Electric double-layer repulsion, differences

The stability of many protected colloidal dispersions cannot be explained solely on the basis of electric double layer repulsion and van der Waals attraction other stabilising mechanisms must be investigated. Steric stabilisation is a name which is used (somewhat loosely) to describe several different possible stabilising mechanisms involving adsorbed macromolecules. These include the following ... 

Another difference lies in the role of electric double-layer repulsion, which is often a key factor in stabilizing aqueous foams with ionic surfactants. The adsorption of ionic surfactant at the liquid surface leads to the formation of a charged surface and a diffuse layer of counterions. As the foam lamellae thin because of the drainage of liquid, these counterions begin to repel each other and retard further thinning. Because ionization is not possible in nonpolar solvents, this double-layer mechanism is not operative in nonpolar foams. 

The situations would be totally different when the two surfaces are put in electrolyte solutions. This is because of formation of the electrical double layers due to the existence of ions in the gap between solid surfaces. The electrical double layers interact with each other, which gives rise to a repulsive pressure between the two planar surfaces as... 

The state of stability under these conditions can be qualitatively described as follows. As two oil droplets approach each other, the negative charge gives rise to a repulsive effect (Figure 7.4). The repulsion will take place within the electrical double-layer (EDL) region. It can thus be seen that the magnitude of double-layer distance will decrease if the concentration of ions in the water phase increases. This is because the electrical double layer region decreases. However, in all such cases in which two bodies come closer, there exists two different kinds of forces that must be considered ... 

It is important to note that the concept of osmotic pressure is more general than suggested by the above experiment. In particular, one does not have to invoke the presence of a membrane (or even a concentration difference) to define osmotic pressure. The osmotic pressure, being a property of a solution, always exists and serves to counteract the tendency of the chemical potentials to equalize. It is not important how the differences in the chemical potential come about. The differences may arise due to other factors such as an electric field or gravity. For example, we see in Chapter 11 (Section 11.7a) how osmotic pressure plays a major role in giving rise to repulsion between electrical double layers here, the variation of the concentration in the electrical double layers arises from the electrostatic interaction between a charged surface and the ions in the solution. In Chapter 13 (Section 13.6b.3), we provide another example of the role of differences in osmotic pressures of a polymer solution in giving rise to an effective attractive force between colloidal particles suspended in the solution. 

Figure 5.6 shows an example of a total interaction energy curve for a thin liquid film stabilized by the presence of ionic surfactant. It can be seen that either the attractive van der Waals forces or the repulsive electric double-layer forces can predominate at different film thicknesses. In the example shown, attractive forces dominate at large film thicknesses. As the thickness decreases the attraction increases but eventually the repulsive forces become significant so that a minimum in the curve may occur, this is called the secondary minimum and may be thought of as a thickness in which a meta-stable state exists, that of the common black film. As the... 

The name, DLYO, originates from the first letter in the surname of the four authors (Derjaguin, Landau, Verwey and Overbeek) from two different groups, which originally published these ideas. The theory is based on the competition between two contributions, a repulsive electric double layer and an attractive van der Waals force [4,5]. The interaction in the electric double layer was originally obtained from mean field calculations via the Poisson-Boltzmann equation [Eq. (4)]. However, the interaction can also be determined by MC simulations (Sec. II. B) and by approximate integral equations like HNC (Sec. II. C). This chapter will focus on the first two possibilities. 

Figure 9.6. Schematic of repulsion or attraction forces (which vary with distance from the particle surface) between particles in suspension. Curves 1 and 6 are examples of repulsion and attraction curves, respectively, which vary with the colloid and the kinds and amounts of electrolytes. A summation of curves 1 and 6 for different conditions produces curves 2-5. In curve 2, the energy of repulsion predominates and a stable suspension is formed. Increasing electrolyte produces curves 3, 4, or 5 owing to suppression of the electric double layer. Curve 3 shows there is still an energy barrier to be overcome prior to flocculation. When the colloids surmount this energy barrier and approach closer than point C, flocculation occurs because the forces of attraction predominate. Curve 5 suggests spontaneous flocculation without redispersion unless there is a shift toward curve 2 by reexpanding the double layer through changing kinds and/or amounts of electrolytes (adapted from Kruyt, 1952).

For the processing of ceramics in liquids, it is important to introduce repulsive forces to overcome attractive van der Waals forces. One type of force is the so-called electric double layer (EDL) force. Some books refer to this force as electrostatic force. To avoid confusion, the term EDL force is used throughout this chapter to clearly show that the physics of particles in liquids strongly differs from particles in air, where electrostatic forces apply that follow Coulombs law. This section describes the chemistry in the development of surface charges on particles and the physics equation that governs the forces. 

A film can only break up into droplets after a disturbance the film locally thins to less than t)q)ically 1000 nm (see Fig. 6.40). In this region the interaction force (van der Waals, electrical double layer, for example) between the liquid-solid and liquid-air surface of the film becomes important. Attraction forces can rupture the thin film and a dry patch is nucleated. Such a film is called a non-wetting film. When the interaction between the two film interfaces is repulsive the so-called disjoining pressure (see also p. 162) of the film, i.e. the pressure difference between the film and bulk liquid, is negative. In the other case of negative disjoining pressures, it may also be called conjoining pressure. 

Thus, the photoisomerization of the monolayer between the 46a-state and the protonated nitromerocyanine 46b-state provides a means to control the electrical features of the electrode surface, thereby regulating electron transfer at the electrode interface. The 46a-monolayer results in a neutral electrode surface while the 46b-monolayer gives a positively charged surface, causing the formation of an electrical double-layer at the electrode interface. Photoisomerization of the command interface resulting from the different electrochemical kinetics of the soluble redox probe can also be probed by Faradic impedance spectroscopy [90]. A small electron transfer resistance is found for the system when there is an attractive interaction between the charged redox probe and the command interface. Much larger electron transfer resistances are found upon photoisomerization to the state when repulsive interactions exist. 

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. 

Double-Layer Interaction. When two charged surfaces approach each other their electrical double layers start to overlap and, as a result, the surfaces experience a repulsive force. For flat surfaces the forces can be explained in terms of the osmotic pressure due to the difference in the ionic concentration in the region between the plates and the concentration in the bulk region. Therefore, we can write (I)... 

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