In the Laboratory
Making Usable, Quality Opaque or Transparent Soap
W
Suzanne T. Mabrouk Department of Chemistry, The Citadel, The Military College of South Carolina, Charleston, SC 29409;
Over the years this Journal has featured articles on the history (1, 2) of soap, the industrial process of saponification (3–5), and laboratory-ready experiments (6–13). The experiments and those in organic laboratory textbooks have described the hot (6–10, 14–16) and cold (10, 11) processes for making opaque soap, eq 1, O O − +
R R
O
O MOH
− +
R⬘
O
O M
+
HO
+
M = Na, K
O R⬙
HO
+
O R⬘
O M
O O
fats and oils (triacylglycerols)
(1)
R⬙
− +
O M
soap (carboxylates)
HO glycerin
and the conversion of commercially-available, opaque soaps into transparent (12) or solid potassium soaps (13). In the experiments, typically one fat or oil (7–9, 11, 14– 16) is saponified with a stoichiometric excess of sodium hydroxide (7, 8, 10, 14–16). In the hot process, excess base is usually removed by filtration and rinsing of the soap precipitated by the addition of aqueous sodium chloride (6–8, 14, 15). Alternatively, Evans suggests adding coconut oil or dilute hydrochloric acid to the melted soap, exposing thin pieces of solid soap to air, or beating the melted soap (10) to remove the excess base. In the latter two methods the base reacts with the CO2 in the air, eq 2: 2NaOH(aq) + CO2(g) → Na2CO3(s) + H2O(l)
(2)
Hill (9) and Lehman (16) rinse the filtered soap with water. As soap containing excess base will irritate the skin (10), students have been prohibited from using the prepared soap. Students will be able to use soaps from this experiment as they contain 5% excess fat. This experiment introduces students to the industrial method of making opaque (formulas 1–8) or transparent soap (formulas 10 and 11) and formulations chemistry (formula 9). It can accompany lectures on personal care products or lipids in a first-year or organic chemistry course. The procedures can be used independently or, as one reviewer noted, as a multiweek project. Over three weeks, students can make opaque soap in one two-hour period or transparent soap in one three-hour period, and formulate a soap and test its effectiveness alongside commercial surfactants. Opaque and transparent soaps are made using the cold and “semiboiled” processes, respectively. In both processes, premelted fats and oils are combined with base. In the “semiboiled” process, heat is applied. Complete saponification 1534
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occurs in one week in the cold process, and a couple hours in the “semiboiled” process. The byproduct glycerin is retained in both methods. As some individuals find soap dehydrates the skin, the presence of the humectant glycerin in the finished soap is advantageous. Soap Formulation
Fats and Oils Commercial manufacturers use 1–7% excess fat (17) to reduce the harshness of soap, to produce a dense creamy lather, and to leave the skin feeling smooth and soft (18). The formulations in this article use 5% excess fat. Effective soaps are made from a mixture of fats and oils because each component contributes different properties. These properties are determined by the type and percentage of the fatty acids in each fat or oil (19–23) (see Tables 1 and 2 in the Supplemental MaterialW). For example lauric acid, the predominant fatty acid in coconut oil (48.0%), produces a hard and effective cleanser with a fluffy lather. Unfortunately, lauric acid does not condition the skin nor provide a stable lather; therefore other fatty acids are warranted. Table 3 in the Supplemental MaterialW gives the percent maximum recommended usage of fats and oils based on therapeutic benefits (24), cost, and fatty acid content. Beeswax (25) and coconut and palm oils (19) harden an otherwise soft soap. Manufacturers preferentially use coconut and palm oils because they are cheap and they produce an effective cleanser with fluffy lather (19). Base, Water, and Scent After selecting the fats and oils, the mass of required sodium hydroxide is determined from the saponification (SAP) value of the soap (26, 27). The SAP value of the soap is calculated from the sum of the product of the SAP value of each fat or oil and the percent of this component in the soap (see Table 3 in the Supplemental MaterialW) (20, 22, 23, 28). Because SAP values are defined as the number of milligrams of potassium hydroxide per one gram fat or oil (29), the SAP value of the soap is converted to mass of potassium hydroxide per gram fat or oil and subsequently multiplied by the total mass of fats and oils. Since potassium hydroxide is typically used to make liquid soap while sodium hydroxide is used to make solid soap, the required mass of potassium hydroxide is converted to mass of sodium hydroxide. Because commercial soap contains excess fats and oils to moisturize the skin, the mass of sodium hydroxide is multiplied by 0.95. The mass of deionized water and scent are determined by multiplying the total mass of fats and oils by 0.375 and 0.0125, respectively; if the manufacturer of the scent provides a usage rate, then this value should be used instead. Additional Ingredients Deionized water is used to make soap because it prevents the reaction of newly formed soap with hard water ions
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In the Laboratory
(17, 30); this unwanted reaction would lower the yield. Deionized water also minimizes discoloration caused by impurities. Milk in soap moisturizes the skin and produces a mild, creamy, and rich consistency (31). Based on fat content, whole milk (4%), half-and-half (10.5–18%), light cream (18– 30%), light whipping cream (30–36%), and heavy whipping cream (36–40%) are commonly used (32). In this article, whipping cream is used for its high fat content. When replacing all water with milk, the milk must be frozen prior to the addition of base (31), otherwise proteins in the milk denature causing a tan-to-brown-colored soap (33). When using small quantities of milk, as in formulas 2, 4, 6, and 8, the milk is added following saponification. To reduce the formation of fibrous crystals and to make transparent soap, castor oil, ethanol, glycerin, and sugar are added to the hot soap solution (34). The molded soap is also cooled slowly in the open. Surfactant Tests The prepared soap and commercial surfactants are tested for pH, feel, and effectiveness (ability to lather) in hard and soft water. The transparency of formulas 10 and 11 is confirmed by reading text written with a 14-point font through a 1/4 in. slice of soap (35, 36). Experimental Procedure
Chemicals Majestic Mountain Sage (37) and From Nature With Love (38) supply most fats and oils. Craft stores sell beeswax
and stearic acid. Supermarkets sell canola, corn, olive, safflower, and sunflower oils, lard, sugar, and whipping cream. Note that the coconut and palm oils should be melted prior to use, as they separate on cooling. Replace any fats and oils that burn on melting.
Cold-Process Soap (Formulas 1–8) Combine and stir the deionized water and half of the sodium hydroxide (Table 1) in a 250-mL beaker on a magnetic stirring plate. After the solid has dissolved, add the remaining sodium hydroxide. Stir and heat all fats and oils until melted in a 400-mL beaker on a magnetic stirring hot plate. Slowly pour the base solution into the beaker of fats and oils and stir until “trace” is observed. To test for trace, dip a stirring rod into the mixture and wave it over the surface. If the drizzled material leaves a pattern on the surface before sinking, the soap has traced. Add scent and cream and stir the mixture until trace is reached again. Pour the soap into molds or weighing boats (either four, 3 in. × 3 in.-square boats or six, 2 5/8 in. × 3 in.-hexagonal boats). Place the molds in an insulated chest for one week so that the evolving heat will harden the soap. Student-Formulated Cold-Process Soap (Formula 9) After selecting the fats and oils and performing the necessary calculations, prepare the soap as described in cold-process soap. Transparent Soap (Formulas 10 and 11) Combine the fats, oils, and alcohol in a 500-mL threenecked round-bottom flask equipped with a condenser and two ground-glass stoppers. Place the flask on a heating mantle
Table 1. Required Ingredients for the Cold-Processed Soaps Formula
Ingredients
1
2
3
4
5
6
7
8
Deionized water
75.00
40.00
75.00
40.00
75.00
40.00
75.00
40.00
Sodium hydroxide
24.70
24.70
29.60
29.60
29.20
29.20
29.50
29.50
Beeswax
10.00
10.00
———
———
———
———
———
————
Canola oil
18.00
18.00
———
———
———
———
60.00
60.00
Castor oil
18.00
18.00
———
———
20.00
20.00
40.00
40.00
Coconut oil
———
———
80.00
80.00
80.00
80.00
———
———
Base Solution
Fats and Oils
Corn oil
18.00
18.00
———
———
———
———
40.00
40.00
Lard
———
———
———
———
80.00
80.00
———
———
Olive oil
100.00
100.00
40.00
40.00
———
———
———
———
Palm oil
———
———
80.00
80.00
———
———
———
———
Stearic acid
———
———
———
———
———
———
20.00
20.00
Safflower oil
18.00
18.00
———
———
———
———
———
———
Sunflower oil
18.00
18.00
———
———
20.00
20.00
40.00
40.00
Scent
02.50
02.50
02.50
02.50
02.50
02.50
02.50
02.50
Whipping Cream
———
35.00
———
35.00
———
35.00
———
35.00
Other
NOTE: All quantities are in units of grams.
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In the Laboratory
on a magnetic stirring plate. Stir and heat the mixture until the fats have melted (Table 2). To make the base solution, stir the deionized water and half of the sodium hydroxide in a 250-mL beaker on a magnetic stirring plate. Following dissolution, add the remaining sodium hydroxide. Slowly pour the base solution through the condenser into the round-bottom flask. Using a permanent marker, draw a line on the outside of the flask to indicate the level of liquid. If the level drops, add more ethanol slowly through the condenser to bring the level to the line. Stir the mixture at reflux for two hours. Table 2. Required Ingredients for the Transparent Soaps
Ingredients
Formula 10
11
Fats and Oils Castor oil
30.00
30.00
Coconut oil
45.00
45.00
Olive oil
15.00
15.00
Palm oil
45.00
60.00
Shea butter
15.00
———
Deionized water
50.00
50.00
Sodium hydroxide
22.48
22.68
Deionized water
24.52
24.10
Sugar
27.94
27.79
Absolute ethyl alcohol
60.37
60.37
Glycerin
37.26
37.05
Scent
03.73
03.73
To make the sugar solution, stir and heat the deionized water and sugar in a 250-mL beaker on a magnetic stirring hot plate until the sugar dissolves. Cover the beaker with a watch glass to minimize evaporation. Following reflux, pour the sugar solution through the condenser into the round-bottom flask. Add glycerin through a neck on the flask. Stir the solution at reflux an additional ten minutes and then allow it to cool to 60 ⬚C. Add scent and food coloring (drop by drop to desired shade) through a neck on the flask. Stir the solution a few minutes and pour it into molds or weighing boats (six 3 in. × 3 in. square boats). To promote transparency, leave the molds in the open. Within a few hours the soap will harden.
Surfactant Tests Measure the pH of the prepared soap, commercial surfactants, the student’s skin, and deionized water with pHydrion paper. Wash hands with a bar of each prepared formulation and compare the quantity of lather, the size of the bubbles, and the feel. Test the effectiveness of the prepared soap and commercial surfactants1 in hard and soft water with 4% aqueous calcium chloride and 9% aqueous sodium phosphate tribasic, respectively. Test for transparency.
Base Solution
Sugar Solution
Other
Hazards Sodium hydroxide is caustic and can cause serious burns. On contact, flush the affected area with water. Because hot oils will burn, avoid contact. Since ethanol and essential and fragrance oils are flammable, keep away from flames. People who are sensitive to coconut oil should not prepare formulas 3–6, 10, or 11. Results and Discussion Since soaps are alkali salts of fatty acids, they were found to be basic. The skin and deionized water were found to be slightly acidic. None of the soaps lathered in hard water, whereas the “combars” did. Combars are combination bars composed of soap and synthetic detergent. All surfactants lathered in soft water. Formulations 10 and 11 produced transparent soap (Figure 1). Generally students liked the feel of the prepared soap. In one organic section, where students devised their own formulation, the majority of the students preferred a soap made from 20% almond, 20% avocado, 35% coconut, and 25% olive oils.
NOTE: All quantities are in units of grams.
Conclusion
Figure 1. Student-prepared transparent soaps. (This image is shown in color on p 1427.)
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According to anonymous surveys, students enjoyed making and testing the soap. From this experiment they learned the chemistry of soap, which helped them in the lecture course: “I learned how soap actually works and how it is chemically formed.” Another student “learned that it takes many oils to be combined together to get soap.” “I learned what transparent soaps are actually made of, and how easy it is to make it.” The students appreciated the freedom to select the scent for their soap and for those who devised a unique formulation, enjoyed the experience and opportunity to create something of their own: “I enjoy making something using chemistry that I can actually use.” The students were especially glad to take their finished soap home and to use
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In the Laboratory
it. “Any time you can use or take home what you make in class you will be more interested in it.” In formulas 1–9, some students had difficulty recognizing trace. With all formulations, students wished the soap would “trace faster”. Trace is reached in five to twenty minutes with formulas 1–9 whereas two hours are needed for formulas 10 and 11.
19. 20.
Acknowledgments I thank the students who tested the formulations; the Chemistry Department, which provided funds; the spring 2004 organic chemistry students whose soaps appear in the photographs; Russell K. Pace who took the photograph; and Kevin Metzger who designed Figure 1. W
18.
21.
22.
Supplemental Material
Detailed instructions for students and notes for the instructor are available in this issue of JCE Online.
23.
Note
24.
1. Commercial surfactants used in this experiment will be classified as soaps, syndets, or combars based on the ingredients: soaps are alkali salts of long chain fatty acids; syndets, or synthetic detergents, are long alkane chains containing at least one functional group; and combars or combination bars contain both soaps and syndets.
Literature Cited
26.
27.
1. Levey, M. J. J. Chem. Educ. 1954, 31, 521–524. 2. Kostka, K. L.; McKay, D. D. J. Chem. Educ. 2002, 79, 1172– 1175. 3. Preston, W. C. J. Chem. Educ. 1925, 2, 1035–1044. 4. Preston, W. C. J. Chem. Educ. 1925, 2, 1130–1139. 5. Snell, F. D. J. Chem. Educ. 1942, 19, 172–180. 6. de Mattos, M. C. S.; Nicodem, D. E. J. Chem. Educ. 2002, 79, 94–95. 7. Phanstiel, O., IV; Dueno, E.; Wang, Q. X. J. Chem. Educ. 1998, 75, 612–614. 8. Preston, W. C. J. Chem. Educ. 1940, 17, 476–478. 9. Hill, J. W.; Solberg, S. J.; Hill, C. S. J. Chem. Educ. 1982, 59, 788. 10. Evans, D. C. J. Chem. Educ. 1937, 14, 534–536. 11. JCE Editorial Staff. J. Chem. Educ. 1999, 76, 192A–192B. 12. Hill, J. W.; Hill, C. S. J. Chem. Educ. 1980, 57, 372. 13. Nelson, A. F. J. Chem. Educ. 1948, 25, 379. 14. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques: A Small-Scale Approach, 1st ed.; Saunders: New York, 1998; pp 134–135. 15. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques: A Contemporary Approach, 3rd ed.; Saunders: New York, 1988; pp 113– 114. 16. Lehman, J. W. Multiscale Operational Organic Chemistry: A Problem-Solving Approach to the Laboratory Course; PrenticeHall: Upper Saddle River, NJ, 2002; p 511. 17. Ghaim, J. B.; Volz, E. D. Skin Cleansing Bars. In Handbook of Cosmetic Science and Technology; Barel, A. O., Paye, M.,
www.JCE.DivCHED.org
25.
•
28. 29.
30.
31.
32. 33.
34.
35. 36.
37. 38.
Maibach, H. I., Eds.; Marcel Dekker: New York, 2001; p 493. Piso, Z.; Winder, C. A. Soap, Syndet and Soap/Syndet Bar Formulations. In Soap Technology For The 1990’s; Spitz, L., Ed.; American Oil Chemists’ Society: Champaign, IL, 1990; p 217. Cavitch, S. M. The Chemistry of Soapmaking. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; p 238. Davidsohn, J.; Better, E. J.; Davidsohn, A. The Fatty Raw Materials: Introduction. In Soap Manufacture; Interscience Publishers: New York, 1953; Vol. I, pp 206–211. Use of Natural Fats and Oils in Cosmetics. In Bailey’s Industrial Oil and Fat Products, 5th ed.; Hui, Y. H., Ed.; Wiley & Sons: New York, 1996; Vol. 5, pp 366–367. Altman, P. L.; Dittmer, D. S. Fats and Oils: Properties and Composition. In Biology Data Book, 2nd ed.; Federation of American Societies for Experimental Biology: Bethesda, MD, 1972; Vol. I, pp 348–353. From Nature With Love, Mango Butter. http://www. fromnaturewithlove.com/soap/product.asp?product_id=butmango (accessed Jun 2005). Cavitch, S. M. An Overview of Soapmaking Oils. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; pp 95– 118. Cavitch, S. M. Answers to Soapmakers’ Most-Asked Questions. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; pp 201–202. Gupta, S. Soap, Chemistry, Chemical and Physical Properties & Raw Materials. In Soap Technology For The 1990’s; Spitz, L., Ed.; American Oil Chemists’ Society: Champaign, IL, 1990; pp 57–58. Cavitch, S. M. The Chemistry of Soapmaking. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; p 246. Cavitch, S. M. The Chemistry of Soapmaking. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; p 247. Milwidsky, S. Analytical Tests on Soapmaking Raw Materials. In Soap and Detergent Technology; HAPPI: Ramsey, NJ, 1980; p 16. Ghaim, J. B.; Volz, E. D. Skin Cleansing Bars. In Handbook of Cosmetic Science and Technology; Barel, A. O., Paye, M., Maibach, H. I., Eds.; Marcel Dekker: New York, 2001; p 494. Makela, C. Preparing To Put Milk into Soap. In Milk-Based Soaps: Making Natural, Skin-Nourishing Soap; Storey: Pownal, VT, 1997; pp 31–32. McGee, H. On Food and Cooking: The Science and Lore of The Kitchen; Fireside: New York, 1997; p 7. Cavitch, S. M. Answers to Soapmakers’ Most-Asked Questions. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; p 190. Jungermann, E. Specialty Soaps: Formulations and Processing. In Soap Technology For The 1990’s, Spitz, L., Ed.; American Oil Chemists’ Society: Champaign, IL, 1990; p 233. Cavitch, S. M. Making Transparent Soaps. In The Soapmaker’s Companion; Storey: Pownal, VT, 1997; p 79. Failor, C. All About Soap. In Making Transparent Soap: The Art of Crafting, Molding, Scenting, & Coloring; Storey: Pownal, VT, 2000; p 7. Majestic Mountain Sage Home Page. http://www.the-sage.com/ (accessed Jun 2005). From Nature With Love, Skin Care Ingredients. http:// www.from-nature-with-love.com/soap/ (accessed Jun 2005).
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