Krafft (Journ. Soc. Chem. Ind., 1896, 206, 601; 1899, 691; and 1902, 1301) and his collaborators, Wiglow, Strutz and Funcke, have investigated this property of soap solutions very fully, the researches extending over several years. In the light of their more recent work, the molecules, or definite aggregates of molecules, of solutions which become gelatinous on cooling move much more slowly than the molecules in the formation of a crystal, but there is a definite structure, although arranged differently to that of a crystal. In the case of soda soaps the colloidal character increases with the molecular weight of the fatty acids.
Soda soaps are insoluble in concentrated caustic lyes, and, for the most part, in strong solutions of sodium chloride, hence the addition of caustic soda or brine to a solution of soda soap causes the soap to separate out and rise to the surface. Addition of brine to a solution of potash soap, on the other hand, merely results in double decomposition, soda soap and potassium chloride being formed, thus:—
| 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.
Rotondi (Journ. Soc. Chem. Ind., 1885, 601) was of the opinion that the basic soap, which he believed to be formed by hydrolysis, was alone responsible for the detergent action of soap, this basic soap dissolving fatty matter by saponification, but, as already pointed out, his theory of the formation of a basic soap is now known to be incorrect, and his conclusions are therefore invalid.
Several explanations have been suggested, based on the purely physical properties of soap solutions. Most of these are probably, at any rate in part, correct, and there can be little doubt that the ultimate solution of the problem lies in this direction, and that the detergent action of soap will be found to depend on many of these properties, together with other factors not yet known.
Jevons in 1878 in some researches on the "Brownian movement" or "pedesis" of small particles, a movement of the particles which is observed to take place when clay, iron oxide, or other finely divided insoluble matter is suspended in water, found that the pedetic action was considerably increased by soap and sodium silicate, and suggested that to this action of soap might be attributed much of its cleansing power.
Alder Wright considered that the alkali liberated by hydrolysis in some way promoted contact of the water with the substance to be cleansed, and Knapp regarded the property of soap solutions themselves to facilitate contact of the water with the dirt, as one of the chief causes of the efficacy of soap as a detergent.
Another way in which it has been suggested that soap acts as a cleanser is that the soap itself or the alkali set free by hydrolysis serves as a lubricant, making the dirt less adherent, and thus promoting its removal.
The most likely theory yet advanced is that based on the emulsifying power of soap solutions. The fact that these will readily form emulsions