Читать книгу The Handbook of Soap Manufacture - H. A. Appleton - Страница 7
CONSTITUTION OF OILS AND FATS, AND THEIR SAPONIFICATION.
ОглавлениеResearches 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 | |||||
| palmitin | sodium hydroxide | sodium palmitate | glycerol |
| CH2OOC18H33 | CH2OH | |||||
| | | | | |||||
| CHOOC18H33 | + | 3NaOH | = | 3NaOOC18H33 | + | CHOH |
| | | | | |||||
| CH2OOC18H33 | CH2OH | |||||
| olein | sodium hydroxide | sodium oleate | glycerol |
Berthelot subsequently confirmed Chevreul's investigations by directly synthesising the fats from fatty acids and glycerol, the method he adopted consisting in heating the fatty acids with glycerol in sealed tubes. Thus, for example:—
| 3C18H35O2H | + | C3H5(OH)3 | = | C3H5(C18H35O2)3 |
| stearic acid | glycerol | tristearin |
Since glycerol is a trihydric alcohol, i.e., contains three hydroxyl (OH) groups, the hydrogen atoms of which are displaceable by acid radicles, the above reaction may be supposed to take place in three stages. Thus, we may have:—
| (1) | C18H35O2H | + | C3H5(OH)3 | = | C3H5(OH)2C18H35O2 | + | H2O |
| monostearin | |||||||
| (2) | C18H35O2H | + | C3H5(OH)2C18H35O2 | = | C3H5(OH)(C18H35O2)2 | + | H2O |
| distearin | |||||||
| (3) | C18H35O2H | + | C3H5(OH)(C18H35O2)2 | = | C3H5(C18H35O2)3 | + | H2O |
| tristearin |
There are two possible forms of monoglyceride and diglyceride, according to the relative position of the acid radicle, these being termed alpha and beta respectively, and represented by the following formulæ, where R denotes the acid radicle:—
Monoglyceride:—
| CH2OR | CH2OH | ||||
| | | | | ||||
| (alpha) | CHOH | and | (beta) | CHOR | |
| | | | | ||||
| CH2OH | CH2OH |
Diglyceride:—
| CH2OR | CH2OR | ||||
| | | | | ||||
| (alpha) | CHOH | and | (beta) | CHOR | |
| | | | | ||||
| CH2OR | CH2OH |
According to the relative proportions of fatty acid and glycerol used, and the temperature to which they were heated, Berthelot succeeded in preparing mono-, di- and triglycerides of various fatty acids.
Practically all the oils and fats used in soap-making consist of mixtures of these compounds of glycerol with fatty acids, which invariably occur in nature in the form of triglycerides.
It was formerly considered that the three acid radicles in any naturally occurring glyceride were identical, corresponding to the formula—
CH2OR | CHOR | CH2OR
where R denotes the acid radicle. Recent work, however, has shown the existence of several so-called mixed glycerides, in which the hydroxyls of the same molecule of glycerol are displaced by two or sometimes three different acid radicles.
The first mixed glyceride to be discovered was oleodistearin, C3H5(OC18H35O)(OC18H35O)2, obtained by Heise in 1896 from Mkani fat. Hansen has since found that tallow contains oleodipalmitin, C3H5(OC18H35O)(OC16H31O), stearodipalmitin, C3H5(OC18H35O)(OC16H31O), oleopalmitostearin, C3H5(OC18H33O)(OC16H31O) (OC18H35O) and palmitodistearin, CH(OC16H31O)(OC18H35O)2, the latter of which has also been obtained by Kreis and Hafner from lard, while Holde and Stange have shown that olive oil contains from 1 to 2 per cent. of oleodidaturin, C3H5(OC18H33O)(OC17H33O)2, and Hehner and Mitchell have obtained indications of mixed glycerides in linseed oil (which they consider contains a compound of glycerol with two radicles of linolenic acid and one radicle of oleic acid), also in cod-liver, cod, whale and shark oils.
In some cases the fatty acids are combined with other bases than glycerol. As examples may be cited beeswax, containing myricin or myricyl palmitate, and spermaceti, consisting chiefly of cetin or cetyl palmitate, and herein lies the essential difference between fats and waxes, but as these substances are not soap-making materials, though sometimes admixed with soap to accomplish some special object, they do not require further consideration.
The principal pure triglycerides, with their formulæ and chief constants, are given in the following table:—
| Glyceride. | Formula. | Chief Occurrence. | Melting Point, °C. | Refractive Index at 60° C. | Saponification Equivalent. |
| Butyrin | C3H5(O.C4H7O)3 | Butter fat | Liquid at −60 | 1.42015 | 100.7 |
| Isovalerin | C3H5(O.C5H9O)3 | Porpoise, dolphin | … | … | 114.7 |
| Caproin | C3H5(O.C6H11O)3 | Cocoa-nut and palm-nut oils | −25 | 1.42715 | 128.7 |
| Caprylin | C3H5(O.C8H15O)3 | Do. do. | −8.3 | 1.43316 | 156.7 |
| Caprin | C3H5(O.C10H19O)3 | Do. do. | 31.1 | 1.43697 | 184.7 |
| Laurin | C3H5(O.C12H23O)3 | Do. do. | 45 | 1.44039 | 212.7 |
| Myristin | C3H5(O.C14H27O)3 | Nutmeg butter | 56.5 | 1.44285 | 240.7 |
| Palmitin | C3H5(O.C16H31O)3 | Palm oil, lard | 63–64 | … | 268.7 |
| Stearin | C3H5(O.C18H35O)3 | Tallow, lard, cacao butter | 71.6 | … | 296.7 |
| Olein | C3H5(O.C18H33O)3 | Olive and almond oils | Solidifies at −6 | … | 294.7 |
| Ricinolein | C3H5(O.C18H33O2)3 | Castor oil | … | … | 310.7 |
Of the above the most important from a soap-maker's point of view are stearin, palmitin, olein and laurin, as these predominate in the fats and oils generally used in that industry. The presence of stearin and palmitin, which are solid at the ordinary temperature, gives firmness to a fat; the greater the percentage present, the harder the fat and the higher will be the melting point, hence tallows and palm oils are solid, firm fats. Where olein, which is liquid, is the chief constituent, we have softer fats, such as lard, and liquid oils, as almond, olive and cotton-seed.
Stearin (Tristearin) can be prepared from tallow by crystallisation from a solution in ether, forming small crystals which have a bright pearly lustre. The melting point of stearin appears to undergo changes and suggests the existence of distinct modifications. When heated to 55° C. stearin liquefies; with increase of temperature it becomes solid, and again becomes liquid at 71.6° C. If this liquid be further heated to 76° C., and allowed to cool, it will not solidify until 55° C. is reached, but if the liquid at 71.6° C. be allowed to cool, solidification will occur at 70° C.
Palmitin (Tripalmitin) may be obtained by heating together palmitic acid and glycerol, repeatedly boiling the resulting product with strong alcohol, and allowing it to crystallise. Palmitin exists in scales, which have a peculiar pearly appearance, and are greasy to the touch. After melting and solidifying, palmitin shows no crystalline fracture; when heated to 46° C. it melts to a liquid which becomes solid on further heating, again liquefying when 61.7° C. is reached, and becoming cloudy, with separation of crystalline particles. At 63° C. it is quite clear, and this temperature is taken as the true melting point. It has been suggested that the different changes at the temperatures mentioned are due to varying manipulation, such as rate at which the temperature is raised, and the initial temperature of the mass when previously cool.
Olein (Triolein) is an odourless, colourless, tasteless oil, which rapidly absorbs oxygen and becomes rancid. It has been prepared synthetically by heating glycerol and oleic acid together, and may be obtained by submitting olive oil to a low temperature for several days, when the liquid portion may be further deprived of any traces of stearin and palmitin by dissolving in alcohol. Olein may be distilled in vacuo without decomposition taking place.
Laurin (Trilaurin) may be prepared synthetically from glycerol and lauric acid. It crystallises in needles, melting at 45°-46° C., which are readily soluble in ether, but only slightly so in cold absolute alcohol. Scheij gives its specific gravity, d60°/4° = 0.8944. Laurin is the chief constituent of palm-kernel oil, and also one of the principal components of cocoa-nut oil.
Fatty Acids.—When a fat or oil is saponified with soda or potash, the resulting soap dissolved in hot water, and sufficient dilute sulphuric acid added to decompose the soap, an oily layer gradually rises to the surface of the liquid, which, after clarifying by warming and washing free from mineral acid, is soluble in alcohol and reddens blue litmus paper. This oily layer consists of the "fatty acids" or rather those insoluble in water, acids like acetic, propionic, butyric, caproic, caprylic and capric, which are all more or less readily soluble in water, remaining for the most part dissolved in the aqueous portion. All the acids naturally present in oils and fats, whether free or combined, are monobasic in character, that is to say, contain only one carboxyl—CO.OH group. The more important fatty acids may be classified according to their chemical constitution into five homologous series, having the general formulæ:—
I. Stearic series CnH2n+1COOH II. Oleic series CnH2n-1COOH III. Linolic series CnH2n-3COOH IV. Linolenic series CnH2n-5COOH V. Ricinoleic series CnH2n-7COOH
I. Stearic Series.—The principal acids of this series, together with their melting points and chief sources, are given in the following table:—
