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Alcohols, Sugars and Cold Process Soap
Author: Kevin Dunn
Thursday, January 11, 2018

In recent installments of this column, we have looked at additives that promote transparency in soap. Chief among these additives were those that belong to the broad class of chemicals known as the alcohols. Non-chemists immediately think of grain alcohol (ethanol) or rubbing alcohol (isopropanol), but these are only two examples of a large class of compounds that can be defined by the presence of one or more OH groups in their formulas. Ethanol, for example, is C2H5OH, and isopropanol is C3H7OH.

Some alcohols contain more than one OH group in their formulas. Glycerol, a by-product of saponification, has three: C3H5(OH)3. Propylene glycol, used in many melt-and-pour soaps, has two: C3H6(OH)2. What many people don’t realize is that sugars also fit this pattern. Glucose and fructose, the most familiar simple sugars, share the same overall formula: C6H7O(OH)5. Lactose (milk sugar) and sucrose (cane sugar or table sugar), also share the same formula: C12H14O3(OH)8. Now, you don’t have to memorize these formulas or become an expert in their molecular structures to understand the most important aspects of their interactions with soap. You only have to know that, like their kindred alcohols, sugars are bristling with OH groups.

The chemistry of soap is dominated by the simple fact that oil and water don’t mix. They don’t mix because water molecules are strongly attracted to one another, but not so attracted to oil molecules. Water molecules are attracted to one another because their hydrogen atoms carry a partial positive charge, their oxygen atoms carry a partial negative charge, and because opposite charges attract. Oil molecules do contain oxygen and hydrogen atoms, but they are bonded to carbon atoms, not to one another. Hence, no positive and negative charges. Hence, no strong attractive forces. Water molecules, like cheerleaders, prefer to associate with one another, and to avoid the company of relatively unattractive, nerdy oil molecules.

A soap molecule bridges the gap between oil and water molecules. One end of the molecule, the watery, hydrophilic end, contains a pair of negatively-charged oxygen atoms that are strongly attracted to the positive hydrogen atoms in water molecules. The other end of the soap molecule, the fatty, hydrophobic end, consists of a long chain of un-charged carbon and hydrogen atoms, and finds comfort in the company of similarly un-charged oil molecules. Soap does what it does by associating with water molecules at one end, and oil molecules at the other.

Now consider the formulas for the alcohols and sugars. Part of those formulas consist mostly of carbon and hydrogen atoms, and the other part of OH groups. Sound familiar? Yep, like soap molecules, alcohols and sugars have hydrophobic parts and hydrophilic parts. Comparatively, soap molecules have long hydrophobic tails and small hydrophilic heads. They are more fatty than watery, and hence, not very soluble in water. A bar of soap dropped in the tub dissolves slowly. Alcohols and sugars have relatively small hydrophobic parts, and one or more hydrophilic OH groups. They tend to be more watery than fatty, and dissolve quickly in water. Alcohols and sugars, then, are more hydrophilic than soaps, and less hydrophilic than water.

If soap molecules bridge the gap between oil and water molecules, alcohols and sugars can bridge the smaller gap between soap and water molecules. In combination, alcohol and sugar molecules cluster around the hydrophilic head of a soap molecule, making it more attractive to water molecules than it would have been otherwise. This leads to quicker emulsification of oils in the soap pot, a hastening of trace, more rapid saponification, more rapid temperature rise, and consequently an earlier entry into gel phase. It also helps soap to dissolve more quickly in the wash basin, affecting the texture of the lather and the speed with which it is raised.

The idea of using sugar as the hydrophobic/hydrophilic bridge is so powerful that new, synthetic surfactants and detergents have been designed around this phenomenon. These new compounds replace the (merely) two oxygen atoms at the head of a traditional soap molecule with one or more sugar molecules. Synthesis of these new detergents would not be accessible to most handcrafted soapmakers, but the simple addition of sugar to soap allows us to easily modify the properties of soap and lather.

Table sugar or honey may be added to raw soap in multiple ways. They may be added to your water portion before or after the addition of sodium hydroxide. They can be dissolved in a little water and mixed into your oil portion before you add your lye. You can put a sugar/water solution into a spray bottle and spray it between the layers of a layered soap. And you can add sugar water to raw soap at trace.

When adding sugar to your lye portion, the sugar may caramelize if the temperature gets too high. This can be avoided by allowing your lye to cool before adding the sugar. In our lab, we master-batch our lye at 50% concentration and then add extra water when making soap. For example, if we wanted to add 12 ounces of sodium hydroxide and 26 ounces of water, we would use 24 ounces of master-batched lye solution (i.e. 12 ounces of sodium hydroxide and 12 ounces of water) and 14 ounces of extra water. The 12 ounces of water in the master batched lye plus the 14 ounces of extra water adds up to our 26 ounce water portion. There are many advantages to this approach, but in this case, the sugar is dissolved in the extra water and the lye is already at room temperature. Thus, the sugar does not scorch.

Another consideration is that sugar may hasten trace and cause the soap to get hotter, quicker. This can be ameliorated by increasing your water portion or lowering the temperature of your oils compared to your usual practice. There is no magic formula for the water portion and temperature. We often use oil at room temperature and we vary the water portion over a wide range, depending on the effect we are trying to achieve.

The final consideration is the amount of honey or sugar to use. Many people advocate a teaspoon of sugar per pound of oil. We have experimented with sugar portions up to 4% of the oil weight. Whether you use table sugar or honey, I hope your results will be Sweeet!





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