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. This is one of many possible molecules, which have varying carbon-chain structures. This one is know as sodium oleate (NaC18H33O2).
A soap molecule functions in two important ways. First of all, it serves as a surfactant. Surfactants are substances which reduce the intermolecular forces of attraction of water when they are dissolved in the water. These intermolecular forces (IMF) bind molecules to one another in the liquid and solid states. They govern how well a substance will dissolve in a solvent such as water, too. The strength of the IMF that binds water molecules to one another is relatively high. And although soap molecules dissolve in water readily enough, water molecules do not bind as tightly to the soap molecules as they do to one another. As a result, the overall strength of forces between molecules in a soap solution is less than in pure water.
This can be understood in terms of surface tension: a droplet beads up due to the attractive forces between water molecules (see fig. 2).
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. This important because some compounds we wish to wash away from dirty hands or clothing is only fat soluble and are well known for being insoluble in water. Fats and oils are collectively known as lipids. Their structure is non-polar and being very large there are strong forces holding molecules of lipids together. These forces keep them from separating as they would have to do to mix with water. Soaps can dissolve in lipids. The long carbon chain is what makes a soap molecule capable of dissolving in lipids since both kinds of molecule have this structure in common. Soaps are, after all, made from lipids to begin with. In this lab you will make soap using vegetable oils. Soaps can also dissolve in water due to the fact that soap molecules have an ionic group at one end. Water is a great solvent for ions as water molecules bind more tightly to ions than they do to one another.
The reason this dual solubility helps soap to clean things is that soap molecules form 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. When soapy water comes into contact with greasy material the molecules of the greasy material become trapped inside of micelles and films. 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. In fig. 5 a micelle is illustrated which has two triglyceride molecules in its center. This is how soap dissolves lipids. When rinsed with water the micelle and its cargo of lipids is washed away.
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 (C3H5(C18H33O2)3). Biologically, lipids play many roles. In our own bodies, 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, such as vitamin D.
This lab activity is concerned with the organic synthesis of soap. This reaction has been carried out by people for millenia. At first, it may have been done by accident when alkaline ashes from a fire were mixed with fat. When natural oils are mixed with a strong alkali such as sodium or postassium hydroxide (NaOH or KOH) a reaction called saponification occurs. For example, the saponification of a triglyceride molecule with three oleate carbon chains (C3H5(C18H33O2)3) proceeds as follows, First, a hydroxide ion (OH–) binds to a carbon atom on the fatty acid. The carbon atom it binds to is the one which sits between the two oxygen atoms. This carbon atom now has three oxygen atoms bound to it in addition to it bond to the neighboring carbon atom. Next, the bond between that carbon atom and the oxygen atom attached to the glycerine backbone is broken. The other two oxygen atoms remain tightly bound. The oxygen atom left behind on the glycerin backbone has a negative charge because it takes the electrons that were in the bond with it when its bond to the fatty acid is broken. As a result it steals the hydrogen atom that was originally part of the hydroxide ion away from the now separated fatty acid, turning the fatty acid into its basic form. This happens for all three carbon atoms on the glycerin backbone, leaving plain glycerin (C3H5(OH)3) and 3 units of sodium oleate (NaC18H33O2).
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. The alkali you use will be sodium hydroxide, which you will need to dissolve in water. 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 chemistry here is fairly easy to follow conceptually but a bit harder to work with quantitatively. This is because of the variable structures of oils. Since natural oils are mixtures, there is no single molar mass that allows the calculation of the number of moles of sodium hydroxide needed to fully saponify them. All natural oils contain a mixture of different triglycerides, each with its own molar mass. Because of this it is not a simple matter to calculate the moles of triglyceride molecules and to work out from that the correct amount of sodium hydroxide. By experimentation and analysis of the structure of the oils a saponification number is assigned. This number is 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) and precise measurements are critical. The amounts specified in the procedure will saponify most of the oil molecules but will also result in a small amount of unreacted lipids. This is by design. The amount of unreacted oils 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 reactant you can be sure that the sodium hydroxide will have been completely used up, leaving none behind in the final soap. 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. 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. Without this waiting period it is possible that the soap will dissolve too quickly in water when used and so be used up faster. If this should happen anyway, remove the soap from service and allow it to dry for a few days before using it again.
The saponification reaction is 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 is not reduced 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.
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. Study the figures in the text for help and inspiration. Most of what you need to know to answer these questions can be found in the introductory text in this lab handout. If you do outside research, cite your sources, make sure they are legitimate, and do not copy and paste. Read and understand and write in your own words, always.
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!
Answer the following questions in a typed document.