Soap Solution-phase Properties

Commercially, soap is most commonly produced through either the direct saponification of fats and oils with caustic or the hydrolysis of fats and oils to fatty acids followed by stoichiometric (equal molar) neutralization with caustic. Both of these approaches yield workable soap in the form of concentrated soap solutions (- 70% soap). This concentration of soap is the target on account of the aqueous-phase properties of soap as well as practical limitations resulting from these properties. Hence, before discussing the commercial manufacturing of soap, it is imperative to understand the phase properties of soap. 

Two solutions to Illustrate these structural changes within one phase were chosen with emphasis both on their practical importance and on the pronounced difference in behavior brought about by a small difference in composition. The two liquids chosen are both soap/water solutions in a) an alcohol and b) a carboxylic acid. The alcohol solutions form the basis for the W/0 micro-emulsions — and the carboxylic acid solutions show a pronounced difference in properties which merit an evaluation of the inter-molecular forces between the solvent and the solute. The primary difference is in the minimum concentration of water that is required to form the solution. 

It is illustrated by the behaviour of a soap (which is the alkali salt of a long-chain fatty acid) at the boundary of benzene and water. Groups such as —COONa favour solution in the water, as evidenced by the properties of sodium formate or acetate, while the hydrocarbon chains favour solution in benzene. The soap molecules make the best of both worlds when they become concentrated in the inter-facial zone so that their carboxylic groups, with or without their associated alkali ions, are in contact with the water, and the rest of the molecule penetrates into the benzene. In the state of equilibrium the concentration of soap at the bomidary exceeds that in either of the liquid phases. 

Lather volume depends upon the amount and the type of soap dissolved in the soap liquor during lathering. The mobility of the soap molecules, in addition to their surface properties, contributes to foaming. It is therefore possible that the soaps with very short chains (sodium caprylate and sodium caprate) may have an additional lather benefit. However, the proposed benefit should be restricted to low wash temperatures. The source of the short chain soaps is the coconut or palm kernel oil component of the fat charge. The amount of soap in solution in the wash liquor increases as the level of soluble soap in the bar increases. However, because the lather depends on the very short transient hydration period, it is the amount of soap which goes into solution over this period that is important. This amount also increases as the rates of dissolution of the solid soluble phases of the bar structure increase. 

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