Surfactants as Wetting and Dewetting Agents

It is often desirable to spread a thin film of solution W) on a hydrophobic surface (5). This does not happen spontaneously. If one tries to spread a thin film in a partial wetting regime, the film tends to fragment in droplets, each of which conforms to the contact angle 6e specified by Young s relation. As discussed in chapter 7, only thicker films, with a puddle thickness 6c = s h 6e/2) in the millimeter range, are stable. One must therefore 

FIGURE 8.9. Third argument used to justify Bancroft s rule Formation of a finger destabilizing into droplets. 

Let us reexamine the condition for total wetting derived in chapter 1. It reads 

To meet this criterion, we can decrease 7 by means of a surfactant soluble in water. Two cases must be distinguished. 

If the surfactant is anionic (negative ion) and if S is a mineral surface charged negatively, the two will repel each other and the surfactant cannot attach itself to 5. As a result, the energy sw remains unchanged and the surfactant acts on the surface tension 7 only. 

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