Electrical double-layer repulsion forces

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. 

The electric double layer repulsion force, FR, for the same system is (53)... 

The effects due to electrical double-layer repulsion forces, electroviscous effects and the physical presence of adsorbed film are neglected. 

Fig. 14.5. Experimental rejection (o) and theoretical prediction of the critical pressure for filtration of BSA in 0.001 M NaCI solution at pH 9 at a membrane of mean pore diameter 84 nm. Rejection is high below the critical pressure as electrical double layer repulsion prevents the protein (effective spherical diameter 6nm) from entering the membrane pores. As the critical pressure is approached, hydrodynamic forces increase and drive the...

Fig. 14.6. Filtration flux as a function of time of filtration for the filtration of O.Oi g/L silica particles in 0.001 M NaCI solution at pH 6 at a membrane of mean pore diameter 84 nm. The particle size was very close to the pore size. The critical transmembrane pressure for these conditions was calculated as 130 kPa. Operation below this pressure gives only a gradual decline in filtration flux with time. Operation above this pressure gives an initially higher filtration flux which declines rapidly with time. In the latter case the intial hydrodynamic force exceeds the electrical double layer repulsion between the membrane and the particles, causing the particles to block the membrane pores.

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. 

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). 

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... 

The release coefficient (a) is shown as a function of flow rate (q) as in Figure 13. The release coefficient (a) was found to be directly proportional to the flow rate (q). This observed proportionality between the release coefficient (a) and the flow rate (q) is unexplained at this point in our study. However, on a speculative basis, the nature of the above relationship is comprehensible, since at higher flow rates the particles attached to the pore wall will be subjected to higher shear force. It should be emphasized that the shear force of moderate flow rates acting alone cannot release particles attached to the pore wall. However, shear force even at slow flow rates can detach particles from the pore walls if they have already been loosened by other means for example, electric double layer repulsion. 

When two nonpolarizable charged surfaces approach, during which their electrical double layers are fuUy relaxed, they do so at constant surface potentials (see Section 16.1.2). It then follows directly from Le Chatelier s principle that the surface charge densities decrease, which implies a reduction of the surface excess of charge determining ions. Thus, the net adsorption decreases upon approach, which requires the force to be repulsive. This is the electrical double layer repulsion. 

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. 

To fit the experimental data it was necessary to set Wgi equal to -0.3 and -0.46 yN for the 3.4 and 8.5 mM SDS solutions respectively (middle and lower curves of Fig. 9). Bell Peterson have developed a comprehensive theory to account for the electrical double layer interaction forces between spheres. Peterson44 has calculated for the asperity radii found in this work, that such values of Wgi can be generated using reasonable estimates for the surface potential at the contact. His calculations also show that the predominate repulsion occurs outside the contact zone. 

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). 

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