Work of cohesion

The two new areas will have a surface tension of 2yLV e work of cohesion is simply 

the work of cohesion is simply equal to twice the liquid surface tension. An important conclusion may be drawn if one considers the work of adhesion given by 

the competition between cohesion of the liquid to itself and its adhesion to a solid gives an angle of contact that is constant and specific to a given system at equilibrium. This shows the importance of Young s equation in defining wetting. 

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Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

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. 

Work of cohesion, 1 506—507 Workover fluids, 9 2-3, 26-30 Workplace air, silver compounds in,... 

The work of cohesion, e.g., of a pure liquid, consists of producing two new interfaces, each of 1 m2 it measures the attraction between the molecules of this phase... 

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. 

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. 

Recall from Section 2.4.1 that there is work associated with creating any new surface. The fracture of the solid at k/2 creates two new surfaces where none existed before (see Figure 5.35c), so that the work of cohesion between the two surfaces, W, is given by Eq. (2.85), =2y, where y is the surface energy of the solid. We can equate... 

The quantity WAA is known as the work of cohesion since it equals the work required to pull apart a column of liquid A apart. It measures the attraction between the molecules of the two... 

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. 

Our starting point in this discussion is the resemblance between Figure 6.8a and Figure 10.6b. The former illustrates the work of cohesion and the latter the interaction between two blocks of material. Suppose we identify by d0 the equilibrium spacing between molecules of a bulk sample of the material under consideration. Then the cohesion process represented by Figure 6.8a can be viewed as one in which two blocks of material are separated from d = d0 to d = 00. In terms of Equation (63),... 

Equating this with the work of cohesion given by Equation (6.56), we obtain... 

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. 

Eq. (8.2) indicates a relationship between surface tension, cohesive energy and molar volume. A quantitative and dimensionally correct relationship between these quantities has been derived by Grunberg and Nissan (1949). For compounds of low molecular weight they defined a quantity Wcoh, called work of cohesion ... 

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