olive.oil.soap.molecule (1K)
Figure 1

Soap Making

Introduction

Soap is an essential part of our everyday lives. Keeping our clothes, our dishes, and our bodies clean is important for our health and comfort. There are many kinds of soaps but they all share certain features in common. Soap molecules consist of long chains of carbon atoms which have at one end an organic acid group. This acid group is missing the hydrogen atom which would make it acidic and instead is accompanied by a sodium or potassium ion. The chemical structure of a single soap molecule, with its accompanying sodium ion, is depicted in fig. 1, at right.

Water.Bead.Showing.Intermolecular.Forces (92K)
Figure 2

A soap molecule functions in two important ways. First of all, it serves as a so-called surfactant. Surfactants are substances which reduce the intermolecular forces of attraction of water when they are dissolved in the water. Usually this is explained in terms of surface tension: a droplet beads up due to the attractive forces between water molecules (see fig. 2). The attractive forces within a water droplet are asymmetrical at the surface: there are no forces pulling water molecules outward from the surface, only inward. When a surfactant reduces the forces of attraction between water molecules it allows the water to spread out and more effectively spread across a surface. In fig. 3 the water droplet on the left has some soap dissolved in it and has spread out much farther than the tap water in the droplet on the right. This is called ‘wetting’ the surface and is the reason why surfactants are also called wetting agents. When water beads up on the surface of, for example, a fabric, it may not make it wet. When it spreads out due to the presence of a surfactant, the fabric gets wet. The water can then carry away water-soluble materials.

Water.Droplets.with.and.without.Soap (106K)
Figure 3

The second way that soap molecules work to help make things clean is that they can mix both with water and with oils and fats. Fats and oils are collectively known as lipids. Lipids are well known for being insoluble in water. Their structure is non-polar and being very large there are strong forces holding molecules of fats and oils together. These forces keep them from separating as they would have to do to mix with water. Soaps, however, can dissolve both in water and in the lipids from which they are made. This is due to the fact that soap molecules have an ionic end, capable of dissolving in water, and a carbon chain capable of dissolving in lipids. In water, soap forms micelles. These are structures in which many soap molecules cling together with their carbon-chain tails inside and their ionic heads outside. In fig. 4 a micelle is shown in cross-section: these are actually spherical structures. A few water molecules have been included to represent the surrounding liquid. In addition to micelles, soap molecules can form films which are much like the lipid bilayer that makes up the membrane of a living cell. In the case of a film the molecules line up next to one another to make extended structures with their tails together and their heads toward the water. This is why soap can make long-lasting bubbles. When soapy water comes into contact with greasy material the molecules of the greasy material become trapped inside of micelles and films. In fig. 5 a micelle is illustrated which has two triglyceride molecules in its center. The surrounding water washes the combination away when the soap is rinsed off.

Micelle (274K)
Figure 4

Soaps are made from naturally occuring fats and oils. These include soybean oil, palm oil, coconut oil, olive oil, beeswax, lard, and dairy fat among many others. Naturally occuring fats have widely varying structures but usually consist of three copies of a long carbon chain (known as a fatty acid), each of which is bound by an ester bond to a molecule of glycerin (C3H5(OH)3, see fig. 6). Lipid molecules with this structure are known as triglycerides or triacylglycerides and they perform a wide variety of functions in a living organism. An illustration of a triglyceride with three oleate acyl groups can be seen in fig. 7. Layers of cells containing large droplets of fat molecules can serve as both insulation and a back-up source of energy. In addition, fat molecules aid in the storage of biologically important molecules which are not soluble in water.

The formation of a triglyceride comes about through a process known as esterification. Ester bonds connect a molecule with an acid group (COOH) to a molecule with an alcohol group (C—OH) by eliminating water between them. When an ester bond forms it does so by binding the acidic hydrogen atom from the acid to the alcohol group while the oxygen from the acid group pushes the alcohol group off of the molecule. When triglycerides are mixed with a strong alkali such as sodium or postassium hydroxide (NaOH or KOH) a reaction called saponification occurs. This reaction is more or less the reverse of esterification. The hydroxide ion (OH) binds to the carbon atom on the fatty acid which sits between the two oxygen atoms.

Micelle.surrounds.2.triglycerides (226K)
Figure 5
This makes for one too many bonds for that carbon atom and results in the breaking of the bond between that carbon atom and the single-bonded oxygen atom that links the fatty acid to the glycerin backbone. A hydrogen atom is transferred from the fatty acid to the glycerin. After this sequence occurs for all three acyl groups (the fatty acids) the result is three units of the sodium salt of the fatty acid and one molecule of glycerin.
glycerin (5K)
Figure 6

Knowledge of the details of soap synthesis has led to the technological development of detergents. Soaps are made from naturally occuring lipids and have just one kind of ionic head. Detergents are substances with similar structure and function to soaps but they may be cationic (positively charged), anionic (negatively charged), or non-ionic. Soaps work best when the temperature is warm or hot. Also, soaps will react with calcium (Ca2+) or magnesium (Mg2+) ions to form an insoluble solid. On bathroom tile this is called soap scum and when it affects clothing it makes the fabric appear dull and gray. Detergents are engineered to avoid the pitfalls of soaps so that they function at low temperature and do not form insoluble solids with magnesium or calcium ions found naturally in water.

In this lab you will be making a few small bars of soap by doing a saponification reaction with soybean oil, coconut oil, olive oil, and sodium hydroxide. The oils are gently warmed to melt the coconut oil, which is high in saturated fatty acids. Mixing sodium hydroxide into water releases a lot of heat, so that solution will initially be very hot. The two mixtures will be allowed to cool to about 45°C and then mixed with constant stirring. The use of a magnetic stirrer reduces the

triglyceride (1046K)
Figure 7
amount of labor involved. After the reaction mixture begins to thicken scented essential oils may be added. If coloring is desired, this may be added at this point as well. After being poured into a mold the soap will harden overnight.

Most oils contain exclusively triglycerides so it should be simple to calculate the number of moles of sodium hydroxide needed to fully saponify the oils. However, different types of oil require different amounts of sodium hydroxide based on their molar mass, which can be difficult to determine because all natural oils contain a mixture of different triglycerides, each with its own molar mass. By experimentation and analysis of the structure of the oils a so-called saponification number is assigned to each oil. This number can be used to find the right amount of alkali to saponify that oil. Online calculators make determining the amount of sodium hydroxide quick and easy, even for mixtures of different oils. The amount of sodium hydroxide has been calculated carefully (using soapcalc.net) so precise measurements are critical. The amounts specified will result in a small amount of unreacted lipids by design. This is called the ‘super fat’ or ‘lye discount’. It is common to have a lye discount (a reduction of the amount of sodium hydroxide used below the stoichiometric amount) of 5% to 8%. By ensuring that the oil is the excess reagent you can be sure that the sodium hydroxide will have been completely used up. Sodium hydroxide is a strong alkali and is much too harsh for use directly on skin. The excess oil also aids in conditioning skin when the soap is used becuase the oils coat the skin and help prevent the loss of water from skin cells. In addition to the cautious excess of fat you will ensure good quality soap by using a curing period of 2 - 3 weeks. This allows moisture to evaporate and gives more time to allow the saponification reaction to go to completion, eliminating any residual sodium hydroxide and finally making the soap ready to use. Without this waiting period it is possible that the soap will dissolve too quickly in water when used and so be used up faster. Saponification is usually complete quite soon after mixing if the mixing is thorough but if there happen to be pockets of material with excess sodium hydroxide the waiting period will help to ensure that it has a chance to react.

The saponification reaction appears to be an exothermic reaction and the heat released in the process helps to speed the reaction. The equilibrium constant for the reaction is very large and though it is exothermic its value does not reduce by very much with increasing temperature. The reason the reaction is carried out with warmed oils is mainly in order to control the rate of the reaction which is several times faster at 45°C than at room temperature (around 20°C). At 45°C the reaction is fast enough to be nearly complete after a few minutes but slow enough that there is time to add color and scent ingredients. This is why the soap making is not usually done at much higher temperatures: it would be so fast that it would be difficult to add color and scent.


Pre-lab Questions

Answer these questions before coming in to the lab. If you do not know how to make the drawing or answer the question then that is a hint that you should go find out. You have learned some chemical drawing skills in class: use these. Talk to your teacher. Do some research to find out. Do not simply find information and copy it down. Find a few articles, decide which ones have the best quality information, read the whole thing and write your answer from your own (new) knowledge.

  1. Draw a cartoon picture of a thin film of soap molecules showing how the carbon-chain tails stick together while the ionic heads dissolve in water.
  2. Using your understanding of intermolecular forces explain how mixing soap molecules with water reduces the strength of intermolecular forces in the mixture as compared to those within water alone.
  3. Draw the structure of a triglyceride molecule based on the fatty acid in figure 1 in the introduction.
  4. Draw the saponification reaction. First, show how hydroxide ions bind to the carbon atom in the C=O double-bond. Next, show how the bond between that same carbon atom and the oxygen atom connecting it to the glycerin backbone is broken, leaving the pair of electrons that made the bond as a third lone pair on the oxygen atom. Finally, show how the hydrogen atom which was originally part of the hydroxide ion is transferred from the fatty acid to the negatively charged oxygen atom on the glycerin backbone, forming an alcohol group. You may use the symbol —C17H 33 to represent the carbon chain of the fatty acid but clearly show the structure of the acid group.
  5. What are the dangers of using sodium hydroxide? What physiological effects does it have on skin? On eyes?
  6. How does the design of this lab activity work toward reducing the risk of having excess sodium hydroxide in your soap?
  7. Based on your knowledge of chemistry what class of chemicals will be best suited to the removal of soap scum? Acids? Bases? Oxidizers? Reducers? These are all chemicals part of common cleaning materials. Explain your answer.
  8. Find the structure of a detergent. Give its name and draw it. Describe how it avoids one of the pitfalls of soaps made from natural lipids.
  9. Some simple scents and colorants will be made available to you for this lab. If you would like to contribute your own some ideas include: cocoa powder for color and vanilla for scent, cinnamon for color and scent, and turmeric or paprika for color with an essential oil (such as sweet orange) for scent. Do some research online at soap-making hobby web sites and find out a few other ideas and write them down in answer to this question. And bring something in if you would like to use it!

Materials

  1. 33.33 g soybean oil
  2. 33.33 g coconut oil
  3. 33.33 g light olive oil
  4. 14.41 g NaOH
  5. 28.0 mL water
  6. 1 g titanium dioxide (optional)
  7. 1 - 2 g other colorant (optional)
  8. 16 - 24 drops essential oil (optional)
  9. Lab Balance
  10. 2 250-mL beakers
  11. 1 100-mL beaker

  1. 50-mL graduated cylinder
  2. thermometer
  3. electric hot plate
  4. magnetic stirrer with stir bar
  5. metal scoop
  6. plastic cup as soap mold
  7. oven mitt
  8. lab goggles
  9. nitrile gloves

Safety

Procedure

Be aware of the chemical hazards as you follow this procedure. If done carefully there is little chance of harm. If carried out carelessly permanent injury could result. Have fun!

  1. Measure 33.33 g of soybean oil into a 250-mL beaker. Add 33.33 g of light olive oil. Add 33.33 g of coconut oil, which (unlike the other oils) is a solid at room temperature due its composition, which has a larger percentage of saturated fats.
  2. Warm the oil mixture on a hot plate at a low setting to about 45°C. Do not exceed a temperature of 50°C. If you do then you must allow it to cool off below this tempmerature before proceeding. This will melt the coconut oil and allow the mixture to be warm enough for a rapid saponification reaction.
  3. Measure out precisely 14.41 g of sodium hydroxide in a 100-mL beaker. Take care in handling the solid as contact with skin can cause injury and the dust is an eye and respiratory tract irritant. Do not exceed 15.1 g of sodium hydroxide as that would be the full stoichiometric amount.
  4. Measure 28.0 grams of water either by volume (28.0 mL) or by mass and pour it into a 250-mL beaker.
  5. Use a glass stirring rod to constantly stir the water while adding small portions of your measured amount of sodium hydroxide. Do not add all of the sodium hydroxide to the water at once! The water will get hot but will not get out of hand as long as you only add a few pellets at a time. As soon as the first few are nearly dissolved you may add a few more. Keep stirring and adding pellets until all of the NaOH has dissolved.

    Note that adding all of the NaOH at once will cause the solid to fuse into a large mass that will be almost impossible to dissolve. In addition, it risks a runaway release of heat which can cause injury.

  6. Place the beaker of warmed oil on a magnetic stirrer and add a stir bar. Set to stir quickly but without splashing.
  7. The NaOH solution will be ready to use when it has become completely or nearly completely clear and when there is no more evidence of solid pieces of material. Add it all at once to the warmed oil mixture.
  8. Watch the mixture carefully while continuing to stir rapidly. Note the stages the reaction goes through. Note how it thickens as the reaction proceeds. The soap still needs to be mixed for 10 - 20 minutes. Stir by hand using a sturdy metal scoop rather than a glass stirring rod if the magnetic stirrer is not available.
  9. The reaction mixture will gradually thicken as the reaction proceeds and water evaporates. When it is thick enough that a small portion of the mixture dribbled on top leaves a trace on the surface you have reached the point where you may add colors and scents. You may add up to 24 drops of essential oil (you may mix scents) or small amounts of scented materials you may have brought from home. Stir thoroughly to mix.
  10. If you wish to add titanium dioxide white pigment to your soap to cover the yellow color of the oil, do it now. Measure out 1 g using a lab balance and add it all at once while letting the magnetic stirrer mix the pigment into the oil. Other colors may be used instead or alongside the titanium dioxide. Use only a gram or two.
  11. Once you have finished mixing it is time to put the soap in a mold. Appropriate molds include old yogurt or pudding containers or plastic weighing boats from the lab. Make sure to pour the soap into a mold before it solidifies.
  12. The next day your soap may be hard enough to take out of the mold. If the shape allows it, this is a good time to slice the soap into convenient bars. Once cut, set the soap out to cure. Or bring it home with you to cure in a cool, dry place with plenty of ventilation. After 2 - 3 weeks the soap will be dry and ready to use. It can be used sooner but it will not last as long since the water mixed in with the soap will make it dissolve quickly.




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Post-lab

Answer the following questions in a typed document.

  1. Evaluate the procedure. Were there any circumstances that you wish you had been told about before-hand? Was there anything that did not work as described? How would you improve this lab?
  2. Describe the process of soap making from a chemist’s point of view. What chemical reactions occur?
  3. Thermodynamically, the saponification reaction is exothermic. That is, it releases heat. However, since the equilibrium constant is so large, the reduction in its value with increasing temperature (according to the Le Châtelier Principle) is not important. Consider how rates of reaction are affected by an increase in temperature and explain why the reaction in this lab is carried out at 45°C rather than at room temperature.
  4. Describe the soap making process based on your own observations. Decide what the key steps and turning points are. Describe them and relate them to your understanding of the chemical transformations that are taking place.
  5. What two consequences are there to using the soap too soon after it is made? In other words, why is there a curing period for the soap?
  6. Glycerin is a by-product of soap production. Was it possible to recover the glycerin from this procedure? Where is the glycerin? Explain.
  7. Glycerin is a valuable chemical for synthesis of useful materials. Find two materials that use glycerin as a starting point in their manufacture. Describe each product, its uses, and how it is made.
Soap recipe calculations were carried out using the calculator at SoapCalc.net.
Here is the recipe.
A useful source of information about soap are the books The Soapmaker’s Companion: A Comprehensive Guide with Recipes, Techniques & Know-how by Susan Miller Cavitch, Storey Communications, Inc., 1997.
Castile Soapmaking by Anne Watson, Shepard Publications, 2016.
An aid to my thinking about the thermodynamics and kinetics of saponification was a paper by some students in a college chemistry course, including Tylar Farmer.
Last updated: May 22, 2019       Home