The Handbook of Soap Manufacture. H. A. Appleton
oils has long been known, and the detergent action of soap has frequently been attributed to it, the explanation given being that the alkali set free by the water emulsifies the fatty matter always adhering to dirt, and carries it away in suspension with the other impurities. Experiments by Hillyer (loc. cit.) show, however, that while N/10 solution of alkali will readily emulsify a cotton-seed oil containing free acidity, no emulsion is produced with an oil from which all the acidity has been removed, or with kerosene, whereas a N/10 solution of sodium oleate will readily give an emulsion with either, thus proving that the emulsification is due to the soap itself, and not to the alkali.
Plateau (Pogg. Ann., 141, 44) and Quincke (Wiedmann's. Ann., 35, 592) have made very complete researches on the emulsification and foaming of liquids and on the formation of bubbles. The former considers that there are two properties of a liquid which play an important part in the phenomenon, (1) it must have considerable viscosity, and (2) its surface tension must be low. Quincke holds similar views, but considers that no pure liquid will foam.
Soap solution admirably fulfils Plateau's second condition, its surface tension being only about 40 per cent. of that of water, while its cohesion is also very small; and it is doubtless to this property that its emulsifying power is chiefly due. So far as viscosity is concerned, this can have but little influence, for a 1 per cent. solution of sodium oleate, which has a viscosity very little different from that of pure water, is an excellent emulsifying agent.
Hillyer, to whose work reference has already been made, investigated the whole question of detergent action very exhaustively, and, as the result of a very large number of experiments, concludes that the cleansing power of soap is largely or entirely to be explained by the power which it has of emulsifying oily substances, of wetting and penetrating into oily textures, and of lubricating texture and impurities so that these may be removed easily. It is thought that all these properties may be explained by taking into account the low cohesion of the soap solutions, and their strong attraction or affinity to oily matter, which together cause the low surface tension between soap solution and oil.
CHAPTER II.
CONSTITUTION OF OILS AND FATS, AND THEIR SAPONIFICATION.
Researches of Chevreul and Berthelot—Mixed Glycerides—Modern Theories of Saponification—Hydrolysis accelerated by (1) Heat or Electricity, (2) Ferments; Castor-seed Ferment, Steapsin, Emulsin, and (3) Chemical Reagents; Sulphuric Acid, Twitchell's Reagent, Hydrochloric Acid, Lime, Magnesia, Zinc Oxide, Soda and Potash.
The term oil is of very wide significance, being applied to substances of vastly different natures, both organic and inorganic, but so far as soap-making materials are concerned, it may be restricted almost entirely to the products derived from animal and vegetable sources, though many attempts have been made during the last few years to also utilise mineral oils for the preparation of soap. Fats readily become oils on heating beyond their melting points, and may be regarded as frozen oils.
Although Scheele in 1779 discovered that in the preparation of lead plaster glycerol is liberated, soap at that time was regarded as a mere mechanical mixture, and the constitution of oils and fats was not properly understood. It was Chevreul who showed that the manufacture of soap involved a definite chemical decomposition of the oil or fat into fatty acid and glycerol, the fatty acid combining with soda, potash, or other base, to form the soap, and the glycerol remaining free. The reactions with stearin and palmitin (of which tallow chiefly consists) and with olein (found largely in olive and cotton-seed oils) are as follows:—
| CH2OOC18H35 | CH2OH | |||||
| | | | | |||||
| CHOOC18H35 | + | 3NaOH | = | 3NaOOC18H35 | + | CHOH |
| | | | | |||||
| CH2OOC18H35 | CH2OH | |||||
| stearin | sodium hydroxide | sodium stearate | glycerol |
| CH2OOC16H31 | CH2OH | |||||
| | | | | |||||
| CHOOC16H31 | + | 3NaOH | = | 3NaOOC16H31 | + | CHOH |
| | | | | |||||
| CH2OOC16H31 | CH2OH | |||||
|
|