The Handbook of Soap Manufacture | Page 5

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:--
CH{2}OOC{18}H{35} CH{2}OH | | CHOOC{18}H{35} + 3NaOH = 3NaOOC{18}H{35} + CHOH | | CH{2}OOC{18}H{35} CH{2}OH
stearin sodium sodium glycerol hydroxide stearate
CH{2}OOC{16}H{31} CH{2}OH | | CHOOC{16}H{31} + 3NaOH = 3NaOOC{16}H{31} + CHOH | | CH{2}OOC{16}H{31} CH{2}OH
palmitin sodium sodium glycerol hydroxide palmitate
CH{2}OOC{18}H{33} CH{2}OH | | CHOOC{18}H{33} + 3NaOH = 3NaOOC{18}H{33} + CHOH | | CH{2}OOC{18}H{33} CH{2}OH
olein sodium sodium glycerol hydroxide oleate
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:--
3C{18}H{35}O{2}H + C{3}H{5}(OH){3} = C{3}H{5}(C{18}H{35}O{2}){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) C{18}H{35}O{2}H + C{3}H{5}(OH){3} = C{3}H{5}(OH){2}C{18}H{35}O{2} + H{2}O monostearin
(2) C{18}H{35}O{2}H + C{3}H{5}(OH){2}C{18}H{35}O{2} = C{3}H{5}(OH)(C{18}H{35}O{2}){2} + H{2}O distearin
(3) C{18}H{35}O{2}H + C{3}H{5}(OH)(C{18}H{35}O{2}){2} = C{3}H{5}(C{18}H{35}O{2}){3} + H{2}O 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 formul?, where R denotes the acid radicle:--
Monoglyceride:--
CH{2}OR CH{2}OH | | (alpha) CHOH and (beta) CHOR | | CH{2}OH CH{2}OH
Diglyceride:--
CH{2}OR CH{2}OR | | (alpha) CHOH and (beta) CHOR | | CH{2}OR CH{2}OH
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--
CH{2}OR | CHOR | CH{2}OR
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, C{3}H{5}(OC{18}H{35}O)(OC{18}H{35}O){2}, obtained by Heise in 1896 Mkani fat. Hansen has since found that tallow contains oleodipalmitin, from C{3}H{5}(OC{18}H{35}O)(OC{16}H{31}O), stearodipalmitin, C{3}H{5}(OC{18}H{35}O)(OC{16}H{31}O), oleopalmitostearin, C{3}H{5}(OC{18}H{33}O)(OC{16}H{31}O)(OC{18}H{35}O) and palmitodistearin, CH(OC{16}H{31}O)(OC{18}H{35}O){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, C{3}H{5}(OC{18}H{33}O)(OC{17}H{33}O){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 formul? and chief constants, are given in the following table:--
[Transcriber's note: Table split to fit on page better.]
--------------------------------------------------------------------- Glyceride. | Formula. | Chief Occurrence. --------------------------------------------------------------------- Butyrin | C{3}H{5}(O.C{4}H{7}O){3} | Butter fat --------------------------------------------------------------------- Isovalerin | C{3}H{5}(O.C{5}H{9}O){3} | Porpoise, dolphin --------------------------------------------------------------------- Caproin | C{3}H{5}(O.C{6}H{11}O){3} | Cocoa-nut and | | palm-nut oils --------------------------------------------------------------------- Caprylin | C{3}H{5}(O.C{8}H{15}O){3} | Do. do. --------------------------------------------------------------------- Caprin | C{3}H{5}(O.C{10}H{19}O){3} | Do. do. --------------------------------------------------------------------- Laurin | C{3}H{5}(O.C{12}H{23}O){3} | Do. do. --------------------------------------------------------------------- Myristin | C{3}H{5}(O.C{14}H{27}O){3} | Nutmeg