| C17H35COOK | + | NaCl | = | C17H35COONa | + | KCl |
| potassium stearate | sodium chloride | sodium stearate | potassium chloride |
The solubility of the different soaps in salt solution varies very considerably. Whilst sodium stearate is insoluble in a 5 per cent. solution of sodium chloride, sodium laurate requires a 17 per cent. solution to precipitate it, and sodium caproate is not thrown out of solution even by a saturated solution.
Hydrolysis of Soap.—The term "hydrolysis" is applied to any resolution of a body into its constituents where the decomposition is brought about by the action of water, hence when soap is treated with cold water, it is said to undergo hydrolysis, the reaction taking place being represented in its simplest form by the equation:—
| 2NaC18H35O2 | + | H2O | = | NaOH | + | HNa(C18H35O2)2 |
| sodium stearate | water | caustic soda | acid sodium stearate |
The actual reaction which occurs has been the subject of investigation by many chemists, and very diverse conclusions have been arrived at. Chevreul, the pioneer in the modern chemistry of oils and fats, found that a small amount of alkali was liberated, as appears in the above equation, together with the formation of an acid salt, a very minute quantity of free fatty acid remaining in solution. Rotondi (Journ. Soc. Chem. Ind., 1885, 601), on the other hand, considered that a neutral soap, on being dissolved in water, was resolved into a basic and an acid salt, the former readily soluble in both hot and cold water, the latter insoluble in cold water, and only slightly soluble in hot water. He appears, however, to have been misled by the fact that sodium oleate is readily soluble in cold water, and his views have been shown to be incorrect by Krafft and Stern (Ber. d. Chem. Ges., 1894, 1747 and 1755), who from experiments with pure sodium palmitate and stearate entirely confirm the conclusions arrived at by Chevreul.
The extent of dissociation occurring when a soap is dissolved in water depends upon the nature of the fatty acids from which the soap is made, and also on the concentration of the solution. The sodium salts of cocoa-nut fatty acids (capric, caproic and caprylic acids) are by far the most easily hydrolysed, those of oleic acid and the fatty acids from cotton-seed oil being dissociated more readily than those of stearic acid and tallow fatty acids. The decomposition increases with the amount of water employed.
The hydrolytic action of water on soap is affected very considerably by the presence of certain substances dissolved in the water, particularly salts of calcium and magnesium. Caustic soda exerts a marked retarding effect on the hydrolysis, as do also ethyl and amyl alcohols and glycerol.
Detergent Action of Soap.—The property possessed by soap of removing dirt is one which it is difficult to satisfactorily explain. Many theories, more or less complicated, have been suggested, but even now the question cannot be regarded as solved.
The explanation commonly accepted is that the alkali liberated by hydrolysis attacks any greasy matter on the surface to be cleansed, and, as the fat is dissolved, the particles of dirt are loosened and easily washed off. Berzelius held this view, and considered that the value of a soap depended upon the ease with which it yielded free alkali on solution in water.
This theory is considered by Hillyer (Journ. Amer. Chem. Soc., 1903, 524), however, to be quite illogical, for, as he points out, the liberated alkali would be far more likely to recombine with the acid or acid salt from which it has been separated, than to saponify a neutral glyceride, while, further, unsaponifiable greasy matter is removed by soap as easily as saponifiable fat, and there can be no question of any chemical action of the free alkali in its case. Yet another argument against the theory is that hydrolysis is greater in cold and dilute solutions, whereas hot concentrated soap solutions are generally regarded as having the best detergent action.