THE ACTION OF SOAP

Evening Post, Volume CXXII, Issue 36, 11 August 1936, Page 14

THE ACTION OF SOAP

A CHEMICAL MYSTERY

METHODS OF CLEANSING

It is recorded that a »soap works, complete with all its apparatus and some .of its products, was found among the ruins of Pompeii, says a T,vriter in the Melbourne "Age." And there are indications that the soap-making technique extended even beyond this distant period. Whatever its origin, the practice of soap making had a tremen-

dous start on the scientific, theoretical explanations. Indeed, it is only in. comparatively recent times that science has made a determined effort to catch up, and although nowadays we are apt to say.that the soap-making process is under scientific control, and even that science has pointed the way to new developments, there are still some features which are by no means as yet quite clear. One of the most interesting facts about soap making is that, in spite of ■ our progress, the ancient 2000-year-old method still stands. By it, various oils and fats—linseed oil, palm oil, coconut oil, lard, tallow, whale oil, and the like —are boiled with caustic soda, whence the molecules are split asunder into the simplified molecules of glycerine and soap.

For our purpose, we might picture each molecule of fat as a sort of octopus with three long arms or tentacles. It is the object of the soap maker to maltreat each octopus by cutting off its tentacles. He uses an extremely efficient chemical tool, namely, caustic soda. The maimed body of the octopus is known as glycerine, and is sold

to the explosive maker to be turned into nitroglycerine, or else it is used for more peaceful purposes.

Each of the amputated tentacles is provided with a head by the caustic soda, and the long, snake-like molecules so formed, which go by the names of sodium palmitate, sodium stearate, sodium oleate, and so forth, when mixed together in varying quantity, constitute soap. • -MAKING SOFT SOAP. If they be provided with a potassium head, instead of one of sodium, a soft soap is obtained, while if the heads are of other metals, such as lead, cobalt, or manganese, one obtains a variety of metal "soaps" which are used in waterproofing, in making lead plaster, or in improving the 4uality of some of the modern synthetic lacquers.

In the plant or animal body, the fat, which is essentially a reserve food supply, is broken up into glycerine and fatty acids in the complete absence ofcaustic soda. From any point of view it would be an advantage if man could imitate this process and avoid the use of caustic soda. Accordingly, various treatments have been developed which seeks to make full use of some catalytic agent—the Twitchell process, for example, uses naphthalene sulphohic acid. Although many methods have been proposed and used, each with some particular advantage, none as yet can offer serious opposition to the caustic soda process, and hence the age-old method still stands. Two of the major problems concerning soap, which have been a continual source of interest to the scientist, are, first, the exact nature of soap solutions, and, second, the manner in which soap carries out its detergent or cleansing action. When soap dissolves in water it does

not behave normally. If, for example, a solution be allowed to stand for some time, it will often turn into a soft transparent jelly. This is by no means a property of ordinary solution. Researches over the past twenty years have shown that, in small quantities, the soap dissolves to form mdi

vidual molecules and then electrically-

charged ions; but when larger quantiI ties are dissolved the molecules collect together into colloidal clusters, which are also ionised, and which, in fact, form one of the best examples of a special class of colloids, known as colloidal electrolytes. . ■ ' TURN INTO JELLIES. , When cold, the strong solutions, or sols, readily turn into transparent, viscous jellies. The change is by no means as deep-seated as appears at first sight. Indeed, except for the obvious change in rigidity, the two forms have very much the same properties, the same refractive index, the same electrical conductivity, and so forth.

It would appear, that, when the sol sets into the pseudo-solid jelly, all that happens is a restriction in molecular movement and an intensification of in-ter-molecular forces. The change is gradual; there is no melting point; nor are there any of the fundamental changes which are evident when an ordinary liquid, like water, sets into a solid, i ' ' If, however, an opaque curd is deposited, this is an entirely different matter. The ultramicroscope reveals that the curd is an aglomerate of crystalline fibres, and that here we have a definite attempt on the part of the soap to crystallise. ■ '.."

The second point of interest concerns the detergent, or cleansing, action of soap. This to many people is- not only the most important, but the sole function of soap. The matter has given rise to a considerable amount of thought and of work, but even so it cannot yet be claimed that the action is completely explained or understood..

At first it was.thought that the soap was hydrolysed to form free caustic soda, which, by reacting with the fat 'on flesh or on fabric, thereby converted it into soap, which 'then dissolved— and the dirt with it. It is now known that soap is not appreciably hydrolysed, and even if it were the above action would be too slow to explain the facts. Nearer to the mark is the explanation in terms of the emulsification of the fat. It is well known that soap can dissolve oils and fats to sform oil-in-water emulsions, and there is no doubt that this behaviour plays some considerable part in the cleansing action of soap. If the layer of fat is removed, then, of course, the attached dirt goes with it SUM OF THE EFFECTS. On the other hand, experiments with clay, soot, red ochre, etc., reveal that soap will remove dirt in the complete absence of fats or oils. It is believed that the soap forms a colloidal absorption compound with various types of ; dirt. In general, there is little doubt \ that both these actions proceed in normal washing. But there is still another possibility. ' Dissolved soap affects both the interfacial tension and the contact angle between different surfaces. Accordingi ly, when a soap solution comes in con- , tact with an oily surface, like that of ! a greasy dinner plate, the water has a I preferential liking for the plate. It thus slips round the oil and completely I displaces it. Emulsification of the dist placed oil may or may not follow. > On top of all this comes the evidence ; that oil.and fats are soluble to a slight I extent in soap solutions. Taking it s altogether, it may well be that the ao - tion of the soap is the sum total of all these different effects.