Synthesizing a "Designer" Soap
The molecular structure of a typical soap is shown below
The molecule consists of a long nonpolar hydrocarbon chain bonded to a highly polar COO- group. The cleaning action of soap arises from the fact that the nonpolar end readily dissolves in nonpolar substances such as oil and grease. The polar end readily dissolves in water, a polar substance. In the washing process, greasy dirt particles are dislodged from the object being washed, and become surrounded by soap molecules. This isolates the particles from one another, prevents them from clumping together, and allows them to be carried away by the wash water.
Soaps are soluble in water. However, in hard water soap forms a scum or precipitate, which is sometimes evident as a "bathtub ring". The origin of this precipitate is the reaction of soap with magnesium and calcium ions which are present in high concentrations in hard water.
Soaps are made from the reaction of fats, derived from either animal or vegetable sources, and a strong base such as sodium or potassium hydroxide. For example:
In the example, tristearin, a fat of animal origin reacts to produce sodium stearate, a soap.
The general formula of a fat is
In tristearin R is C17H35 and its structure is
Tristearin, is an example of a saturated fat. In a saturated fat all of the C-C bonds in the R group are single bonds. Another example of a saturated fat has R with the structure CH3CH2CH2 . This fat is found in milk and butter. Animal fats contain mostly saturated fats. Vegetable fats, on the other hand,are mostly unsaturated fats. In an unsaturated fat the R group contains C-C double bonds. For example, in an unsaturated fat found in olive oil, the R group is CH3(CH2)7=CH(CH2)7 with the structure:
Animal fats are solids, whereas vegetable fats are usually liquids.
Common substances which are high in fat include: bacon grease, lard, vegetable shortening, butter, cheese, steak drippings, peanut butter, and cooking oils such as olive oil and corn oil. Does this mean we can make soap from bacon grease? YES!
The purpose of this experiment is to synthesize a "designer" soap using the fat of your choice, to observe its properties, and to compare them with the properties of soaps made by your fellow students using other fats. If you like peanut butter, here's an opportunity to see if you like the soap made from peanut butter.
Synthesis of soap: To a 250 mL beaker add 10 g of the fat source and 40 mL of an sodium hydroxide-ethanol-water solution (containing 5 g of sodium hydroxide in 40 mL of a 50/50 water-95% ethanol mixture). Heat the mixture by partially submerging the 250 mL beaker in a larger beaker of boiling water for about 45 min. Use a hot plate to boil the water. During the heating process some of the ethanol will evaporate. Add 50/50 water-95% ethanol solution as necessary to maintain a constant volume of liquid in the beaker. Upon completion of the heating step, allow the reaction mixture to cool to room temperature. Add 100 mL of a saturated solution of NaCl to the reaction mixture, and stir the mixture vigorously with a stirring rod. The precipitate which forms is soap. The remaining mixture contains water, glycerol, ethanol and excess sodium hydroxide.
Filter the precipitated soap using a Buchner funnel and filtering flask, and wash it once with ice water. Dry the soap as completely as possible.
Testing the properties of soaps
Test 1: Observe and record the properties of your soap such as texture, smell, appearance, etc. Observe and record the same properties of soaps prepared from other fat sources by your fellow classmates. Record your observations in a table for easy reading. Be sure to specify the fat source for each soap.
Test 2: Behavior in hard water: Dissolve about 1 g of your soap in 50 mL of warm water in a 100 mL beaker. Pour about 5 mL of the solution into each of three test tubes. To one test tube, add a few drops of a 5% CaCl2 solution. To the next one, add a few drops of a 5% MgCl2 solution, and to the last one add a few drops of a 5% FeCl3 solution. Let these solutions stand until you have finished the rest of the experiment. Clearly record your observations.
Test 3: Washing effectiveness: Spread a thin coat of the original fat on a watch glass and test the ability of your soap to wash it off. How effective is your soap? Try the same test with several other soaps obtained from your classmates. How do they compare with your soap? Record your observations.
In your conclusion answer the following questions using complete sentences.
1. Summarize the properties of your soap observed in Test 1.
2. Discuss and explain your results with hard water in Test 2. Compare and contrast the results obtained with CaCl2, MgCl2 and FeCl3.
3. Compare the washing effectiveness of your soap with all of the others tested.
4. Based on your observations in Tests 1 and 3, which fat source produces the best soap? Which one produces the worst one? Please explain.