Soap-making with alkali

Liquid/hquid reactions of industrial importance are fairly numerous. A hst of 26 classes of reactions with 61 references has been compiled by Doraiswamy and Sharma Heterogeneou.s Reactions, Wiley, 1984). They also indicate the kind of reactor normally used in each case. The reactions range from such prosaic examples as making soap with alkali, nitration of aromatics to make explosives, and alkylation of C4S with sulfuric acid to make improved gasoline, to some much less familiar operations. 

Acylglycerols can be hydrolyzed by heating with acid or base or by treatment with lipases. Hydrolysis with alkali is called saponification and yields salts of free fatty acids and glycerol. This is how soap (a metal salt of an acid derived from fat) was made by our ancestors. One method used potassium hydroxide potash) leached from wood ashes to hydrolyze animal fat (mostly triacylglycerols). (The tendency of such soaps to be precipitated by Mg and Ca ions in hard water makes them less useful than modern detergents.) When the fatty acids esterified at the first and third carbons of glycerol are different, the sec-... 

Sodium carbonate is an alkali, a strongly basic compound that has a pH of 9 or more in solution. Boiling an alkali with fat makes soap. Chemically, soap contains a long chain of hydrocarbons that repels water but has an affinity for other fatty substances. When treated with alkali, the ends of the fatty chains are modified so that they are attracted to water. A molecule with one end that attracts fatty substances and another end that likes to dissolve in water is ideal for coating dirt particles and floating them away in water. Soap can be made from any kind of fat, whether from kitchen scraps or whale, olive, or palm oil. 

Once the volatile prodiicts have been pyrolyzed, one is left with a material that is mostly carbon, and these are called coke (from coal) and charcoal (from wood). Coke was of course used in blast furnaces to make iron (the volatiles would make the process less reproducible in reducing Fc203) by our ancestors, while charcoal was used in soap (mainly the alkalis in the ash) and as an adsorbent. 

Such reactions can have a considerable effect on the stability of aldehydes and ketones in bases that are even mildly alkaline, such as soaps and detergents. Before soap making reached the standards of quality that are achieved today, the base would often contain, and still does in poorer qualities, a surplus of alkali from the process of saponification. This made it almost impossible to use any of the less stable types of aldehyde with any degree of confidence. Now, however, with good quality bases being available most aldehydes, including many of the straight chain aliphatic aldehydes, can be used quite safely. 

A major step toward large-scale commercial soap making occurred in 1791 when a French chemist, Nicholas Leblanc, patented a process for making soda ash, or sodium carbonate, from common salt. Soda ash in the alkali obtained from ashes combines with fat to form soap. The Leblanc process 5uelded quantities of good quality, inexpensive soda ash. 

Another approach is to make use of the fact that in contact with alkali, chlorophyll derivatives also react to form a precipitate (88). This can be used in the course of alkali refining. The precipitate can be removed together with the aqueous soap phase, but at this time, specihc details are still proprietary. Experience so far has shown that as much as about 70% of the chlorophyll derivatives can be removed in the course of alkah refining. 

AP is made by saponification, the reaction between alkalis and animal or vegetable fats. Soaps made with austic potash are liquid, while caustic soda makes soaps that are solid. The most popular fats used are lard, goat suet, beef tallow, olive oil and palm oil. 

If we wish to start the stoiy where it began, we should travel back in time to an Ancient Age when soap-making was first discovered. What is soap If fats are boiled together with some aqueous alkali for a lengthy time, they will decompose. Chemically, they undergo alkaline hydrolysis (Fig. 2.6). Glycerol and soap are formed in this process soap is a mixture of the sodium (or potassium) salt of different fatly acids. In an ever-popular novel, The Mysterious Island L lle mysterieuse), French science-fiction writer Jules Verne very nicely described that soap could be made even on a deserted island (the movies supposedly based on this novel seldom make a point of this part). 

There are several soap-making processes which utilize the continuous neutralization of fatty acids. In the MILLS process (McBride, 1947), distilled fatty acids at 75 °C are fed with a metered supply of sodium hydroxide solution to a high-speed mixer. Sufficient sodium chloride is added to the alkali so that the final soap composition is similar to that obtained by the triglyceride saponification route. 

Since the treatment of fatty acids with carbonated iJka-lies, instead of employing them in the caustio state, as in ordinary soap-making, involves the escape of carbonic acid, and a consequent swelling up of the materials when brought in contact, even without boiling, ample room most be left in the pan to allow for the great increase in bulk which occurs after repeated additions of alkali. 

The use of ammonia as an alkali for soap-making purposes has often been attempted, but owing to the ease with which the resultant soap is decomposed, it can scarcely be looked upon as a product of much commercial value. 

The manufacture of alkali was at one time carried on in conjunction with soap-making, but of late years it has become more general for the soap manufacturer to buy his caustic soda or carbonated alkali ficom the alkali-maker. 

Patent Textile Soaps.—Stockhausen (Eng. Pat. 24,868, 1897) makes special claim for a soap, termed Monopole Soap, to be used in place of Turkey-red oils in the dyeing and printing of cotton goods and finishing of textile fabrics. The soap is prepared by heating the sulphonated oil (obtained on treatment of castor oil with sulphuric acid) with alkali, and it is stated that the product is not precipitated when used in the dye-bath as is ordinary soap, nor is it deposited upon the fibres. 

---