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).

My Vegan "Lard" Bar

Tried a new soap this morning in the shower, and I just had to write about it, so I'm taking a mini-break from the chemistry "lectures."

I bought some lard this past winter to make suet cakes for the bird feeders at school (I have a bird club that collects and sends data to the Cornell Institute of Ornithology through their Project FeederWatch Program).  Found out the hard way that was a mistake.  Lard melts WAY too easily. I ended up with an extra one pound container of lard for which I had no use.  I prefer to make soap from plant oils, but I didn't want the lard to go to waste, so I made a four bar batch of lard soap, concocting a recipe on Soapcalc.net that had the properties I wanted.  I also tested out a new FO - Nature's Garden Exotic Amazon Teakwood, as I wanted to try out a more manly scent for my husband.

Some people really love lard soap, and now I can see why. It makes a hard bar with thick, creamy lather. Not as bubbly as my usual recipe, but I really liked it for a change.  My husband liked the soap and the scent.  I loved the scent too; manly enough for a "men's line," yet it didn't make me feel like I bathed with Old Spice or something like that.  Now I had a problem.  I liked the soap, but still preferred to use plant oils and butters.  Lard is also a bit smelly to make soap out of, much like milk soaps can stink until they cure. When researching fatty acids for one of my earliest posts, I discovered that mango butter (as well as shea and cocoa butters) has a fatty acid profile similar to lard. The plant butters, however, have much higher amounts of ricinoleic acid, which should increase the lather compared to lard.  So I developed a mango butter soap recipe similar to my lard recipe (I had some mango butter on hand).  I tried a different FO (Nature's Garden Pineapple Paprika) that I thought would work well with the mango theme.  Finally tried the soap this morning, after a four or five week cure.  Impression?  Love it!  It has a lot of lovely, creamy lather much like the lard soap, maybe even better, and the FO smelled terrific. It feels just as hard as the lard soap, which is what I expected from my Soapcalc numbers.   The only downside is that mango butter isn't cheap, especially compared to lard.  But I think it makes a great vegan "lard" bar substitute.

I'm going to keep one of the mango butter bars around for about a year, as well as a lard bar,  to see what happens.  Mango, shea, and cocoa butters all have high amounts of linoleic acid, as does lard.  This fatty acid can make soap that is prone to getting DOS.  I want to see how long those bars will last for me.

Unfotunately, I'm not posting a picture of the soap.  The FO made the lye/oil mixture seize on me, so the bars are not especially pretty.  But that's another post.

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.

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.

Fragrance Oil Tests

I've done several rounds of fragrance oil (FO) tests in the past few weeks. I bought at least 30 samples of different FOs from several online sites, all to see which ones might be candidates for selling in the future. I made a one pound batch of my simplest recipe, and poured about one ounce per paper cup at light trace (hence more than one test batch). I used plastic droppers to put in approximately 1.5 ml of FO in each cup, and used popsicle sticks to stir in the FO.  That roughly equates to 0.7 ounces per pound of oil. Since I knew the FO's might discolor the soap, I made sure one cup had no FO.  Wow! It is amazing how much the FO can change the color of soap. One of the cups was as white as the cup without any FO, while the rest ranged from slightly creamier to dark brown. I knew that vanilla in the FO makes the soap turn various shades of tan to brown, depending upon the concentration of the vanilla, and I found that it doesn't take all that much to turn the soap brown.  Ironically, the "Wedding Cake" scent was the darkest of all. I wanted to make soap cupcakes for my students with that one, but I guess they'll just have to look like chocolate cupcakes!  I didn't take a picture of all 30 scents, but I did take a picture of the finalists, showing the range of discoloration.

The soap on the far left by itself has no FO. The darkest is the "wedding cake" FO.

This little experiment taught me that unless I don't care about the color of my soap, it is very important to see how the FO will change it. It is also important to know that the color change can occur over several days - to almost a week, so wait at least that long to determine the final effect.

As for scent, it became apparent that it too, changes over time, again over several days.  What it smells like out of the bottle is often not a good indication of what it will smell like once soaped. And several scents changed significantly for the better after a few days.

Lesson learned?  Test! Test! Test!

Note: My tests were done with CP soap.  Melt and pour is an entirely different animal. I'll get to that sometime in the future.