Soap, Fatty Acids, and Synthetic Detergents
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SOAP AND FATTY ACIDS Introduction
The origin of the word "soap" is traced to sacrificial Mount Sapo of ancient Roman legend. The mixture of fat and wood ashes that reacted to form soap was carried by rain to the banks of the Tiber River and was found as a clay deposit useful for cleaning clothes. 1 The boiling of fats with ashes was recorded as early as 2500 B.c. Commercial soap-making was a widespread art in the Middle Ages in Europe. The invention of the soda ash process by LeBlanc in 1791, and the discovery by Chevreul in 1811 that soap was composed of a mixture of fatty acids paved the way to modem soap-manufacturing processess. 1•2
and ammonium salts of fatty acids containing from 8 to 22 carbon atoms. These fatty acids are generally a mixture of saturated, unsaturated, and polyunsaturated moieties: CH3(CH2 ) 10COOM: saturated soap CH3(CH2) 6CH 2CH =CHCHiCH2 ) 6COOM: mono-unsaturated soap CH3 (CH2MCH2 CH =CH) 2CHiCH2) 6COOM: di-unsaturated soap CHiCH2CH =CH)3CHiCH2) 6COOM: poly-unsaturated soap [M = Na, K, (HOCH2CH 2) 3NH+]
*Colgate Palmolive Company, Piscataway, NJ-Soap and Fatty Acids. **Huntsman Corporation, Austin, TX-Synthetic Detergents.
In general, two types of chemical reactions are utilized in the manufacture of soap: the saponification of triglycerides (fats and oils) and the neutralization of fatty acids (which themselves are produced from the triglycerides by a variety of methods, most notably splitting or hydrolysis of fats and oils with steam under high pressure). Sodium hydroxide (the predominant alkali employed in the manufacture of soaps), potassium hydroxide, sodium carbonate, and triethanolamine are
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Riegel Handbook of Industrial Chemistry, I Oth Edition Edited by Kent. Kluwer Academic/Plenum Publishers, New York 2003
Chemistry
The modem definition of soap relates to cleansing agents derived from fats, oils, and other fatty derivatives: the soaps are alkali
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SOAP, FATTY ACIDS, AND SYNTHETIC DETERGENTS
Saponification (NaOH)
3 RCOONa + Glycerin (Soap)
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Neutral !'at saponification Process
RCO-OCH 2 RCO-OCH RCO-OCH 2 Triglyceride Fat Splitting
3 RCOOH + Glycerin
l
Fatty Acid Neutralization (NaOH)
l'atty Acid Neutralization Process
3 RCOONa (Soap) Fig. 27 .1. The chemistry of soap manufacture.
Methanol Triglyceride --------------~~ Catalyst
RCOOMe + Glycerin Fatty Methyl Ester
l
Saponification (NaOH)
RCOONa + MeOH (Soap) Fig. 27 .2. Fatty methyl ester process for soap manufacture.
the most commonly utilized alkaline moieties in these processes (Fig. 27.1). 3' 4 In recent years, soap manufacture by an alternate route, the saponification of fatty methyl esters, has been under development, most notably in Japan (Lion Corporation) and Italy (Ballestra). The fatty methyl esters are obtained from the methanolysis of triglycerides; inorganic alkali, quaternary ammonium salts, and enzymes (lipase) have been used as catalysts for methanolysis in commercially practiced processes
(Fig. 27.2). The methyl ester process for soap manufacture is typically more capital and cos
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