Soap Chemistry 101

I recently bought Kevin Dunn's book, Scientific Soapmaking. It was written both as a reference for soapmakers who are interested in learning about the chemistry behind their craft, and as a potential textbook for chemistry students. Having two such disparate audiences does cause some problems, however. Unless you have a basic chemistry background (recent high school chemistry class or a college introductory class), it has a lot of technical information that could make it a tough read. There are also quite a few “experiments” and demonstrations that most people would find tedious and unecessary. That said, I thought it was a fabulous book. Luckily I have a pretty strong background in chemistry (albeit many years ago), so I found it to be a fascinating read. There were quite a few “aha!” moments where observations I've made while soaping, or situations I've read about, suddenly were explained and made sense. The information I gleaned from this book is valuable to any soapmaker, so in the next few posts I plan on summarizing what I've learned, hopefully presenting it in a manner that is more accessible to non-chemists. Note to those who are comfortable with chemistry concepts: in my explanations below and future posts I will simplify some descriptions in order to avoid unnecessary confusion. Please keep that in mind! I'm not going to go into the particle/wave duality of electrons for example, or detail what an electron cloud is, as these concepts are not germane to this discussion. They are just going to be particles.

Before we can begin to talk about the chemistry of making soap, we need to understand the chemistry of water and oils. We all know water and oil don't mix, but why? Let's start with an examination of the chemistry of water. When we compare that to the chemistry of oils, the differences will explain why oil and water don't mix.

Water is made up of two hydrogen atoms and one oxygen atom. Each atom of a specific element (the basic building blocks of all chemicals; examples are hydrogen, carbon, oxygen, sodium, potassium, and nitrogen) has an equal number of particles called protons and electrons. Protons have a positive charge and are found in the nucleus of the atom, while electrons have a negative charge and are found in different levels (also called orbits) of the periphery of the atom. Because there are equal numbers of protons and electrons in an atom, the atom as a whole is neutral. Even though the atoms are neutral, most elements have atoms with enough physical space in their outermost level to accommodate more electrons, and this is the basis for how all elements combine with each other to form molecules. Every hydrogen atom has space for one more electron, and oxygen has enough space for two electrons. What happens is that two hydrogen atoms share their single electron with oxygen, which shares two of it's electrons with the hydrogen atoms. So the hydrogen atoms now appear to have all the electrons they can hold, and the same is true for the oxygen. When atoms bond together and share electrons, it is called a covalent bond. Oxygen, however, is a bit greedy. It is a much bigger atom than hydrogen, and doesn't share the electrons equally. So the electrons spend more of their time closer to the oxygen atom, giving it a slightly negative charge, while the hydrogen atoms end up with a slightly positive charge because their electrons are being hogged by the oxygen. When atoms share electrons unequally, this is called a polar covalent bond, since the molecule has a slight positive charge on one side, and a slight negative charge on the other. This polarity has a huge effect on the chemistry of the resulting molecule. In the case of water, the slightly positive hydrogens are attracted to the slightly negative oxygens of other water molecules, and will form weak bonds with them. This is called a hydrogen bond. It is not as strong as a bond that holds the atoms in molecules together, but is strong enough so that water molecules are very attracted to each other. Hydrogen bonds are responsible for the fact that water seems to form a “skin”, which is why you can fill a glass so that the water actually bulges up above the rim. Hydrogen bonds are important in more than just water too. Hydrogen bonding is responsible for much of the physical shape of a protein molecule, for instance. It takes energy to break hydrogen bonds. This is why sweating cools you down. The hydrogen bonds must be broken before the liquid water can evaporate off your skin, so energy in the form of heat is drawn from your body to power the evaporation.

Fats and oils, on the other hand, are made up almost entirely of chains of carbon and hydrogen atoms. Carbon and hydrogen atoms also form covalent bonds, but they are non-polar covalent bonds. The electrons are shared equally between the carbon and hydrogen atoms, so the entire molecule is neutral – there is no area of positive or negative charge. Each carbon atom is looking for four extra electrons, so it will share an electron with up to four other hydrogen atoms.

There is a third type of molecular bond that we will eventually need to know about, and that is the ionic bond. In ionic bonds, one of the atoms has the ability to completely steal an electron from the other atom. When you dissolve a substance with an ionic bond, the atoms will separate, with one atom (or group of atoms) stealing an electron from the other. Table salt, for example, is made of sodium and chlorine atoms, and has the chemical formula NaCl. When you dissolve salt in water, the sodium and chlorine dissociate to form sodium ions and chlorine ions. Ions are atoms that are missing or have an extra electron attached to them. Hence the name, “ionic” bond. These ions are very reactive.

You may have heard the saying “like dissolves like.” Water, having polar covalent bonds, can dissolve substances with polar covalent or ionic bonds. Oils, on the other hand, can't dissolve in water, because it has non-polar covalent bonds. The oily dirt on our skin can't be easily washed off with just water, because it can't dissolve in water.

This is where soap comes in. The chemistry of a soap molecule is such that one end of it has a polar covalent bond, while the tail is make up of carbon and hydrogen atoms that have non-polar bonds. The oily dirt on our skin dissolves in the non-polar part of the molecule, while the polar end dissolves in water, blending or emulsifying the oil/water mixture, and allowing the dirt to be washed away.

That's enough for this post. This information gives us the background to understand the structure of fatty acids, our next “lesson” in soap chemistry.