Antifoam Effects of Calcium Soaps

Soaps have often been incorporated in detergents to control levels of foam, especially in the context of products designed to be used in foam-intolerant washing machines. Utility as antifoam additives in this context alone invites attention with respect to their mode of action. We therefore explore the possibility that they function as hydrophobic particles according to the principles outlined here. Of particular interest is the possibility that formation of highly insoluble calcium soaps represent the key to their mode of action. 

More than half a century ago, Peper [195] considered the effect of sodium soaps and fatty acids on the foamability and foam stability of dilute solutions of anionic surfactant solutions. In this work, these soaps and fatty acids were present at a concentration of only 0.02 g dm (in the range of 0.6 X 10 -1.0 x 10 M depending on molecular weight). Peper does not make clear the state of these materials dispersed in the surfactant solution. However, the concentrations of surfactant appeared to exceed the CMC in most cases and it seems likely that both sodium soaps and fatty acids were solubilized. Foam measurements were made using hand shaking of measuring cylinders at 25°C. 

Peper showed [195] that in the presence of 1.8 X 10 M calcinm chloride, the adverse effect of these sodium soaps on both foamability and foam stability of a 3 X 10 M (1.0 g dm ) sodium dodecylbenzene sulfonate solution was in the order sodium stearate sodium palmitate sodium oleate sodium laurate, which follows the order of solubility products of the corresponding calcinm soaps (calcium 

It would therefore seem Ukely that these antifoam effects are largely associated with the formation of calcium soap precipitates. Curiously, Peper [195] ignores this possibility and interprets his resnlts in terms of formation of rigid islands of calcium soap monolayer interspersed with gaseous film of adsorbed surfactant. Peper [195] asserts that these islands wiU make the film unstable because of their inflexible brittle nature. No theoretical arguments are given for why these should be unstable. 

Peper [195] found that fatty acids reduce the surface tension of solutions of sodium dodecylbenzene sulfonate containing calcium chloride. The effect was found to be least pronounced for the fatty acid that forms the least soluble calcium soaps and the best antifoam. It is difficult to reconcile this finding with a Marangoni spreading mechanism for antifoam action (see Section 4.4.2). 

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As in the example of the detergent formulation used to obtain the foam profiles shown in Figure 8.4, early attempts to develop praetieal washing powder formulations suitable for foam-intolerant, front-loading drum-type laundry washing machines utilized the antifoam effect of calcium soaps in mixed anionic and ethoxylated alcohol surfactant systems. The approach appears to have been entirely empirical. The foam behavior of ternary mixtures of anionie surfaetants, sueh as sodium alkylbenzene sulfonates, various ethoxylated alcohols, and soaps, was optimized for low foamabil-ity in relevant machines by systematie trial. Obviously, polyvalent metal ion (especially calcium) activity as determined by temperature, water hardness, and builder type and concentration represented an additional variable. Wash temperature and pH... 

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