Electrical double layer repulsion attraction

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 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 ... 

If the balance of van der Waals attraction, electric double layer repulsion, capillary pressure, structure propagation, etc., favours an equilibrium film thickness, random fluctuations in film thickness will, in any case, tend to be neutralised. 

Addition of soluble macromolecules (polymers) in the colloidal dispersion can stabilize the colloidal particles due to the adsorption of the polymers to the particle surfaces. The soluble polymers are often called protective agents or colloids. If the protective agents are ionic and have the same charge as the particles, the electrical double-layer repulsive forces will be increased and thus the stability of the colloidal particles will be enhanced. In addition, the adsorbed polymers may help weaken the van der Waals attraction forces among particles. However, the double-layer repulsion and the van der Waals attraction cannot account for the entire stabilization of the particle dispersions. 

Values of e, n and ve and Hamaker constants for two identical types of a material in a vacuum, which are calculated from Equation (567) by taking e3 = 1 and 3 = 1, are given in Table 7.1. Unfortunately, the lack of material constants, such as the dielectric constant, as a function of frequency for most of the substances, and also the complexity of the derived formulae have hampered the general use of the Lifshitz model. However, Lifshitz theory made possible the advent of the first theories on the stability of hydrophobic colloids as a balance between London attraction and electrical double-layer repulsion. Later, these theories were further elaborated by Derjaguin and Landau, and independently by Verwey and Overbeek. The general theory of colloidal stability (which is beyond the scope of this book) is based on Lifshitz theory and has become known as the DLVO theory, by combining the initials of these four authors. 

Two approaching emulsion droplets may be resisted by electrostatic forces. Electrostatic forces consist of Coulombic repulsion between two like charged objects and attractive van der Waals forces. These two forces are accounted for by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory. A third force. Born repulsion, occurs at very small separation distances when electron clouds overlap [1,6,20,21], In emulsion systems an electrical double-layer may form around the disperse phase droplets. While electrical double-layer repulsion is certainly important in o/w emulsions, it does not play a large role in the stabilization of w/o emulsions due to the low dielectric constant of oil [55,56],... 

The first process involved in particulate removal from water is coagulation. In this step, chemical salt coagulants (most commonly aluminum sulfate) are added to the water to destabilize suspended particles, causing them to aggregate or precipitate and form larger particles. The stability of particles in water can be described by Dejaguin-Landau-Verwey-Overbeek (DLVO) theory, where the total force between particles is the sum of the van der Waals attraction (d>vdw) and the electrical double layer repulsion (<1>edl). 

In the absence of external forces (gravitational, centrifugal, electrical), uncharged particles dispersed in a quiescent liquid should be distributed homogeneously. Actually, there is al vays an interaction between particles electrostatic repulsion (for charged particles surrounded by electric double layers), molecular attraction (Van der Waals forces), hydrodynamic forces (forces arising due to the mutual influence of the velocity fields of liquid and particles). 

The first attempt to compile all the factors contributing to the stability of a microemulsion is due to Ruckenstein and Chi [15], who summarized calculations of enthalpic components (van der Waals attractive potential, electrical double layer repulsive potential, and the interfacial stretching and bending free energy) and entropic contributions from the location of droplets. These calculations as well as those following [16] were useful because they revealed the importance of extremely low interfacial tension. 

Pashley and Israelachvili measured the force as the gap was varied. Fig. 10.7(b), and compared the results with the sum of the van der Waals attractive force and the repulsion given by Equation (10.3). They obtained good agreement, but when the gap was reduced to about 5 nm, the surfaces jumped into contact, because the van der Waals force exceeded the electrical double layer repulsion. 

This method of formulation by von Smoluchowski and Fuchs is limited to small concentrations of particles. Then the fixed particle can at most feel the presence of one other particle, and (p is equal to the sum of the van der Waals attraction and the electrical double-layer repulsion poteitial, or, as discussed in previous sections. In this limit it is also legitimate to model the reaction as a second-order reaction (i.e., only two-particle collisions can occur and the higher body collisions are virtually nonexistent). In aerosols, which arc colloidal dispersions in air, there is no significant electrical repulsion betwerai particles. Hence the effect of interparticle forces on the initial coagulation rate is negligible, and we find... 

At high ionic strength, (Figure 16.5c), electrical double layer repulsion is strongly suppressed, so that there is a net attraction between the particle and the substratum at all separation distances. The cells deposit in the primary minimum. 

Electric double layer repulsion (reduces the rates of film thinning and rupture) Dispersion force attraction (increases the rates of film thinning and rupture)... 

Aggregation involves adhesion between colloidal particles, and a detailed consideration of interparticle attraction and bonding has been written by Visser (222) with 295 references. Special attention is given to immersed systems where London-van der Waals force and electric double layer repulsion as well as ionic attraction between surfaces of opposite charge are considered. 

Around the isoelectric point the electrical double-layer repulsion was reduced, finally vanished, and the additional attraction appeared. The observed attraction was unexpectedly strong, being several to ten times stronger than the conventional van der Waals force estimated by the equation,... 

A very useful tool for understanding the stability of colloids is provided by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which was named after the four scientists responsible for its development. The theory allows for both the forces between electrical double layers (repulsive for similarly charged particles) and long-range van der Waals forces that are usually attractive. 

Long-range effects in clay-water systems should be discussed in terms of the detailed structure of the diffuse electrical double layer and of long-range interparticle forces. These forces are electrical double-layer repulsion, van der Waals attraction between the microscopic... 

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). 

The physicochemical forces between colloidal particles are described by the DLVO theory (DLVO refers to Deijaguin and Landau, and Verwey and Overbeek). This theory predicts the potential between spherical particles due to attractive London forces and repulsive forces due to electrical double layers. This potential can be attractive, or both repulsive and attractive. Two minima may be observed The primary minimum characterizes particles that are in close contact and are difficult to disperse, whereas the secondary minimum relates to looser dispersible particles. For more details, see Schowalter (1984). Undoubtedly, real cases may be far more complex Many particles may be present, particles are not always the same size, and particles are rarely spherical. However, the fundamental physics of the problem is similar. The incorporation of all these aspects into a simulation involving tens of thousands of aggregates is daunting and models have resorted to idealized descriptions. 

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