Work of adhesion and cohesion

An important property connected to interfacial tensions is the thermodynamic or ideal work of adhesion, which is defined as the work needed to separate an interface into two separate surfaces (in contact with air). 

The work of adhesion is defined in all cases where we have two immiscible interfaces, e.g. liquid-liquid, solid-liquid and solid-solid. 

Similarly, we can define the so-called work of cohesion for a single surface (work needed to break the surface), Wcoh, which is equal to two times the value of its surface tension (Wcoh = 2y). 

In reality, the actual contact surface area between two different materials is (often much) less than the 

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Derive the expression (in terms of the appropriate works of adhesion and cohesion) for the spreading coefficient for a substance C at the interface between two liquids A and B. 

Substituting in the Dupre equation, the spreading coefficient can be related to the work of adhesion and cohesion... 

It needs to be noted here that since the surface tension fundamentally originated from the intermolecular forces and can be related to the thermodynamic work of adhesion and cohesion, the slip length can be estimated by appealing to the pertinent molecular theories as [8]... 

Wetting and spreading M E R SHANAHAN Young s equation, work of adhesion and cohesion, spreading coefficient... 

The work of cohesion, the material with itself, is equal to 2y" and has a contact angle of 0°. In general for a simple fluid the work of adhesion and cohesion are equal and for 9 < 90°, the liquid is wetting the solid and when 9 > 90° the liquid... 

For polymer surface properties controlled by the chemical composition, thermodynamic (equihbrium), non-equihbrium, and technical terms and definitions play an important role. These are not always used in a consistent way, hence a short recapitulation seems appropriate. The thermodynamic work of adhesion (Wa) is defined as the reversible work (the free energy change) required to separate two phases with unit area of contact, from contact to infinity. The corresponding work of adhesion (and cohesion for similar bodies) can be easily expressed with surface tension values. In general, for surfaces of two intimately contacting solids ( l and 2 , respectively) each with a unit area, are separated in a medium ( 3 ), a work VT132 is required which can be expressed as ... 

HPMC produced the most friable and smallest granules owing to its lower work of adhesion and cohesion (53). Gelatin produced the strongest granules that needed more work to compress due to their higher resistance to deformation (53). 

The work of adhesion and cohesion, and VTjp respectively, can be used to explain the spreading of a liquid on a solid surface. Combining equation 7.14 with Young s equation (equation (7.7)), yields the following ... 

The spreading coefficient, S, as follows from Eqs 2.4-2.T, is equal to the difference in the work of adhesion and cohesion of hquid ... 

Notice that the Harkins coefficient is related to the works of adhesion and cohesion positive spreading coefficient and thus spreading (S > 0) are obtained only when the work of adhesion between oil and water is higher than the work of cohesion of the oil, or, in other words, when oil likes (adheres to) water more than it likes itself (coheres to itself) (see Problem 3.11). Similar concepts are true for solid-liquid interfaces, as discussed in Chapter 6. 

Usually tj/ is very much larger than Fq. This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than works of adhesion or cohesion. However, a modest increase in Fq may result in a large increase in adhesion as and Fo are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

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.

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

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

The work of adhesion (see Chapter 1,1) reflects the degree to which unsaturated molecular interactions between solids and liquids in contact are balanced. The value of cos 9, which is symbatic to the work of adhesion, is also a measure of the degree of similarity between the solid surface and a liquid (liophilicity). Polar surfaces that are wetted by water well are hydrophilic, while those poorly wetted (solid hydrocarbons, and particularly fluororinated polymers) are hydrophobic. Since the value of 0 is determined by both the work of adhesion and the work of cohesion, a comparison of the contact angles formed by different liquids at the same solid surface does not allow one to compare the works of adhesion (the degree of similarity in the nature of the liquid and solid) directly. For example, polar surfaces are equally wetted well by both water and hydrocarbons. 

To rernm to the definitions of the work of adhesion and the work of cohesion, one notes an interesting feature, that is, that the work done in moving apart molecules can be related to interfadal tensions. The important energy that is dependent on the intermolecular distances is the intermolecular potential. On assuming that the work done is solely the change in intermolecular potential on tearing apart a column of matmal, one finds a means for relating interfadal tensions to molecular effects. A simple form of intermolecular potraitial between molecules i and j is... 

For secondary bonds. Dispersion forces, Polar forces and Hydrogen bonding, various explicit forms of Eqn. 2 have been proposed. Their use requires familiarity with Young s equation and the definition of the work of adhesion and the work of cohesion (Fig. 1), discussed in Wetting and spreading. It is convenient to qnote these relationships here ... 

It is important to realize that the work of adhesion and work of cohesion defined by Eqns. 4 and 5 refer to the hypothetical changes described they may not, and usually will not, be closely related to practical measures of fracture energy of an adhesive bond or of a single-phase material. This is because in the context of practical fracture, much larger amounts of work are dissipated by other mechanisms, such as the plastic deformation of the materials around the fracture zone. 

Work of Cohesion, Work of Adhesion and Spreading Coefficient... 

The work of adhesion and work of cohesion have been define and discussed in detail in O Chap. 6, Thermodynamics of Adhesion. 

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