Introduction: The Science of Soap Making in a Lab
Making soap doesn't seem like something you'd do in a lab, but it's actually more scientific than you'd think. Saponification is the soap making process, which uses the basic solution lye and different types of fats. The science behind soap making is in the structure of the fats, the properties of the lye, and the chemical reaction that produces cleaning molecules. Not only is it a process that uses science, but it's also just a fun activity to make your own soap with the properties that you want.
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.
Step 1: Ingredients
The first step is to select your ingredients. For the soap that we made, we used coconut oil, shea butter, olive oil, and canola oil.
The type of fat that you use impacts the properties of the soap that they produce. For example, coconut oil produces more of a hard soap while olive oil makes it extremely soft. This is due to the qualities of the fat. Qualities on a molecular level such as chain length and the amount of saturatedness determine their melting points.
For example, a fat that is saturated will have no double bonds in it. It will be a straight chain of molecules with a strong intermolecular attraction. The stronger the intermolecular attraction, the higher the melting point of that fat is because it takes more energy to break those intermolecular bonds.
If it has a high melting point, the fat is likely to be a solid at room temperature. If a fat is polyunsaturated, meaning that there are multiple double bonds within it, it will probably have a curved shape due to those double bonds. The curved shape makes it harder for there to be intermolecular attraction and for the fat chains to be stacked close to each other, which means that the fat has a low melting point and will most likely be a liquid at room temperature. Similarly, the longer the length of the fat chain, the higher the melting point and the more solid it will be. With all of this in mind, having a good balance of fats that will give you a solid or liquid soap is important when choosing your ingredients.
You can use an online soap calculator (click here) to find the amounts and types of fat that you want to use in order to yield the right properties of soap such as hardness, cleansing, and how much lather or bubbliness it gives.
We will also be using lye in this lab. Lye is a strong, basic solution that can be used for cleaning and that is used in the process of making soap. The two most common types of lye used in soap are potassium hydroxide and sodium hydroxide. When we made our soap, we used sodium hydroxide as our lye.
Step 2: Additional Ingredients
You can also add additional ingredients that don’t affect the quality of the soap, but make it more pleasant, such as herbs, essential oils, and textures. You can use things like coffee grounds or oatmeal to add texture. Other things like herbs and essential oils can add pleasant scents. You can also add coloring through natural products like turmeric or food coloring.
We added lavender essential oil for scent, chamomile for texture and scent, and red food dye for a nice pink color.
Step 3: Calculations
There has to be a specific amount of lye added to the soap, and in order to know how much lye has to be used, you have to first know how much of each type of fat you are using.
- The soap calculator that you used earlier to determine your ingredients will tell you your SAP value, which is how many grams of lye are necessary to react with one gram of fat. Once you have your SAP value and the total amount of fats in grams, then you can calculate the amount of lye you will need. First, multiply the weight of the fats by the SAP value.
- For example, if I had 500 grams of fats total and a SAP value of 0.128, then I would multiply 500 by 0.128 and that would give me the amount of lye that I need in grams. In the example, I would need 64 grams of lye.
- However, if you don’t have a pure amount of concentrated lye and you have it in a molar solution instead, then you have to find out how many moles are in your amount of lye. In this lab, we used sodium hydroxide as our lye. If I need 64 grams of sodium hydroxide, then I can convert grams to moles by dividing the amount in grams by the molar mass of sodium hydroxide, which is 40 grams per mole. 64 divided by 40 gives you 1.6 moles of sodium hydroxide.
- If you have a molar solution of, for example, 10.3 moles per liter, then you can divide 1.6 moles by 10.3 moles to get how many liters of solution you will need. You would need 0.15 liters of solution, or 155 mL of solution.
- Through this calculation process, you can find out how many grams of lye you will need, or if you have your lye in a diluted solution, how many mL of solution you will need to use.
Still there? Great. It’s necessary to calculate and use the correct amount of lye because if you use too much of it, then the soap will have leftover lye in it and be basic, which can cause skin irritation. However, if you don’t have enough lye, then the soap will not really be effective in cleaning because it has excessive unsaponified fats and oils.
Step 4: Preparation
Lye, which in our case was sodium hydroxide, is extremely basic, reactive, and harmful to human skin. It causes severe burns and eye damage. When handling it, wear safety goggles, gloves, long-sleeved shirts, and use any precautions that would ensure that it doesn’t touch your skin and eyes. Furthermore, if any spills occur, you should use vinegar to neutralize the spill.
- The lye solution should be in a separate container away from your workspace. You want to measure out the appropriate amount in a beaker that you can bring to your workspace and use.
- The equipment that you use may vary, but you will need a beaker, a hot plate, a thermometer, soap molds, and a stir stick or several magnetic stir bars. The beaker will hold the mixture that will become the soap and the hot plate will be used to heat up the beaker and melt the fats. The thermometer will measure the temperature of the mixture to make sure it is within an acceptable range while the magnetic stir bars and stir stick are to aid in mixing the mixture. The soap molds that you use can be made from simply cutting off the bottom of a shampoo bottle. You can basically use anything that you would want your soap to be shaped like as long as it can hold the soap while it solidifies.
- The soap calculator that you used earlier to find the right ingredients as well as the SAP value will also help you make the recipe for how much of each product you will need. Gather those ingredients and measure them out by weight using a scale. This will ensure that you use the right amount of each ingredient.
Step 5: Melting Process
- Using the thermometer to keep tabs on the temperature, mix all of the ingredients together in the beaker and melt them over the hot plate until they are all in their liquid state. You can stir the mixture to quicken the pace.
- Use the thermometer to make sure that the temperature of your mixture of fats and oils doesn’t exceed 49-54 degrees Celsius, or 120-130 degrees Fahrenheit. This ensures that when you add the lye, the mixture will not be overly hot.
- If you do exceed the necessary temperature, don’t worry. Just wait for the temperature of the mixture to drop down to the right temperature before proceeding to add lye.
Step 6: Adding the Lye
- When the temperature is between 49-54 degrees celsius, slowly add the calculated amount of lye.
- Clean out the containers that you used for the lye with acetic acid, or vinegar. The lye is extremely dangerous because of how basic it is. If vinegar, which is something that is extremely basic, is added to the containers it will help bring the pH to a more neutral level, which will then be safe enough to handle and clean.
When you add the lye to the mixture of fats, the saponification process starts. The fats within the mixture all have the structure of a triglyceride, which is three fatty acid chains attached together by a glycerol. However, when lye is added, it changes the structure of the triglyceride during the reaction and the produce a glycerol and soap. The reaction goes as (if using sodium hydroxide): one molecule of fat reacts with three molecules of sodium hydroxide to produce one glycerol and three molecules of sodium stearate, or soap.
Sodium stearate is a soap, which is a surfactant. Surfactants are compounds that reduce surface tension when dissolved in water or an aqueous solution. They are able to do this because one end of a surfactant is polar, which makes it soluble in water, while the other end is nonpolar, which means that it will not dissolve in water. This nature of surfactants makes it easier to wash away dirt or oil.
Step 7: How to Know When It's Done
- Mix the mixture with a stir stick or magnetic stir bars. It’s important for it to be properly mixed until it starts to become even more thick. It will most likely take 20-40 minutes for the soap to be thoroughly thickened.
- The pH of a solution is how acidic or basic it is. It’s important to test the pH of the soap to make sure that it is neutral and not too basic or acidic. You can use pH test strips to determine the pH, which should be between 5 and 9 to be safe.
- You’ll know that it’s done when the sides start to bubble and collapse in on themselves. The surface will be bubbly and thick.
Step 8: Finishing Up
- Now is the time to add your additional ingredients. The mixture has already gone through most of the saponification process so it’s safe to add additional ingredients because they won’t affect the important reaction taking place.
- Quickly pour or spoon the soap into the molds.
- Wait at least 24 hours for the soap to harden and cure. However, the longer you wait, the better the final product will be because the soap has had even more time to settle and solidify.
Step 9: Reflection
Compared to regular soap, the soap that we made in the lab was, in my opinion, similarly effective. It was slightly oily to the touch, but when used and mixed with water, it created a medium lather. It was hard enough to be considered a "bar" of soap (although the shape is more like a muffin), but soft enough to dig your nails in (if you're into that sort of thing). The selection and percents of ingredients that we chose gave us good properties, most likely because they were within the recommended range of qualities the soap calculator suggested.
Compared to another group’s soap, our soap was softer and more oily to the touch while theirs looked and felt more like traditional soap. Ours did smell better, though. Their recipe included 50% crisco and 50% coconut oil with a total of 340 grams. Our recipe had a total of 500 grams of fats, with a range from hard to soft oils/fats. Although our soap recipes were different and the properties vary, the soap that we made was effective, smelled semi-pleasant (as pleasant and home-made soap can smell), and was overall a success.