Soap Chemistry 103

Before we get to the actual saponification reaction, there is one more chemical concept we need to explore. This is the ester. Remember from the last chemistry post that the reaction of an acid and a base produces a salt and water. Alcohols will also react with acids, but instead of producing a salt and water, they create an ester and water. This is a kind of condensation reaction, because of the formation of a water molecule from the original reactants. Esters are essentially insoluble in water, a property that we will come back to later. This is because the carbon/hydrogen tail is long enough that it counteracts the solubility of the head formed from the alcohol. If you mix an ester and a base, you get back the original alcohol and a salt in a type of hydrolysis reaction. If you look at the different parts of that word, you can see the hydro, meaning water, and lysis, which is to break apart. A hydrolysis reaction reinserts the water molecule lost in the condensation reaction to break apart the molecule. If the base is sodium hydroxide (lye) you will get a sodium salt. An important alcohol in soapmaking is glycerol, better known to soapmakers as glycerin. We'll talk much more about glycerin below.

Now that we've covered the chemistry of water and oils, acids and bases, fatty acids, and a little bit about esters, we're ready to tackle the reaction that creates soap: saponification. Though we talk about the fatty acids found in oils, in reality it isn't the fatty acids that are in coconut and olive oils, for example, but their esters. The fatty acid ester structure found in oils consists of three fatty acids attached to one glycerol molecule, formed by a condensation reaction. Chemists call these molecules triacylglycerides, the acyl part (or group) being the part that is formed from the fatty acids. The three fatty acids don't have to be all of the same type, either. Olive oil contains stearic, ricinoleic, linoleic, and linolenic fatty acids, and the triacylglyceride molecules can contain any combination of those fatty acids as the acyl parts of the molecule. The proportion, however, of all the acyl groups matches the proportion you find on Soapcalc for the fatty acids. You've heard of triacylglycerides as triglycerides, the very fatty molecules that you don't want a lot of in your blood. Trigylcerides, being esters, have non-polar covalent bonds and are not soluble, which explains why oils do not mix with water. I mentioned above that if you mix an ester and a base, a hydrolysis reaction occurs, resulting in a salt and the original alcohol. Therefore, when you add lye (sodium hydroxide), to your oils when making soap, the reaction forms glycerol and sodium salts of the fatty acids in the esters. That's the source of the glycerin in handcrafted soap. Fatty acid salts are soaps. Remember, the fatty acid has a water soluble head, and an insoluble tail. They are also alkalis, which have a basic (greater than 7) pH. Knowing how the saponification works also gives us the information on how to calculate the amount of glycerin formed from the reaction. For every three fatty acid molecules produced, one glycerol molecule will be produced. So the ratio of glycerin to fatty acid molecules is 1:3. You can actually calculate the weight of glycerin in a batch of soap. I refer you to Kevin Dunn's Scientific Soapmaking, pages 218 and 219 if you want to calculate the weight (or more correctly, mass) of glycerin in your finished soap.

We aren't finished with soap chemistry. To come later: how and why some FO's accelerate trace, why FO's often change in the finished soap, soaps versus detergents, soap scum, and those dreaded orange spots (DOS).

Soap Chemistry 102

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.

Fatty Acid Composition of Oils

Fatty Acids, Part 2

Here is a list of oils commonly used in soap making, their major fatty acid make up and contribution to the finished properties of soap:

Almond Oil:  Very high in oleic acid, so it contributes conditioning properties.  Low to moderate amount of linoleic acid, so don't use large amounts to avoid rancidity and DOS.

Avocado Oil:  High in oleic acid, moderate amount of palmitic acid, and low to moderate amount of linoleic acid.  Good conditioning, contributes a bit to hardness.  Don't use in large amounts to avoid DOS and rancidity.

Canola Oil:  High in oleic acid with a moderate amount of linoleic.  While conditioning, this oil would make soap prone to DOS and rancidity. Very little saturated fatty acids, so it will contribute to a soft bar of soap.

Castor Oil: Extremely high in ricinoleic acid, so it adds much conditioning and fluffy lather properties.  Contains no saturated fatty acids, so too much will make a soft, sticky bar of soap.

Coconut Oil:  Moderate amounts of saturated fatty acids, so it contributes to a hard bar of soap. High in lauric acid, which contributes to a rich, fluffy lather. Low to moderate amount of oleic acid.  The amount of saturated fa;tty acids mean that generally speaking, the more coconut oil in the soap, the more drying it will be.

Cocoa Butter: Total amounts of fatty acids are high (palmitic and stearic).  Would make a hard bar of soap with good conditioning due to the oleic acid.  Similar to lard. 

Crisco:  Moderate amounts of palmitic acid, with some stearic acid, so it will contribute to a harder bar. High in oleic, so it is also a bit conditioning. Moderate amount of linoleic, so too much will make the soap prone to DOS (it happened to me!).

Grapeseed Oil: Very high in linoleic acid, so more than a token amount will make the bar very prone to rancidity and DOS. The amounts of other fatty acids don't make up for the large amount of linoleic, so I'd stick with bath and body products for this one. 

Hemp Oil:  Very similar to grapeseed. Very high in linoleic acid, so it will tend to go rancid and promote DOS.  Even less of other fatty acids than grapeseed, so there is little to recommend it for soap.

Jojoba Oil:  The only fatty acid in joboba oil is oleic, and there isn't that much to begin with, so unless you are using it as a superfatting oil (it is roughly made up of 50% unsaponifiables, so you could assume only half will react with the lye), there is little it offers to soap.

Lard:  High in saturated fatty acids, so it will produce a hard bar, and very high in oleic acid, so it is conditioning.

Mango Butter:  Very similar to lard in ratio of saturated to unsaturated fatty acids, so it would make a  good veggie substitute. Hmmm....will have to try that!

Olive Oil:  Very high in oleic acid, so it is very conditioning. Low to moderate amounts of palmitic (mostly) and stearic acid,  so it will not create a very fluffy lather.  Moderate amount of linoleic, which probably explains why some 100% castile soaps get DOS.


Palm Kernel Oil/Flakes:  You want fluffy lather?  This is the oil for you. Very high in lauric acid, with moderate amounts of myristic and palmitic acids. Moderate oleic acid as well.  Be careful of using too much or the soap may be drying.

Palm Oil:  Similar in properties to palm kernel oil, though the major saturated fatty acid is palmitic acid, not lauric acid. High amounts of oleic acid, so it should be more conditioning than PKO. Low to moderate amount of linoleic acid.

Rice Bran Oil:  This is an interesting oil.  Moderate to high amount of palmitic acid, so it would contribute to a harder bar, very high in oleic, so it is conditioning as well, but unfortunately also moderately high amount of linoleic acid, so unless used in tiny amounts, it should promote DOS and rancidity.

Safflower Oil:  Extremely high in linoleic acid, with little else in its favor.  Just avoid it, unless you want to use it in bath and body products.

Shea Butter:  Very similar to mango butter, so therefore a good veggie substitute for lard. High in stearic acid (hard bar), and oleic acid (conditioning). A tad more linoleic acid than mango butter.

Soybean Oil:  Another one very high in linoleic acid.  Stick to bath and body products for this one.


There are many other oils out there, and Soapcalc has lots of other oils in its database.  Feel free to use the information from my last two blog posts to determine the contributions some of the more esoteric oils would make to your soap. Use this information, and Soapcalc (or other lye calculator), to develop soap recipes that make a soap with YOUR desired properties.  That's the art and science of making soap!

Feel free to copy and paste this information for your use, but please reference this page as your source.

Oil fatty acid compositions found at www.soapcalc.net

Fatty Acids in Soap

There are so many different oils we can use to make soap, choosing which oils to use in a new recipe can get really confusing to the newbie soapmaker (and some of us who've been around the block a couple of times too!).   Soapcalc (my favorite lye calculator, and topic of a future post), lists the fatty acid make up of all the oils in its database.  However, unless you know what each fatty acid brings to the party, the information is only so helpful.  Soapcalc does list the hardness, cleansing, conditioning, and lather properties of each oil. However, I find that as I develop new recipes to try out, I'm looking more and more often at the amounts of specific fatty acids in my oils, because I want to increase or decrease specific properties in my soap.

This post will describe the different fatty acids found in soap, and how they affect the properties of our finished soap.  The next post will look at some of the more common oils we use in soap making, and the major fatty acid make up of each one. I'll start by explaining some basic chemistry of fatty acids.  If high school or college chemistry gave you a headache, feel free to skip this next paragraph and go right to the list of fatty acids found in our soap making oils.

What IS a fatty acid?  As the name implies, it is both a fat and an acid.  The head of the molecule is a carboxylic acid,  and the tail is a chain of carbon and hydrogen atoms of varying length. There may be some double bonds between the carbon atoms, making the fatty acid unsaturated, or there may be no double bonds, making it saturated (it has as many hydrogen atoms attached as is possible). The number and location of the double bonds contributes to the flexibility of the molecule, and the melting temperature of the oil.  A non-scientific way of determining the relative saturation of oils is to see how solid the fat is at room temperature.  The more solid the fat, the fewer double bonds it has. Saturated fatty acids (like stearic acid and palmitic acid), are quite hard at room temperature, and have high melting temperatures, while highly unsaturated oils (like olive oil), are liquid at room temperature, and stay liquid until they get fairly cool.  Because of saturation/non-saturation, one can see that the more saturated the fatty acids in the oils and butters you use, the harder the finished bar of soap will be.  Unsaturated oils will add more conditioning properties to soap.

Fatty Acids Found in Soap and Their Properties

• Lauric Acid:  A saturated fatty acid that contributes to a hard bar, fluffy and stable lather, and high cleaning. Too much of this fatty acid can make a soap drying.
• Linoleic Acid:  An unsaturated fatty acid adds conditioning to the soap.  Oils high in this fatty acid tend to go rancid quickly, and more easily develop DOS (dreaded orange spots).
• Linolenic Acid:  Not to be confused with linoleic acid.  Adds conditioning properties and is very mild. 
• Myristic Acid:  A saturated fatty acid, so it adds to the hardness of the bar, fluffy lather, and cleansing ability.  Soaps high in myristic can be very drying to the skin.
• Oleic Acid:  An unsaturated fatty acid, so it adds conditioning properties.  This fatty acid does not contribute much to the lather of a soap, but it does contribute to the "slippery" feeling of soap.
• Palmitic Acid:  Another saturated fatty acid, so it will contribute to a hard bar and can by drying if too much is used.  It contributes to a creamy, rather than fluffy lather. 
• Ricinoleic Acid:  Unsaturated, it adds conditioning properties, and is great for a fluffy, bubbly lather.  Found almost exclusively, and in great quantities in castor oil. 
• Stearic Acid:  Saturated, so it contributes to a hard bar. It is very similar to palmitic acid, and is generally interchangeable with it.  Contributes to a creamy lather. 

Feel free to copy and paste this information for your future use. However, please reference this page as your source. 


A list of my sources of information: 
Wikipedia entry on fatty acids
Wikipedia entry on carboxylic acids
Wikipedia entry on ricinoleic acid
Buzzle article on fatty acids
Fatty acid information