Use of Fatty Acids and Soaps

It is well known that precipitation of calcium soaps (i.e., calcium alkyl carboxylates) is often accompanied by reductions in foamability and foam stability of aqueous solutions of other surfactants. The available evidence, as we have discussed in Section 4.7.6, suggests that these effects concern the bridging mechanism of hydrophobic particles. 

Practical laundry detergent formulations usually include builders for control of polyvalent metal ion activities so that the relevant salts of the anionic surfactant, which is usually present, are not precipitated out of solution. The presence of builders also has implications for the effectiveness of antifoam action because they may prevent the precipitation of calcium soaps. We can illustrate the relevant issues if we consider, for example, the concentration of calcium harduess uecessary to just initiate calcium soap precipitation in the case of a solution built by sodium tripolyphosphate at a pH 10.5. If we iguore the effect of solubilizatiou of soaps and calcium binding in anionic surfactant micelles, then, according to Irani and Callis [4], 

Applying a simple mass balance, ignoring activity coefficients,and setting the calcium soap concentration at a vanishing small level (effectively zero) means that we can easily show that 

Clearly, long-chain soaps and fatty acids are likely to form calcium soap antifoams even in the presence of large amounts of sodium tripolyphosphate builder. However, if builders with complexation constants significantly lower than those of sodium tripolyphosphate are used, then it is possible that calcium soap formation may be inhibited and antifoam effects diminished. Observations of Schmadel and Kurzendorfer [5] comparing the antifoam effect of soaps in the presence of nitrilotri-acetic acid and sodium tripolyphosphate are consistent with that expectation. 

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