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.
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.
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.
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.
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
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.
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.
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!
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.
Answer the following questions in a typed document.