Work adhesion 339, cohesion

The statement was made that the work of adhesion between two dissimilar substances should be larger than the work of cohesion of the weaker one. Demonstrate a basis on which this statement is correct and a basis on which it could be argued that the statement is incorrect. 

Because the work of cohesion, of the adhesive is 2 CJ, equation 3 follows. 

Surface energies are also associated with failure of an adhesive bond, because failure involves forming new surfaces and the appropriate surface energies have to be provided. The surface energy term may be the work of adhesion, VTa, or the work of cohesion, VTcoh. depending on whether the failure is adhesive or cohesive. For phases 1 and 2, these are defined as follows [lOj ... 

C) Illustration of the effect of work of cohesion of water and work of adhesion to mineral on the mineral wettability. 

Harkins defines two terms, the work of cohesion We which is the work done when a bar of liquid of unit cross sectional area is pulled apart against the cohesive forces. It is thus numerically equal to twice the sur ce energy of the liquid. The work of adhesion Wa is similarly defined as the work required to pull apart a composite bar consisting of half of one liquid and half of the other, at the place of junction. 

Differentiate between adhesion, cohesion, and spreading, and calcnlate the work associated with all three. 

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.

Person 1 Calculate the work of cohesion within the epoxy adhesive. This is the work required to separate epoxy bonds from one another. 

It is informative to apply Equation (60) to low-energy surfaces for two extreme values of 0, 0° and 180°, for which cos 0 is 1 and — 1, respectively. For 0 = 0°, WSL = 2yLV = WAA the work of solid-liquid adhesion is identical to the work of cohesion for the liquid. In this case interactions between solid and solid, liquid and liquid, and solid and liquid molecules are all equivalent. At the other extreme, with 0 = 180°, WSI = 0. In this case the liquid is tangent to the solid there is no interaction between the phases. 

Define the following terms and their relation to surface energies (a) work of adhesion, (b) work of cohesion, and (c) spreading coefficient. 

The free surface of pure liquids. There is some evidence in favour of a tendency for the hydrocarbon ends of molecules to be oriented outwards when one end of the molecule is hydrocarbon in character and the other has a greater residual affinity.1 The most direct evidence is gained by comparing the work of cohesion, or twice the surface tension (Chap. I, 8), of compounds of related constitution with their work of adhesion to water. The surface tension is half the work that must be done in order to pull apart a bar of the liquid of 1 sq. cm. cross-section, for 2 sq. cm. of fresh surface are formed in this operation. The work of cohesion therefore measures the intensity of the attraction between two free surfaces of the same liquid about to come into contact. 

Comparison of Work of Cohesion with Work of Adhesion to Water, at 20°... 

If (5) is combined with Dupr6 s equation (Chap. I (2)), we see that WAB = 2y i.e. the work of adhesion for two liquids mutually saturated with each other is equal to the work of cohesion of the liquid of lower surface tension, after saturation with that of higher tension. This must mean that the surface of B> after saturation, becomes very similar to that of A as regards external field of force— not necessarily identical as regards molecular composition. 

Fig. 6.10 Work of adhesion and work of cohesion. For a liquid that wets the solid ...

For further thermod)mamic elaboration it is useful also to bring the work of cohesion into the picture. Generally speaking, upon spreading, as in fig. 5.8a, the internal cohesion of the liquid has to be overcome in order to achieve adhesion between liquid and solid. Figure 5.9 Indicates how the work of cohesion is defined. An infinitely long column of unit cross-section is cut into two halves and is the isothermal reversible work required to achieve that. Only for liquids or c... 

Figure A9.4. (a) Work of adhesion and (b) work of cohesion for the oil-water interface.

Capillary action When water is placed in a narrow container such as a graduated cylinder, you can see that the surface of the water is not straight. The surface forms a concave meniscus that is, the surface dips in the center. Figure 13-15 models what is happening to the water at the molecular level. There are two types of forces at work cohesion and adhesion. Cohesion describes the force of attraction between identical molecules. Adhesion describes the force of attraction between molecules that are different. Because the adhesive forces between water molecules and the sihcon dioxide in glass are greater than the cohesive forces between water molecules, the water rises along the inner walls of the cylinder. 

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