Basic Soap Chemistry, Part 1

Perhaps I'm a little biased, but chemistry has never been a difficult subject for me.  It's fascinating how different elements or compounds combine to create brand new things.  (Full disclosure, I went to a specialized high school and concentrated my studies in chemistry, including a year of research.  I also published a scientific article about my research my senior year.  I followed this by going onto college to major in chemistry.  In addition, both of my parents have chemistry degrees, as do several other members of my family.  So "scary" chemistry has never actually been scary for me.)

But I also understand that it can be a little tricky figuring out exactly how different things will combine and in what quantities.  Then add in trying to produce a specific product that is safe for use on your delicate skin.  Maybe that is where the scary part starts to kick in!

Don't worry, I'll break it down a little into (hopefully) easier concepts.  

Lye (Sodium Hydroxide)

This is the big one.  Lye, or sodium hydroxide, is one of the compounds that are required to make soap. It typically comes in a solid form, but may be flakes or pellets.  All of them work the same way.  It's a very strong base, and is therefore highly caustic in a pure form.  This means that it can severely burn your skin or eyes if it comes into contact with them.  For this reason, I handle lye with extreme caution, and wear proper safety equipment (gloves, an apron, safety goggles, shoes, hair tied back, and long sleeves) when using it.  These safety precautions are really not optional, and are part of what makes soap making a scary idea for many people. However, if you handle lye cautiously, it is no more dangerous than household drain cleaners.  


We dissolve the solid lye in water to break it down into its constituent atoms.  Sodium hydroxide (NaOH) is a highly polar molecule, meaning that is has naturally positive and negative "sides," similar to a magnet. The sodium side (Na+) donates it's extra valence electron to the hydroxide side (OH-), making each part of the molecule charged.  Since this happens easily, this substance readily dissolves in water. Water has this same property, so they mix well together.  Chemists call this the "like dissolves like" principle, and it also explains why oil and water don't mix.  Oils are non-polar molecules, and have no true positive or negative areas.  Our lye water is now really water with sodium and hydroxide ions swirling around in it.  This will make it easier for the mixture to fully incorporate into our oils.  The creation of this solution is an exothermic one, meaning that it produces heat. 


The last ingredient we need to make soap is a fat, usually in the form of oil(s).  The choice of oil (or combination of oils) is completely up to the soap maker.  Generally, the soap maker chooses specific oils for the qualities they will bring to the soap.  For example, some oils help to create a harder bar, others may increase bubbles and later, and other may add moisturizing benefits.  To determine what your specific preferences are, you will need to try several different recipes.  Be sure to keep good notes so that you can remember how you achieved your desired results!  If you are stumped on where to find a beginner recipe, I suggest looking at the Soap Queen blog.  Anne-Marie has many different recipes posted, and you should be able to find one that suits your needs. 

Lye + Water + Oils = Soap

Perhaps this seems like a bit of a simplification, but this really is the basic formula for soap making.  

So how do we get numbers for this formula?  First we need to figure out what container, or mold, we will fill with our finished soap and how big it is.  We measure our ingredients by weight, not volume.  After we have a mold size, we need to have a recipe for the oils.  I feel that it is best to start out with percentages of each oil.  For example, you might have a soap that has 40% olive oil, 30% coconut oil, and 30% palm oil as the total oils.  

Take these percentages and go to what is called a Lye Calculator.  Here are two examples:

Bramble Berry Lye Calculator

SoapCalc Lye Calculator

In each case, you will need to choose "percentage" and then input the percentages beside each individual oil.  Take an educated guess at the total weight of the oils based on how much your mold will hold.  It's generally around two-thirds of the final weight your mold will hold.  You can change this later to get a better recipe to fit your mold.

These calculators also have a spot to enter a superfat level.  Superfatting is adding in some extra oil to be doubly sure all lye molecules are completely neutralized and the soap will be safe to use.  It also means that there will be extra oil left in the finished soap to help nourish your skin.  You don't want it to be too high though, because it can the soap make go rancid over time.  I suggest a 5% superfat to start, then you can decide how you like it and adjust from there.  

Once you hit calculate, the next page will show your recipe, with water and lye amounts.  It will also show a total weight.  Compare this to the weight your mold will hold and make adjustments if needed.  

These are the very basic ideas behind soap making.  Obviously there is much more to it, and we will delve deeper into it next week.  Join us then as we explore more of the science behind the suds!

Leave a comment