Last month we explored seizing,
a phenomenon in which an oil/lye mixture solidifies so quickly after mixing that it is too thick to transfer easily from the mixing pot to the molds. The root cause of seizing is the presence of a surfactant (like soap), which hastens the conversion of an unstable emulsion (pre-trace) to a stable colloid (post-trace). Normally, lye reacts slowly with oil, soap forms gradually, and the mixture becomes thicker as the emulsion becomes a colloid. Rancid oil, however, may contain “free” fatty acid (i.e. not bound to glycerol), which reacts almost instantly with lye to form soap, cutting the time short available for crafting and pouring. Last month's article gave directions for detecting free fatty acid in oil.
There are eight fatty acids that come from the oils used most often in handcrafted soapmaking. They differ in length and in the presence and number of double bonds, but they all share the same basic shape. At one end is a hydrophilic “head” consisting of two oxygen atoms and an acidic hydrogen atom. At the other end is a hydrophobic “tail” consisting of a chain of carbon atoms and non-acidic hydrogen atoms. Figure 1 shows a structural formula for stearic acid, one of the main components of solid fats like lard, tallow and palm oil. The two oxygen atoms and the acidic hydrogen atom are shown explicitly, and the chain of carbon and hydrogen atoms appears as a squiggly line. Each kink in the line represents a carbon atom, in this case, eighteen of them. The carbon atoms in the middle of the chain are implicitly bound to two hydrogen atoms, and the carbon at the end (the omega carbon) is implicitly bound to three hydrogen atoms. When a fatty acid reacts with a strong base like sodium or potassium hydroxide, the acidic hydrogen atom reacts with the hydroxide ion to form water, and the sodium or potassium ion takes its place at the head of what is now a molecule of soap. The hydrophilic head of a soap molecules is attracted to water and the hydrophobic tail isn't. This combination is what allows soap to bring oil and water together.
Soap is only one example of a surfactant, however, and any molecule that has a similar structure can prematurely stabilize the oil/lye emulsion. This month, we look at one in particular—eugenol, shown in Figure 2. Similar to a fatty acid, eugenol has an acidic hydrogen atom bonded to oxygen at one end of the molecule and a hydrophobic carbon-hydrogen tail at the other. The shape of the tail is quite different from that of a fatty acid, but eugenol's behavior in an oil/lye emulsion is similar. The acidic hydrogen atom reacts almost instantly with sodium or potassium hydroxide, producing water, and leaving sodium or potassium at the hydrophilic head. This combination of hydrophilic head and hydrophobic tail allows eugenol to bring oil and water together, just as soap does.
Eugenol is present in essential oils such as clove, cinnamon, pimento, bay and basil. The composition of essential oils can vary, however, from one variety to another. Clove oil, for example, contains between 74% and 97% eugenol. Cinnamon leaf oil contains 69-87% eugenol, but cinnamon bark oil contains only 2-13%. Pimento berry and leaf oil contains 66-84% eugenol, and bay 44-56%. Bay laurel, however, is only 1-3% eugenol. Some varieties of basil contain as much as 63% eugenol, while others contain none at all. A simple test can determine whether an essential oil contains eugenol or even another molecule with similar surfactant properties.
You will need a small transparent bottle or vial with a cap, essential oil to be tested, 25-50% sodium hydroxide solution (the lye you use for soapmaking is fine), and distilled water. Add five drops of the essential oil to the vial and note the scent. Add five drops of sodium hydroxide solution. If eugenol or a similar molecule is present, it will react instantly. Eugenol will solidify, turn bright yellow or orange, and lose its characteristic scent. The vial may become warm. Add some distilled water (about 60 mL or 2 oz) to the vial, screw on the cap, and shake it up. The product, sodium eugenolate, will form suds just as soap does. An essential oil that behaves this way is likely to cause seizing when used to make cold process soap.
You might think that the problem could be avoided by adding clove oil to hot-process soap at the end of the cook. Indeed, this avoids the problem of seizing, but HP soap remains alkaline enough that it will still react with eugenol, drastically reducing its scent.
Several years ago, we tried to protect clove oil in CP soap by encapsulating it with waxes such as beeswax and carnauba wax. We melted the wax, added the clove oil, and then dispersed it in water to form tiny beads of scented wax. When added to CP soap, the encapsulated oil did not cause seizing, but as with HP soap, the scent was much reduced because of the high alkalinity of all true soaps.
Melt and pour soaps, however, generally contain non-soap surfactants that render them less alkaline than pure soap. Thus clove and cinnamon oils will retain their scents in MP soap, and there is no problem with seizing. I've been meaning to try making a strongly-scented MP soap and using it as shreds or embeds in CP soap. In principle, the MP soap should protect the eugenol, and if the MP portion is high enough, the pH might be low enough to allow the scent to survive.
Is there any silver lining to this eugenolic cloud? I love turning problems into solutions. Many people complain that pure olive oil soap is maddeningly slow to trace. Adding 0.1% or less of clove oil turns two wrongs to a right, hastening trace in a slow-tracing soap without adding an appreciable scent.