Colloid stability diffuse double-layer repulsion

Table 7.3 Colloid Stability as Calculated from van der Waals Attraction and Electrostatic Diffuse Double-Layer Repulsion >b)...

See Chapter IV in E.J.W. Verwey and J.T.G. Overbeek, Theory of the Stability of Lyophobic Colloids. Elsevier, Amsterdem, 1948. E. P. Honig and P. M. Mul, Tables and equations of the diffuse double layer repulsion at constant potential and at constant charge, /. Colloid Interface Sci. 36 258 (1971). 

Kitchener and coworkers (12, 16) have shown that when colloidal size particles with the same charge sign as that of the planar surface diffuse to and sorb on the latter, the rate of uptake may be considerably less than that predicted by simple Fickian diffusion across the unstirred liquid film, principally because of the double layer repulsive forces, as treated in the Derjagin-Landau-Verwey-Overbeek theory of colloid stability. It was found in some systems, particularly at high ionic strength, that the sorption rate decreased with time, eventually becoming negligible. 

In this case, as indicated above, the colloid stability is controlled by the form of the interaction free-energy curve as a function of particle separation. The DLVO theory in its original form considers just two contributions to this energy, namely the attractive van der Waals potential and the repulsive potential that arises when the diffuse double layers round the two particles overlap. To put this in a quantitative form we need to examine more closely the origin of the curves shown in Figures 3.6 and 3.7. 

Electrostatic stabilization is of importance in solution synthesis as another way to stabilize dispersions.1 1 Colloidal particles almost always have charged surfaces that tend to repel each other. One of the most common charging processes is the adsorption of charged species on the surface of the particle. To maintain electroneutrality, a diffuse cloud of counter ions forms in the fluid around the suspended particle. This phenomenon is described by the diffuse double-layer theory. When the diffuse ion clouds of particles interpenetrate, the particles tend to repel each other electrostatically. The electrostatic repulsive forces are opposed by attractive van der Waals forces that are always present between particles in suspension. The description of the potentials created by these two opposing forces is known as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The DLVO theory predicts... 

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

Two kinds of barriers are important for two-phase emulsions the electric double layer and steric repulsion from adsorbed polymers. An ionic surfactant adsorbed at the interface of an oil droplet in water orients the polar group toward the water. The counterions of the surfactant form a diffuse cloud reaching out into the continuous phase, the electric double layer. When the counterions start ovedapping at the approach of two droplets, a repulsion force is experienced. The repulsion from the electric double layer is famous because it played a decisive role in the theory for colloidal stability that is called DLVO, after its originators Derjaguin, Landau, Vervey, and Overbeek (14,15). The theory provided substantial progress in the understanding of colloidal stability, and its treatment dominated the colloid science literature for several decades. 

A quantitative treatment of the effects of electrolytes on colloid stability has been independently developed by Deryagen and Landau and by Verwey and Over-beek (DLVO), who considered the additive of the interaction forces, mainly electrostatic repulsive and van der Waals attractive forces as the particles approach each other. Repulsive forces between particles arise from the overlapping of the diffuse layer in the electrical double layer of two approaching particles. No simple analytical expression can be given for these repulsive interaction forces. Under certain assumptions, the surface potential is small and remains constant the thickness of the double layer is large and the overlap of the electrical double layer is small. The repulsive energy (VR) between two spherical particles of equal size can be calculated by ... 

The Derjaguin and Landau and Verwey and Overbeek, DLVO, theory, the most widely accepted for colloidal stability (13,14), is based on a model in which the rate of coagulation is determined by the diffusion of particles toward each other in the presence of a potential field. This field is the result of molecular attractive forces of the Van der Waals type and repulsive forces due to the interaction of the electric double layer around the particles. The attraction between particles immersed in a fluid is considered in this theory to result from London dispersion forces. Hamaker (15) has shown that the magnitude of the potential due to these forces increases rapidly as the particles are brought closer together. 

States (a) to (c) in Figure 7.33 correspond to a suspension that is stable in the colloid sense. The stability is obtained as a result of net repulsion due to the presence of extended double layers (i.e. at low electrolyte concentration), the result of steric repulsion produced adsorption of nonionic surfactants or polymers, or the result of a combination of double layer and steric repulsion (electrosteric). State (a) represents a suspension with small particle size (submicron) whereby the Brownian diffusion overcomes the gravity force, producing a uniform distribution of the particles in the suspension, i.e. 

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