Colloidal forces electrostatic

Since the first report by Ducker et al. on the direct measurement of colloidal forces using AFM [43], a number of investigations have been carried out to measure attractive van der Waals forces, electrostatic forces (double-layer forces) [44], hydrophobic forces [45-50], intermolecular forces between ligands and receptors [51,52], such as the avidin-biotin complex... 

These trajectory methods have been used by numerous researchers to further investigate the influence of hydrodynamic forces, in combination with other colloidal forces, on collision rates and efficiencies. Han and Lawler [3] continued the work of Adler [4] by considering the role of hydrodynamics in hindering collisions between unequal-size spheres in Brownian motion and differential settling (with van der Waals attraction but without electrostatic repulsion). The results indicate the potential significance of these interactions on collision efficiencies that can be expected in experimental systems. For example, collision efficiency for Brownian motion will vary between 0.4 and 1.0, depending on particle absolute size and the size ratio of the two interacting particles. For differential... 

Effects of colloidal forces (i.e. van der Waals, electrostatic, and Brownian forces). 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

We group the forces that control the suspension rheology into two main categories colloidal forces and viscous forces. The colloidal forces include Brownian diffusion forces and the surface forces of electrostatic repulsion and van der Waals attraction. In order to define the dimensionless scaling parameters that characterize the relative magnitude of these forces we assume the particles are separated by a distance of the order of the particle radius a, which is in turn assumed to be close to the smallest particle separation... 

The unimportance of van der Waals forces Electrostatically stabilized dispersions are readily coagulated by the addition of electrolyte. This reduces the spatial extension of the double layers to the point where instability is induced as a consequence of the London attraction between the colloidal particles. The question then arises as to whether the van der Waals forces are also responsible for the flocculation that is observed in sterically stabilized systems. A crude calculation of the magnitude of the attraction between the core particles in typical cases casts considerable light upon this question. 

In the absence of external hydrodynamic forces, the stability of a colloid depends on partides interaction caused by surface forces electrostatic repulsion and molecular attraction [52]. In order for the partides to interact with each other under influence of these forces, they need to be sufficiently close to one another. The partides approach in a liquid occurs under the action of Brownian motion, due to the influence of external forces, for example, gravity, or due to hydrodynamic forces. Studies of stability of the colloid systems should be carried out with due consideration of all the factors listed. Generally, this problem is very difficult, and therefore we consider first the interaction of particles under the action only of electrostatic and molecular forces. The theory of stability of a colloid system subject to such interactions is called DLFO theory as an acronym of its founders - Derjaguin, Landau, Ferwey, and Overbeck [53]. 

A helpful presentation which illustrates the effect of shear rate on dispersions is to use dimensionless quantities as the axes. In this way the colloidal forces can be represented as the ratio of electrostatic repulsive terms to the attractive term, i.e. ereo i R/A and the hydrodynamic terms as the ratio of the shear term to the attractive term, i.e. (mr)R y/A. Following Zeichner and Schowalter [90] this is illustrated in Figure 3.30. Thus as illustrated by the arrow the impact of a shear gradient can move particles out of the secondary minimum association into a region of stability. However, as the shear rate increases still further primary minimum coagulation can occur until at even higher shear rates the particles are redispersed again. 

Figure 12.3 Interaction energies in colloidal dispersions. electrostatic repulsion force depending on particle charge, van der Waals attraction force, and sum of electrostatic and van der Waals forces. The maximum is the activation energy preventing coagulation.

Because the interparticle interactions in those sols are dominated by physical forces such as van der Waals forces, electrostatic forces and Brownian motion, the colloidal sol-gel method is termed a physical gel route [2]. The next two sections are restricted to two chemical gel routes (i) inorganic polymerization and (ii) organic polymerizahon. 

Figure 10 Force-distance curves for colloidal forces van der Waals attraction forces balanced against increasing steric or electrostatic repulsion forces.

To distinguish between colloid stability/instability and physical stability one must consider the state of the suspension on standing as schematically illustrated in Fig. 3.39. These states are determined by (i) magnitude and balance of the various interaction forces, electrostatic repulsion, steric repulsion and van der Waals attraction (ii) particle size and shape distribution (iii) density difference between... 

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