The Handbook of Soap Manufacture | Page 4

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 with 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