Cohesion Colloid

Cohesive soils have the ability to absorb water and develop plasticity they also have limit liquid. All three characteristics were briefly defined in Chapter 1. Cohesive colloids can form lumps. Forces on such lumps are shown in Figure 6-16. 

An aqueous base is the least expensive vehicle and poses no toxicity problems. A solution of the drug in water or water and cosolvent is made. Glycerin, glycols, natural and synthetic gums, and/or polymers are used to increase viscosity, cohesiveness, and plasticity. To overcome syneresis, or water separation in the gel, a common problem with aqueous bases, one can use absorbing materials such as microcrystalline cellulose, kaolin, colloidal silicon dioxide, starch, etc. 

Figure 2.35 Schematic illustrations of the processes for which AG equals (a) the work of cohesion, (b) the work of adhesion, and (c) the work of spreading. Reprinted, by permission, from P. Heimenz, Principles of Colloid and Surface Chemistry, 2nd ed., p. 316. Copyright 1986 by Marcel Dekker, Inc.

We have already seen from Example 10.1 that van der Waals forces play a major role in the heat of vaporization of liquids, and it is not surprising, in view of our discussion in Section 10.2 about colloid stability, that they also play a significant part in (or at least influence) a number of macroscopic phenomena such as adhesion, cohesion, self-assembly of surfactants, conformation of biological macromolecules, and formation of biological cells. We see below in this chapter (Section 10.7) some additional examples of the relation between van der Waals forces and macroscopic properties of materials and investigate how, as a consequence, measurements of macroscopic properties could be used to determine the Hamaker constant, a material property that represents the strength of van der Waals attraction (or repulsion see Section 10.8b) between macroscopic bodies. In this section, we present one illustration of the macroscopic implications of van der Waals forces in thermodynamics, namely, the relation between the interaction forces discussed in the previous section and the van der Waals equation of state. In particular, our objective is to relate the molecular van der Waals parameter (e.g., 0n in Equation (33)) to the parameter a that appears in the van der Waals equation of state ... 

The class of methods used for preparing colloidal dispersions in which precipitation from either solution or chemical reaction is used to create colloidal species. The colloidal species are built up by deposition on nuclei that may be of the same or different chemical species. If the nuclei are of the same chemical species, the process is referred to as homogeneous nucleation if the nuclei are of different chemical species, the process is referred to as heterogeneous nucleation. See also Dispersion Methods. An empirical or qualitative term referring to the relative ease with which a material can be deformed or made to flow. It is a reflection of the cohesive and adhesive forces in a mixture or dispersion. See also Atterberg Limits. 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond cellulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many applications is employed as an additive to other polymer systems to improve the cohesive strength, film flexibility, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective colloid in emulsion p olymerization. 

Atomic beams, 31 Nanoparticles, 31 Force microsopy on coated surfaces, 31 Glass surfaces, 31 Mica, 32 Bilayers as thin films and as vesicles, 32 Cells and colloids, 33 Aerosols, 34 Bright stuff. Sonoluminescence, 34 Fun stuff, 34 Slippery stuff, ice and water, 34 What about interfacial energies and energies of cohesion Aren t van der Waals forces important there too, not just between bodies at a distance 35... 

Figure 18-82 illustrates the relationship between solids concentration, interparticle cohesiveness, and the type of sedimentation that may exist. Totally discrete particles include many mineral particles (usually greater in diameter than 20 pm), salt crystals, and similar substances that have little tendency to cohere. Flocculent particles generally will include those smaller than 20 pm (unless present in a dispersed state owing to surface charges), metal hydroxides, many chemical precipitates, and most organic substances other than true colloids. 

Aggregative stability is the resistance to cohesion of particles, to coagulation, whereas kinetic stability is resistance to settling, related only to particle size. In colloid chemistry, stability usually is understood to mean the aggregative stability of a given system. 

The answer has to be found in the stability of the thin liquid film, formed at the interface between solid and vapour. Colloquially stated, what is stronger, the interned cohesion of the adsorbate or the adhesion between adsorbate and surface In colloidal parlance, is the disjoining pressure /7(h) across a film of thickness h positive or negative, and how does 77 change as a function of h ... 

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