It has been said that the use of soap is a gauge of the civilisation of a nation, but though this may perhaps be in a great measure correct at the present day, the use of soap has not always been co-existent with civilisation, for according to Pliny (Nat. Hist., xxviii., 12, 51) soap was first introduced into Rome from Germany, having been discovered by the Gauls, who used the product obtained by mixing goats' tallow and beech ash for giving a bright hue to the hair. In West Central Africa, moreover, the natives, especially the Fanti race, have been accustomed to wash themselves with soap prepared by mixing crude palm oil and water with the ashes of banana and plantain skins. The manufacture of soap seems to have flourished during the eighth century in Italy and Spain, and was introduced into France some five hundred years later, when factories were established at Marseilles for the manufacture of olive-oil soap. Soap does not appear to have been made in England until the fourteenth century, and the first record of soap manufacture in London is in 1524. From this time till the beginning of the nineteenth century the manufacture of soap developed very slowly, being essentially carried on by rule-of-thumb methods, but the classic researches of Chevreul on the constitution of fats at once placed the industry upon a scientific basis, and stimulated by Leblanc's discovery of a process for the commercial manufacture of caustic soda from common salt, the production of soap has advanced by leaps and bounds until it is now one of the most important of British industries.
Definition of Soap.—The word soap (Latin sapo, which is cognate with Latin sebum, tallow) appears to have been originally applied to the product obtained by treating tallow with ashes. In its strictly chemical sense it refers to combinations of fatty acids with metallic bases, a definition which includes not only sodium stearate, oleate and palmitate, which form the bulk of the soaps of commerce, but also the linoleates of lead, manganese, etc., used as driers, and various pharmaceutical preparations, e.g., mercury oleate (Hydrargyri oleatum), zinc oleate and lead plaster, together with a number of other metallic salts of fatty acids. Technically speaking, however, the meaning of the term soap is considerably restricted, being generally[Pg 2] limited to the combinations of fatty acids and alkalies, obtained by treating various animal or vegetable fatty matters, or the fatty acids derived therefrom, with soda or potash, the former giving hard soaps, the latter soft soaps.
The use of ammonia as an alkali for soap-making purposes has often been attempted, but owing to the ease with which the resultant soap is decomposed, it can scarcely be looked upon as a product of much commercial value.
H. Jackson has, however, recently patented (Eng. Pat. 6,712, 1906) the use of ammonium oleate for laundry work. This detergent is prepared in the wash-tub at the time of use, and it is claimed that goods are cleansed by merely immersing them in this solution for a short time and rinsing in fresh water.
Neither of the definitions given above includes the sodium and potassium salts of rosin, commonly called rosin soap, for the acid constituents of rosin have been shown to be aromatic, but in view of the analogous properties of these resinates to true soap, they are generally regarded as legitimate constituents of soap, having been used in Great Britain since 1827, and receiving legislative sanction in Holland in 1875.
Other definitions of soap have been given, based not upon its composition, but upon its properties, among which may be mentioned that of Kingzett, who says that "Soap, considered commercially, is a body which on treatment with water liberates alkali," and that of Nuttall, who defines soap as "an alkaline or unctuous substance used in washing and cleansing".
Properties of Soap.—Both soda and potash soaps are readily soluble in either alcohol or hot water. In cold water they dissolve more slowly, and owing to slight decomposition, due to hydrolysis (vide infra), the solution becomes distinctly turbid. Sodium oleate is peculiar in not undergoing hydrolysis except in very dilute solution and at a low temperature. On cooling a hot soap solution, a jelly of more or less firm consistence results, a property possessed by colloidal bodies, such as starch and gelatine, in contradistinction to substances which under the same conditions deposit crystals, due to diminished solubility of the salt at a lower temperature.
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[Pg 3] 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[Pg 4] 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.
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