilution of this type of system will redisperse it (due to a decrease in electrolyte concentration). 

Colloid Stability of Disperse Systems Containing Electrical Double Layers... 

Flocculation is the result of van der Waals attraction that is universal for all disperse systems. The van der Waals attraction was described previously and, as shown in Figure 10.6, it is inversely proportional to the droplet-droplet distance of separation h it also depends on the effective Hamaker constant A of the emulsion system. One way to overcome the van der Waals attractions is by electrostatic stabiUsation using ionic surfactants this results in the formation of electrical double layers that introduce a repulsive energy that overcomes the attractive energy. Emulsions... 

Many properties of disperse systems are related to the distribution of charges in the vicinity of the interface due to the adsorption of electrolytes. The adsorption of molecules is driven by the van der Waals attraction, while the driving force for the adsorption of electrolytes is the longer-range electrostatic (Coulomb) interaction. Because of this, the adsorption layers in the latter case are less compact than in the case of molecular adsorption (i.e., they are somewhat extended into the bulk of the solution), and the discontinuity surface acquires noticeable, and sometimes even macroscopic thickness. This diffuse nature of the ionized adsorption layer is responsible for such important features of disperse systems as the appearance of electrokinetic phenomena (see Chapter V) and colloid stability (Chapters VII, VIII). Another peculiar feature of the adsorption phenomena in electrolyte solutions is the competitive nature of the adsorption in addition to the solvent there are at least two types of ions (even three or four, if one considers the dissociation of the solvent) present in the system. Competition between these ions predetermines the structure of the discontinuity surface in such systems -i.e. the formation of spatial charge distribution, which is referred to as the electrical double layer (EDL). The structure and theory of the electrical double layer is described in detail in textbooks on electrochemistry. Below we will primarily focus on those features of the EDL, which are important in colloid... 

Structural features of disperse systems, in particular the existence of the electrical double layer (EDL), are responsible for a number of peculiar phenomena related to heat and mass transfer and electric current propagation in such systems. The description of electromagnetic radiation propagation is also included in this chapter. These features are utilized in numerous practical applications and underlie methods used to study disperse systems. These methods include particle size distribution analysis, studies of the surface structure and of near-surface layers, the structure of the EDL, etc. In the most general way the most transfer phenomena can be described by the laws of irreversible thermodynamics, which allow one to carry out a systematic investigation of different fluxes that originate as a result of the action of various generalized forces. 

Let us continue the discussion of transfer processes taking place in the free disperse systems. We will primarily focus on the role that electrokinetic phenomena and electrical double layer play in these processes. 

The nature of stability of disperse systems with solid dispersed phase and liquid continuous phase against coagulation is determined by phase composition, particle size and particle concentration. The stability of hydrosols at low electrolyte concentrations is usually related to the electrostatic component of disjoining pressure (Chapter VII), arising from the overlapping diffuse parts of electrical double layers. 

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