Saponification and Soapmaking Background: Soapmaking has been carried out for thousands of years, long before the process was understood at the chemical level. Virtually any kind of fat or oil can be converted to soap by combining it with an aqueous base. In its simplest form, soapmaking is a very “green” process: the starting materials can be produced naturally and sustainably, there are no waste products formed, soap is easily biodegraded, and the process requires little to no energy input. Vegetable oils and animal fats are both comprised of triglycerides: molecules of glycerol that are esterified to three long chain carboxylic acids (fatty acids). When triglycerides are combined with an aqueous base such as NaOH or KOH, hydrolysis of the triglyceride esters occurs in a process called saponification (figure 1). The product of this reaction is soap, which contains the salts of the fatty acids and free glycerol. Figure 1: Saponification The salts of the fatty acids are what give soap its characteristic properties. While the carboxylate group interacts well with water, the long hydrocarbon chains of the fatty acids do not. In water, fatty acid salts form micelles which solubilize grease and grime and form a monolayer on the surface of water, which lowers its surface tension making the water “wetter”. Bubbles are formed when a very thin layer of water becomes sandwiched between two monolayers of fatty acid salts. The mechanism for saponification is shown in figure 2. First, the hydroxide ion attacks the carbonyl carbon producing a tetrahedral intermediate with a negatively charged oxygen atom. Next, the electrons on this oxygen reform the carbonyl causing the alkoxide ion to leave the carbonyl carbon. This step is unfavorable, because strong bases such as R-O¯ are very poor leaving groups. However, the two resulting species – an alkoxide ion and a carboxylic acid – rapidly exchange a proton in a very favorable acid-base reaction that makes the overall reaction favorable. Figure 2: Saponification Mechanism The triglycerides in fats and oils are comprised of many different fatty acids, so a fat or oil has no uniform molar mass. Because of this we cannot use a molar mass to calculate exactly how much NaOH we need to completely saponify a sample, without having excess NaOH present in our soap. Instead we use an experimentally determined value known as the saponification value. The saponification value is the number of grams of NaOH required to completely saponify all the triglyceride esters in one gram of a particular fat or oil. In this procedure, a 95% saponification value will be used so that no base remains in the soap and there is a 5% excess of unsaponified triglycerides. Required Materials: 1. Fats and oils 2. Plastic tray to use as soap mold 3. NaOH pellets (100% lye) 4. Deionized water 5. 400, 150, and 100 mL beakers 6. Glass stirring rod or stainless steel spatula 7. Hotplate 8. Small piece of crayon or food coloring (optional) 9. Fragrance (optional) Procedures: 1. Using a 450 mL Beaker, weigh out a combination of fats/oils so that the total amount is between 100-120 g. Record the mass of each fat/oil (not just the total) in the table below. Tare (zero) the balance after each fat/oil is added. Set this beaker aside. Note: most good recipes consists of fairly even mixtures of soft medium and hard oils. Use the following table to formulate a recipe of your choosing. Fat/Oil Mass of Each Fat/Oil (g) 95% Saponification Value1 Mass of NaOH (g) Canola Oil (soft) 0.126 Olive Oil (soft) 0.128 Suncoco Oil Blend (soft) 0.130 Crisco Shortening (med.) 0.131 Lard (med.) 0.134 Tallow (med.) 0.135 Palm Shortening (med.) 0.135 Coconut Oil (hard) 0.174 Total Mass of NaOH 2 Vol. Deionized Water 1 95% Saponification value for NaOH: this is the number of grams of sodium hydroxide to saponify 95% of one gram of this oil. To get total mass of NaOH you need, multiply the mass of each fat/oil (in grams) by the 95% saponification value and add up the values for all fats/oils. 2 Volume of deionized water should be approximately 1.5 times the mass of NaOH. For example, if you calculate 17.2 g of NaOH, you should use 17.2 x 1.5 = 26 mL of water. 2. Using the table above, calculate the required amount of NaOH and deionized water needed for the reaction using table above. Weigh the NaOH pellets out in the 100 mL beaker. Weigh out the deionized water in a 150 mL beaker. Slowly and carefully add the NaOH pellets into the 150 mL beaker containing the water and stir until dissolved. BE CAREFUL! The solution will get very hot as the NaOH dissolves and it will be very caustic (40% by mass, 10 M NaOH and pH 15!). Allow this solution to cool until it is no longer too hot to touch (you may use a cool water bath to speed up this process). 3. While the NaOH solution is cooling, heat the 400 mL beaker containing the fat/oil mixture on a hotplate using medium heat while stirring. You may add a small pea-sized piece of a crayon at this step to color your soap. Do not heat the oils too much! Heat only until all the oils and crayon (if used) are completely melted. Once melted, remove from the heat and allow mixture to cool until it is warm to the touch. 4. While stirring the oil mixture, slowly pour in the NaOH solution and continue stirring until the mixture just begins to thicken. This can take a few minutes or quite a bit longer. If you are stirring for longer than 15 minutes and do not observe any thickening, you may gently heat up the mixture slightly using a hotplate to speed up the process. Once you have a mixture that is about the consistency of paint, you are ready to pour it into the plastic mold. 5. If you are adding fragrance you should do so right before pouring and mix it well by stirring. WARNING: some fragrances will cause the mixture to rapidly harden, so work fast once you add the fragrance. Pour the mixture into the plastic mold provided and tap it a little on the bench top to smooth out the surface of the mixture. Until the mixture has completely hardened it is very caustic and should be handled with care. Set your tray aside in a safe place until the next lab meeting. Next lab: 6. Carefully pull the sides of the mold outward on all sides and gently push from the bottom of the mold to remove your soap. At this point it is ready to use, but it will become harder if you let it cure longer. Since your soap contains glycerol, it will be more hygroscopic (water absorbing) than many commercial soaps. Because of this, you should try to keep your soap dry when not in use. If you leave your bar of soap on a flat surface while it is still wet, it will absorb water and become very soft. Enjoy your soap! If you would like to make your own soap at home: First of all, be careful! Working with a strong base can be very dangerous and you can damage kitchen appliances, cabinets, your skin, eyes, etc. You should not allow anything to come into contact with anything that contains the dissolved NaOH. You may also want to have a large jug of vinegar handy in the event you need to neutralize a spill. Use only stainless steel and thick Pyrex glass containers for weighing and mixing. Use only stainless steel or silicone spatulas/spoons for mixing. The dissolution of NaOH in water can easily get hot enough to melt a plastic container or spoon! What you need (~price and where to buy): 1) Kitchen balance (~$15 at Bed Bath and Beyond with a $5 coupon or many other places) 2) Fats and oils (can buy anywhere) 3) NaOH – must be 100% lye drain opener (~$5 per lb. at most Ace hardware or Smart and Final stores. They also have it at Lowes, but it is much more expensive) 4) A stick blender is not necessary, but it makes the mixing process a lot easier! You can use a stovetop on low heat to melt your oils. Deionized water works best, but ordinary tap water will work just fine. The potential problem with tap water is that certain dissolved ions can precipitate out hydroxide ions making the NaOH less concentrated. There are many soap resources on the web, so do your research and have fun! .