The chemistry of soap making

Soap is the result of the saponification of fat and lye. Although the chemistry of soap may sound complicated, making soap is actually simple.

The Chemistry of Soap Making

Soap is the result of a chemical reaction between oil or fat and lye called the saponification process. The saponification process is defined as the reaction of combining a base (fat) with an alkali (sodium hydroxide) to produce a salt (soap) and a free alcohol (glycerin). The goal of the process is to create soap with no leftover fat or lye.

The oil or fat used in the soap making process may be either animal or vegetable. Animal fats that are used include tallow and lard. Vegetable oils used either alone or in combination include avocado oil, coconut oil, castor oil, olive oil, palm oil, soybean oil, vegetable oil and numerous others. Each type of fat has its own saponification value. The saponification value determines the amount of lye needed to turn the fat to soap.

The saponification (SAP) value is determined by the length of the fatty acid chain. While it takes the same amount of lye to saponify a short fatty acid molecule as it does a long fatty acid molecule, the longer fatty acid chains have a higher molecular weight and thus require less lye. For example, coconut oil has shorter fatty acid molecules than castor oil, and thus requires the addition of more lye in the soap making process.

Fats are composed of several different types of triglyceride molecules. Three fatty acid molecules attach to one molecule of glycerine to form a triglyceride molecule. The triglyceride molecules are combined with lye, otherwise known as sodium hydroxide molecules. The result of combining a triglyceride molecule with three sodium hyrdroxide molecules is three soap molecules and one glycerin molecule. The resulting soap molecules are negatively-charged, polar molecules. One end of the soap molecule is hydrophilic, and attracts water and repels fats. This end is the carboxylate group (-CO2). The other end of the soap molecule, the hydrocarbon chain, is hydrophobic, and attracts fats while repelling water.

These hydrophilic and hydrophobic characteristics of soap are what allows soap to act as a cleaning agent. When mixed with water, the hydrophobic ends of soap molecules attach to dirt and grease on an object, prying it loose and allowing water to seep underneath.

The saponification process occurs when the lye is mixed into the melted fat or oil. The lye and fat must be thoroughly mixed together, often through the use of a blender. Di Propylene Glycol (DPG) may be used to mix the oil and lye molecules together if mechanical means of mixing are unavailable or undesirable, such as in a demonstration of old fashioned soap making techniques.

The addition of scents, such as essential oils, to the soap mixture may affect the saponification process. Some oils inhibit the saponification process whereas others speed up the process. Once it is learned through trial and error the effect various scents have on the saponification process, DPG can be added to correct the process.

Because lye is a toxic chemical, and thus can be very dangerous to work with, it is frequently asked whether soap can be made without lye. The answer is no. Some craft stores sell a "spaghetti" which can be melted down to create bars of soap. This spaghetti is already soap, meaning lye has already been used in the process of creating the spaghetti. Someone using the spaghetti is not actually making soap, but rather is remelting or rebatching soap that has already been made.

© High Speed Ventures 2011