Colloidal forces relative importance

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

While in the case of noninteracting dispersions one needed to consider only the effect of the particle concentration, in interacting dispersions one needs to consider the time over which the flow behavior is observed and its magnitude relative to the time scales over which either shear or colloidal forces alter the local structure of the dispersions. What the flow behavior is, which interaction effects dominate the behavior, and how they do depend on the competing influences of the applied shear and interaction effects. In this section, we outline some of the important parameters one can formulate to judge the relative effects of various colloidal interactions and the physical significance of those parameters. 

Because colloidal particles have finite size, their mobility and diffusion coefficients depend not only upon their size but also upon, the distance from the collector surface. This variation with distance stems from friction between the collector surface and the fluid which increases the force required to push the fluid out of the path of the approaching particle. In the usual transport equation containing only convective and diffusive terms, the size of the molecules is small enough for the thickness in to be small compared to the length du, where fim and Bp are explained in Table 1. Other situations arise In which these conditions are not met, or in which London or gravitational forces are important To identify the limiting cases, it is useful to seek some quantity for each mechanism which allows the ordering of its relative importance. 

For practical ceramic colloidal processing it is important to understand the relative importance of the interactions discussed below, how mutual combinations and combinations with other forces which occur during processing evolve. 

The excellent ability of nonionics to solubilize and disperse hydrophobic soils such as fats, mineral oils, etc, in water leads to extensive use of this e of emulsifier. Their often superior detergency with respect to solids surfaces is due to a combination of relatively low critical micellar concentration (CMC), allowing emulsification to take place at low emulsifier concentrations, and an ability to adsorb hydrophobically to interfaces and thus, by steric repulsion forces, to disperse hydrophobic liquid or colloid matter. An important group of nonionic emulsifiers is based on ethoxylated alkyl alcohols. Increasing demands for biodegradability and low aquatic toxicity of degradation products of industrial chemicals is expected to make fatty alcohols ethoxylates and nonionic emulsifiers based on natural raw materials an even more important group of chemicals in the future. 

In the colloidal realm, given the large surface-to-volume ratio and the relatively small range of force that can sway the disposition of a colloidal particle, it is easy to appreciate the importance of controlling surface properties. Research literature abounds with the characteristics of colloid systems and model systems that mimic colloid surfaces. Applications permeate the fields of materials processing, adhesion, coatings, food science, and medicine. 

Flow or deformation involves the relative motion of adjacent elements of the material. As a consequence such processes are sensitive to interatomic or intermolecular forces. In the case of liquids containing dispersed particles, interparticle forces are also important. Because the rheological properties of colloidal suspensions exhibit such a rich variety of phenomena, rheological studies not only provide information on medium-particle and particle-particle interactions but also arc of immense technological importance. 

Extended phases of liquid or solid are more stable than droplets or minute crystals. Yet the highly dispersed phases which play so important a part in nature (foams, emulsions, gels, and so on) may in certain circumstances achieve a relative degree of permanence. The tendency of phase boundaries to reduce their area to a minimum is compensated in some degree by their capacity for taking up foreign substances which partially neutralize the unbalanced forces responsible for the contractive mge. These boundary effects manifest themselves in different ways, and dominate that part of the subject called colloid chemistry. 

Since the colloidal particle is dispersed in a viscous fluid, the relative motion between the particle and the viscous liquid medium plays an important role in the flow behavior of a whole colloidal suspension. For a single spherical particle of radius r in a state of relatively moving in a Newtonian liquid of viscosity r, the frictional force F exerted on the particle can be expressed by Stokes law ... 

We know that Va is inversely proportional to d, whereas Vr decreases exponentially with the distance 2d. Thus, at short and long distances of d, Va becomes larger than Vr, but at intermediate distances the two particles repel each other strongly due to the repulsion of Bom and Mayer. Thus, Vt has two minima, as shown in Fig. 7.7 a deep one at a short interparticle distance and a shallow one at a relatively long interparticle distance. Coagulation takes place at the first deep minimum. The secondary minimum plays an important role for plate- or rod-like particles that have a wide interparticle contact area. However, because the second minimum is relatively shallow, the coagulation induced by it is easily broken by an external force. This effect is closely related to rheological phenomena of colloidal suspensions. 

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