Mechanisms of detergent action

It is clear that the solid or liquid dirt particles are adsorbed on a substrate, because it is energetically more favorable to adsorb. If we can reverse the process, so that it is 

We need to decrease the value of W1D to separate the dirt particle from the surface. In thermodynamic terms, the quantity AG D must be negative for the separation to be spontaneous. When a detergent is dissolved in water, due to the positive surface excesses of detergent molecules at DWs and SWs interfaces, the values of y)Ws and ySWs are reduced, so that the (W DM) term decreases, from Equation (723). The reduction of W1D increases the ease of removing the dirt particles with mechanical agitation. 

On the other hand, if the dirt is liquid, it may form a layer or a spherical cap shape on the substrate surface, especially in warm solutions. Then, it is possible to apply the Young-Dupre equation (Equation (645)) to the detergent action 

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The mechanism of detergent action will be considered in more detail in Chapter 18. 

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