Chemistry Everyday for Everyone
Secondary School Chemistry
Synthesis of Exotic Soaps in the Chemistry Laboratory Otto Phanstiel IV,* Eric Dueno, and Queenie Xianghong Wang Department of Chemistry, University of Central Florida, Orlando, FL 32816
History of Soap The earliest accounts of soap production date back more than 5000 years (1). Indeed, historical studies have revealed that soap was utilized in both ancient Egypt and Babylonia (1 ). Several millennia ago crude mixtures of animal fats and alkaline plant ash were found to generate crude soaps, which lathered and cleaned efficiently. Modern soaps most likely evolved from further experimentation with these mixtures. How soaps clean and the influence of cleanliness on developing society has been discussed by several authors (2– 4). What Is Soap? Soap-making has remained virtually unchanged for several thousand years. The procedure involves the basic hydrolysis (saponification) of a fat or oil. Most fats and oils (triglycerides or triacylglycerols) are triesters comprising three long-chain aliphatic carboxylic acids appended to a single glycerol molecule. The process of saponification involves heating either animal fat or vegetable oil in an alkaline solution. Lye (sodium hydroxide) is the primary source of alkali. As shown in the following generalized reaction scheme, the alkaline solution hydrolyzes the triglyceride to its component parts—salts of long-chain carboxylic acids (general structure, RCOO᎑ Na+) and glycerol: O
O x
O
NaOH
O
O O
H 2O
y O
O-Na+
x
HO
"alkali"
HO
+
O
z O
triglyceride or fat
z
glycerol
O-Na+
y
HO
O-Na+
carboxylate salts
The carboxylic acid salts are frequently precipitated from solution by a “salting out” process involving aqueous sodium chloride. The precipitated soap is then isolated by filtration. The carboxylates tend to be structurally diverse, typically having linear chain lengths ranging from 8 to 18 carbons. Evennumbered carbon chains predominate (especially C14, C16, and C18). Some of these chains contain sites of unsaturation. The degree of unsaturation and the number of carbon atoms present in the chain (x, y, and z in the reaction scheme) depend on the triglyceride source. The Problem Many students entering chemistry labs for the first time lack confidence in manipulating matter at a molecular level. *Corresponding author.
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A key aspect of their laboratory training is to develop their ability to observe changes with their senses. Therefore, a qualitative experiment was needed to start them on their journey of discovery. The new experiment needed to test their observation skills and their understanding of at least one aspect of chemistry. The Plan The synthesis of soap via the hydrolysis of animal fat is a typical experiment encountered in a high school or college organic chemistry laboratory and has been described in many soap-making articles in this Journal (5–14 ). The standard experiment hydrolyzes vegetable shortening and gives a reliable white soap (15). We recognized that different fats should give different soaps. Since the physical characteristics of a soap are derived from its fat source, a “designer” soap experiment using a vast array of triglycerides met our earlier criteria for experimentation. This paper describes the exotic soap experiment and presents actual class feedback after its implementation. The assignment for my organic chemistry laboratory class was to read the following experimental procedure and to bring in their own “unique” triglyceride source, which would be saponified into a “designer” soap. Experimental Procedure Fats and oils are supplied by the students. The fats used (listed in Table 1) range from an exotic Vietnamese garlic oil to the grill sludge from a local restaurant. Five grams of sodium hydroxide was dissolved in 40 mL of a 50/50 water– 95% ethanol mixture. CAUTION: Aqueous sodium hydroxide solution is corrosive and it is very dangerous to the eyes. Skin burns are possible. Appropriate safety precautions must be observed. Students should be supervised by the chemistry teacher when carrying out this procedure. This alkaline solution was combined with 10 g of the fat source in a 250-mL beaker. The mixture was heated by partially submerging the 250mL beaker in a larger beaker of boiling water for 45 min. During this heating period the mixture was stirred frequently and 40 mL of additional water–95% ethanol solution was added to make up for evaporation of the solution. The saponification mixture was poured directly into a cooled solution of 25 wt % aqueous sodium chloride. The mixture was stirred vigorously and allowed to cool to room temperature. The precipitated soap was collected by vacuum filtration and washed with ice-cold water. The solid was allowed to air-dry and was then inspected for color, texture, and smell. The results are listed in Table 1. Each soap generated suds when dispersed in water. Quantification is problematic, as diverse mixtures of triglycerides are often present in natural sources of fat. It is
Journal of Chemical Education • Vol. 75 No. 5 May 1998 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
Table 1. Designer Soap Experimental Data Fat Source (Color)
Color of Soap
Texture
Smells like...
Butter (yellow)
beige
waxlike, hard
waxy
Peanut butter (tan)
cream, brown specs
smooth
burnt peanuts
Peanut oil (light yellow)
white
flaky
oatmeal
Sunflower oil (yellow)
vanilla white
smooth, creamy
bread dough
Walnut oil (brown)
cream, beige
smooth, creamy
oily
Vietnamese garlic oil (brown)
yellow
thick paste
strong, oily
Sesame oil (caramel)
yellow
large granules
faint sesame
Olive oil #1 (yellow)
ivory
crumbly grains
wax crayons
Olive oil # 2 (light yellow)
white
flaky
paper maché
Vegetable oil (yellow)
creamy white
powderlike
potato chips
Corn oil (light yellow)
beige
sandy
Playdough
Local restaurant grill sludge (brown)
brown
chunky
plastic
Bacon grease (beige)
white
fluffy flakes
rancid, burnt butterscotch
Provolone cheese (white)
white
paste
sweet butter
Steak drippings (white)
white
chunky
milk
virtually impossible for students to calculate the exact moles of starting triester and to purify each unique carboxylate salt product. In addition, the precise fatty acid makeups of the more exotic triglyceride sources are unknown. Therefore, students do not quantify the yields from each fat source, but use their powers of observation to deduce the properties of their own unique soap. Results and Discussion Product evaluation via sensory inspection not only hones observation skills but allows the student to proceed without fear of the “dreaded” low yield or low product purity constraints. The outcome of each designer experiment is unique. Since the physical characteristics of the soap product are directly related to the fat source selected, students were in control of their own destiny. In a sense, their fate was sealed by their choice of fat. Because students were held accountable for explaining their results, many important questions were asked, such as: 1. Why does my soap not smell like what I started with? 2. Why does my soap crumble, whereas my neighbor’s soap is hard? 3. Why is one soap yellow and another white or brown?
4. Why does my soap stink and the one at home smells so good?
Students were directed to consult the following table (Table 2) in explaining their results. These questions led to class discussions of how chemical changes such as esters being converted to carboxylate salts (soaps) often result in changes of texture, color, and odor. At first glance, texture differences seem to be related to units of unsaturation (compare Tables 1, 3). For example, butter has a high percentage of saturated fats and gives a very hard waxy soap, presumably due to the presence of large amounts of saturated carboxylate salts. This is in contrast to the oils listed in Table 3, which contain higher percentages of unsaturated fats and give more flaky, easily crumbled soaps. It is possible that the observed soap textures may also be influenced by occluded impurities. Further studies are necessary to be conclusive. Surprisingly, there was no obvious relationship between triglyceride unsaturation and soap color in these samples (Tables 1, 3). Sunflower oil, with nearly 90% unsaturated fat, gave a vanilla-white soap. Butter, with only 35% unsaturated fat, gave a beige soap. A priori one may have expected yellowing of the unsaturated carboxylate salts after air dry-
Table 2. Fatty Acid Composition of Selected Fats and Oils Weight % of Total Fatty Acids Fat or Oil
Saturated < C10 C12 – C16
Monounsaturated
C18
Other
C16,C18
Other
Polyunsaturated C18
Other
Butter
9.2
41.0
12.5
2.5
30.1
1.2
Beef tallow
0.1
28.9
21.6
3.0
42.1
1.1
Bacon
0.1
26.4
12.3
0.9
48.2
1.6
10
Olive oil
–
13.7
2.5
0.9
72.3
–
10.6
–
Corn oil
–
12.2
2.2
0.1
27.6
–
57.9
–
Sunflower oil
–
7.5
4.7
0.4
18.7
–
68.7
–
Peanut oil
–
2.3
0
51.0
–
30.9
4.8
11
3.4
0.1
2.8
0.4 0.5
N OTE: Data are from ref 16.
JChemEd.chem.wisc.edu • Vol. 75 No. 5 May 1998 • Journal of Chemical Education
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Chemistry Everyday for Everyone
ing. As the yellowing process may be time dependent, future studies will monitor soap color changes during further aging. Odor changes were perhaps the most striking finding for the students. Only one of the soaps (sesame) smelled like the starting fat or oil. As shown in Table 1, student descriptions ranged from smells like “burnt peanuts” and “milk” to “plastic” and “crayons”. While their descriptors may not be precise, they were certainly diverse. Olfactory comparisons of the product and starting material revealed differences, which reflected the fact that odor changes are related to changes in chemical structure. Another possibility is that the initial odor was lost during the isolation of the soap. Olfactory inspection of the filtrates, however, did not support this premise. It was also mentioned that many esters smell nice, whereas acids are often more pungent and acrid. Since most of the soaps had unappealing smells, the students concluded correctly that commercial soap vendors must add fragrances to mask the true odor of their products. This assumption was confirmed by reading the labels on household bars of soap. In conclusion, this experiment was deemed the favorite lab of many who took the course. Moreover, the freedom to choose their own starting material introduced a well-received component of flexibility into an often rigid curriculum. In addition, the lab increased student awareness of the organic chemistry of soaps and sharpened their observation skills. Additional experiments are needed to further define the relationship between soap texture and the unsaturation levels of their component fatty acid salts. Literature Cited 1. Levey, M. J. Chem. Educ. 1954, 31, 521–524. 2. Mettler, C. C.; Mettler, F. A. History of Medicine; Blakiston: Philadelphia, 1947; pp 245–248, 363.
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Table 3. Saturated and Unsaturated Fatty Acid Content of Fats and Oils Fat or Oil Butter
Saturated (%)
Unsaturated (%)
Ratio, S/U
65
35
1.86
Beef tallow
54
46
1.17
Bacon
40
60
0.67
Olive oil
17
83
0.20
Corn oil
15
85
0.18
Sunflower oil
13
87
0.15
Peanut oil
13
87
0.15
N OTE: Data are from ref 16.
3. Magner, L. N. A History of Medicine; Dekker: New York, 1992; pp 260–267. 4. Ainsworth, S. J. Chem. Eng. News 1996, 74(4), 32–54. 5. Preston, W. C. J. Chem. Educ. 1925, 2, 1035, 1130. 6. Lowman, O. E. J. Chem. Educ. 1932, 9, 1809. 7. Evans, D.C. J. Chem. Educ. 1937, 14, 534. 8. Cook, G. A. J. Chem. Educ. 1938, 15, 161. 9. Preston, W. C. J. Chem. Educ. 1940, 17, 476. 10. Snell, F. J. Chem. Educ. 1942, 19, 172–180. 11. Nelson, A. F. J. Chem. Educ. 1948, 25, 379. 12. Bossert, R. G. J. Chem. Educ. 1950, 27, 10. 13. Mangold, M. C. J. Chem. Educ. 1951, 28, 266. 14. Hill, J. W.; Soldberg, S. J.; Hill, C. S. J. Chem. Educ. 1982, 59, 788. 15. Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Organic Laboratory Techniques; Saunders College Publishing: Orlando, FL, 1988; pp 112–114. 16. Food Fats and Health; Task Force Report No. 118; Council for Agricultural Science and Technology: Ames, IA, December 1991; pp 6, 58.
Journal of Chemical Education • Vol. 75 No. 5 May 1998 • JChemEd.chem.wisc.edu
