Spreading tension

Figure 9. Adhesion tension and HLB vs. spreading tension, Tween surfactants. Symbols as in Figure 8 ([J) mica f ) spreading tension against a 20% (w/v) solution of bitumen in kerosene.

In appendix 1 examples of liquid-vapour surface tensions and of liquid-liquid interfacial tensions are given. From this tabulation spreading tensions can be computed. As a rule, a liquid of low surface tension spreads over a liquid of high surface tension. For instance, from the tabulated vcdues it follows that at 293 K benzene initially spreads over water, because (= 72.8 - 28.9 - 35.0) > 0 mN m L However, benzene and water are not completely immiscible after some time there will be mutual saturation. As the surface tension of water saturated with benzene equals 62.2 mN m cind that of benzene saturated with water 28.8 mN m , the corresponding value for < 0. This explains why a drop of pure benzene,... 

In this section we consider the statics of wetting, emphasizing the roles of spreading tensions and contact angles for ideally flat surfaces. Only one-component liquids will be discussed. 

Spreading wetting, or just sprecuiing, stands for the process which occurs when the solid is spontaneously wet by the liquid. Recall that in 5.1.1 we defined the spreading tension as 5 " s - y . According to Ross and Becher this... 

Recall that we prefer the term spreading tension over the less precise spreading coefficient and spreading parameter. Our spreading tension is also known as work of spreading,... 

The notion of spreading tension deserves further specification. It represents the Helmholtz energy gain if a unit area of dry SG interface is replaced by a liquid film, which is thick enough for its Helmholtz energy to be characterized by the sum of the two tensions see fig. 5.8a. Spreading will occur only if S > 0, For... 

For the spreading of one liquid on another S i 2 (in and eq) are accessible from the constituent three interfacial tensions (The tables for liquid-liquid interfacial tensions in appendix 1 refer to mutually saturated systems so from these only S(eq) is obtainable). Examples of initial and equilibrium spreading tensions... 

As for the meaning of in [5.2.2b and c], we proceed as in connection with the spreading tension, see [5.2.1]. When, before adhering to the liquid, the solid particle is completely dry, is. In that case, is Identified as the initial work of adhesion, a> (ln). However, in practice this situation is not easily realized mostly the solid will have been exposed to the vapour of the liquid and hence has to be replaced by y - n[t). For solids at equilibrium with the vapour before adhesion, y is Just the equilibrium solid-gas interfacial tension. Only under that condition is = 0 and does Young s equation apply. 

Between the spreading tension and the various types of work, simple relationships can be derived, provided they all apply to exactly the same conditions. For instance, for initial spreading and adhesion on an SG surface in its pristine state, from 5.2.2a and 4b],... 

Here yj is the specific surface free energy of liquid A, 7g that of liquid B and 7 5 the specific interfacial free energy of the contact surface. The quantity S, defined as 7g — 7 "TaB known as the spreading tension or spreading coefficient. In practice, liquid A will, in time, become saturated with liquid B, even though they do not mix, and a distinction should be made between the initial spreading tension and the equilibrium value when the liquids are mutually saturated. 

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