Fatty acids are the most important chemicals found in soap. They are what makes soap soapy. I talked about fats in the last post, but haven't talked about acids. Because fatty acids are both fats and acidic, I'm going to start by taking a little detour to explain acids and bases (lye is a base, and you already know you can't make soap without it!).
Acids are chemicals that can donate a hydronium ion to another substance in a reaction. We already met the hydronium ion in the last post – it is a hydrogen atom that had it's electron “stolen”. Because a hydrogen atom has only one proton and one electron, a hydronium ion is simply a lone, positively charged proton. A base is a chemical that can accept a proton in a chemical reaction. Sodium hydroxide has the chemical formula NaOH. It has one sodium atom, one oxygen, and one hydrogen atom. The oxygen and hydrogen are bonded together with a covalent bond; it is called a hydroxide group when part of a larger molecule. The hydroxide group has an ionic bond to the sodium. When dissolved in water, the sodium dissociates from the hydroxide. The hydroxide group steals an electron from the sodium, so the dissociation forms sodium and hydroxide ions. It is the hydroxide group that can accept a proton, since it is negatively charged due to the "stolen" electron. Sodium hydroxide is a strong base, which means it completely ionizes when dissolved in water. Potassium hydroxide (also called lye) works the same way. When acids and bases react, they form a salt and water. It may seem hard to believe, but if you mix just the right amounts of sodium hydroxide and hydrochloric acid (which is a strong acid), you will get plain old salt water – table salt (NaCl) and water. It is the reaction of an acid and a base that is the basis of the saponification reaction – but more about that later. The pH scale is a measure of the concentration of hydronium ions in a solution. The lower the number, the more concentrated it is with hydronium ions, and the more acidic. The pH scale ranges from 1 to 14, with 7 being neutral. Bases have a pH greater than 7. The higher the number, the more basic the chemical is. Soap, of course, is a base. We expect our soap to have a pH of 8 to 10 if it is safe and not lye-heavy.
Fatty acids are molecules that have a carboxylic acid group on one end of the molecule. This means they have a positively charged hydrogen ion bonded to a negatively charged oxygen ion. The oxygen is also bonded to a carbon atom, and the carbon atom also shares a double bond with another oxygen atom. A double bond is when two atoms share two electrons, rather than just one. This is the acidic end of the molecule, as it can donate a proton to another atom or molecule. If you remember from the last post, carbon is looking to share four electrons, and so far we have accounted for only three: the single bond to the oxygen with the hydrogen, and the double bond to the other oxygen. This is where the fatty tail of the molecule is found. Again, a fat is simply a chain of carbon atoms bonded to each other with attached hydrogen atoms to fill up the need for four shared electrons. Each carbon is generally bound to two other carbons, and shares electrons with two hydrogen atoms. This is called a saturated fat, since no more hydrogen atoms can bond to the molecule. In some fatty acids, one, two, or three of the carbons shares a double bond with a carbon next to it, so each carbon involved in the double bond is bonded to only one other hydrogen. This is called an unsaturated fat, because it is possible to break the double bond and add two hydrogen atoms to the molecule for each double bond it has. These double bonds will become important in a later post. And yes, when you are reading a nutrition label this is exactly what it is talking about when it lists the amount of saturated and unsaturated fats. I'll be talking more about saturated and unsaturated fatty acids later as well.
So a fatty acid has a carbon-hydrogen tail attached to the carbon of the carboxylic acid group. Remember, carbon and hydrogen form non-polar covalent bonds, while hydrogen and oxygen form a polar covalent bond. The two ends of the molecule exhibit very different chemistries. The difference between fatty acids is due to the number of carbons in the fatty tail, as well as the presence of any double bonds between the carbons. The smallest fatty acid is acetic acid – its fatty acid tail contains only one carbon. Because the tail is so short, it is relatively soluble in water, and acts much more like an acid than a fat. We all know acetic acid as vinegar. The longer the carbon chain, the less soluble the fatty acid is in water.
The saturated fatty acids we are familiar with in soap making are lauric acid, myristic acid, plamitic acid, and stearic acid. Lauric acid has the shortest carbon chain, so it is the most soluble in water. Soaps with a lot of lauric acid (coconut and palm kernel oils) will produce a lot of lather quickly because of this. Stearic acid is the longest chain, so is least soluble. Soaps high in stearic acid take more time to lather (palm, lard, and tallow) and the lather is not as fluffy.
The unsaturated fatty acids common to soap making are oleic acid, ricinoleic acid, linoleic acid, and linolenic acid. All of these contain 18 carbons in the chain, so the difference between them is how many double bonds they have. Both oleic and ricinoleic acid have one double bond; ricinoleic acid has a hydroxide group attached to a carbon near the double bond. The length of the chains means that these fatty acids are less soluble than all of the saturated fatty acids except stearic acid, which also has 18 carbons in the chain. Therefore they will lather slowly with the exception of ricinoleic. The polar hydroxide group on the chain increases it's solubility significantly, giving soaps that contain it lots of quick, fluffy lather.
Now we are ready to make soap. Next up is the chemistry of the saponification reaction.