Electrical double-layer structure principles

Since this book is dedicated to the dynamic properties of surfactant adsorption layers it would be useful to give a overview of their typical properties. Subsequent chapters will give a more detailed description of the structure of a surfactant adsorption layer and its formation, models and experiments of adsorption kinetics, the composition of the electrical double layer, and the effect of dynamic adsorption layers on different flow processes. We will show that the kinetics of adsorption/desorption is not only determined by the diffusion law, but in selected cases also by other mechanisms, electrostatic repulsion for example. This mechanism has been studied intensively by Dukhin (1980). Moreover, electrostatic retardation can effect hydrodynamic retardation of systems with moving bubbles and droplets carrying adsorption layers (Dukhin 1993). Before starting with the theoretical foundation of the complicated relationships of nonequilibrium adsorption layers, this introduction presents only the basic principles of the chemistry of surfactants and their actions on the properties of adsorption layers. 

This chapter will provide a comprehensive discussion of the fundamentals of double-layer supercapacitors, including the electric double-layer charging and discharging mechanism, the theoretical principles that govern their operation, and their structural designs. 

The electrical double layer arising at the ITIES has been studied by measuring the surface tension [4, 7-16, 25] or the impedance [17-26] mainly of water/nitrobenzene [4, 7-15, 17, 19-24] and water/l,2-dichlorethane [12, 16, 18, 25, 26] systems. This contribution reviews the principles and the results of the impedance measurements, in particular those based on the AC impedance or galvanostatic pulse techniques, which have been used most frequently for the study of the double layer at the ITIES. The quantity, which can be inferred from the impedance measurements, and which is related to the double-layer structure, is the interfacial capacitance. We shall discuss first the thermodynamic background for the capacitance of the electrical double layer at the ITIES. 

Among different liquid-solid interfaces, fhe boundary between an electrolyte and a metal electrode is the one which has been most investigated in surface science. This is dictated by its importance for elecfrochemisfry and by a rich variety of interesting phenomena. In some respect the relevant processes are similar to those at the gas-solid interface. On the other hand, the electrified character of the electrolyte-solid interface resulfs in some peculiarities. One can control interface properties through external manipulation of the interfacial potential difference. All reactions that involve charge transfer respond directly to this quantity. In this section we shall consider the structure of the electric double layer which takes place at an electrolyte-solid interface and the basic principles of control for various reactions at this boundary. 

Chapters 10 and 11 are devoted entirely to aspects of colloid stabihty. First, the essential concepts of the electrical and van der Waals forces between colloid particles are presented with special emphasis on the concepts of the zeta potential, double-layer thickness and Hamaker constants. Then, the DLVO theory for colloidal stability is presented. This is a major tool in colloid chemistry and we discuss how stability is affected by manipulating the parameters of by the classical DLVO theory. Chapter 11 closes with a presentation of kinetics of colloid aggregation and structure of aggregates. Chapters 12 and 13 are about emulsions and foams, respectively - two important categories of colloid systems where DLVO and other principles of colloid and surface science are apphed. In this case, DLVO is often not sufficient. Steric forces and solvation effects are not covered by the classical DLVO and their role in colloid stabihty is also discussed in Chapter 12. 

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