Breaking Down the Chemistry of Soap

Created by Matthew Tittensor, Nicholas Lang, and Sohum Sanghvi

Introduction

We have examined many aspects of soap and shampoo thus far; Creation, Chemical Composition, interaction with water, and even why soap bubbles. This section will take a closer look into some chemistry aspects involved with soap, as well as as the creation and mass production of soap products on a small and large scale.

A Closer Look at the Soap-Making Process

In our previous blog post, we discussed some basic properties of how soap is made using the saponification reaction. With the help of La Shonda Tyree, owner of Handmade Soap Coach, we were able to understand various thermodynamic and kinetic properties of this reaction.

Thermodynamics

The first part of the saponification reaction is ionization, which takes place when sodium hydroxide is mixed with water. This ionizes, or breaks down the sodium hydroxide into separate sodium ions and hydroxide ions. When this happens, the water temperature increases to as much as 200°, and thus the reaction is exothermic to get rid of this excess heat. Dissolution of NaOH Demo

In the previous blog post, we discussed the formation of triglyceride molecules from fatty acids and glycerol. For the next step of saponification, the triglyceride needs to be broken down into fatty acids and glycerol through a two step process called steam hydrolysis. The steam hydrolysis yields a fatty acid without its salt as well as glycerol. Then, the sodium ions (from the ionization) hook up with the fatty acids to form a fatty acid salt, or a sodium soap. The hydroxide ions attach to the glycerol to form glycerin. Note that because of the high-temperature steam hydrolysis, the overall enthalpy of the saponification reaction isendothermic.Saponification Reaction
The fat/oil can be considered as the triglyceride being treated by the Na+ and OH- ions. Note that heat is required for the reaction to be completed. 

Temperature is an important aspect of the reaction. When combining the triglyceride and sodium hydroxide solution, having a temperature of less than 120° is ideal, since a higher temperature will speed up the reaction too much. The addition of scent ingredients, such as honey, milk, cinnamon, and clove can affect the reaction’s heat by increasing the temperature of the raw soap. These ingredients should be handled properly, since having too many ingredients will cause the heat to increase too much and cause the soap to separate during the hottest phase of saponification (called the gel phase) during which the soap is in a mold. If the ingredients are properly added to the soap, the soap will harden without falling apart in the cooling and hardening phase.

Kinetics:

As mentioned before, the use of heat definitely impacts the rate of the saponification reaction. If the reaction takes place at a temperature higher than 120°, the raw soap will saponify too quickly and become thick. Additionally, the crafter would not have enough time add scent, color, and herbs to the raw soap. Having a thick raw soap may make it difficult to pour into molds.

The reaction rates for saponification are based on the method that is used to create the soap. The two common methods for producing soap are the cold process and the hot process. The cold process for making soap takes 18 to 24 hours to complete the saponification process. The hot process requires only two hours for the saponification reaction because it is reheated by a double boiler. The soap from the cold process requires at least 2 weeks to “cure” in which the soap loses water to eventually become hard. The hot process usually requires only one week to harden. Cold process soap tends to be of higher quality, and the remaining glycerin from the saponification reaction is usually added to the soap as a natural skin softener. The cold process soap also tends to have more designs since it is not heated in extremely high temperatures.

The Difference Between Liquid and Solid Soap:

While soaps come in many varieties, the most easily distinguishable types are hard and soft soaps.  Soap is made during the process in which a base reacts with a fat, either a vegetable oil or an animal fat.  This base ultimately determines the final state of the soap.  A sodium hydroxide base results in a harder soap unlike the soap produced by potassium hydroxide. Another factor is what kind of fat is used.  A soft oil will make liquid soap easier to form more purely and with a more clear complexion.   The difference of physical states between the two caustic bases is practical because it allows for different functions.  For instance KOH is used often in making shaving cream as it is very soluble in water.  Other reasons for having both varieties of soap include catering to a wider market of people; many people have a distinct preference in soap and having variety allows for many people to be reached.  makes a liquid soap as opposed to 

Mass Production of Soap:

    In large amounts, soap is created in factories for commercial purposes, but through a variety of different ways. One such fashion, as described by the Alabu Soap Company, begins with over an ounce of goat milk, coconut oil, food-grade oils, and soybean oil. After the oils are melted, olive oil is added. In some specific soaps, exotic oils are added as well, such as Squalane and Shea Butter. The mixture is mixed with lye and poured into molds, to set for 12 hours. After this period, it is put through a conveyor belt with high-tension strings on the end to be cut into individual bars, as the mold created a solid about 5 feet long. After 4 weeks, the soap is ready to be packaged and sent away.

Another way of creating soaps is called the hot process.In contrast to cold processes of making soap, hot processed soap saponifies during or immediately after the mixture is handled by people/machines, whereas the cold process takes a while longer to saponify. In hot processes, the hydroxide and lipids are mixed just below the boiling point of the solution, allowing them to saponify faster. The advantage of doing so is that one does not need to know the exact amount of Hydroxide in the mixture.

Performing the cold processing method of soap-making requires more careful consideration and planning. The exact amount of oils and fats must be known. Also, the saponification values of the oils and fats must be consulted in the corresponding saponification chart. Too much lye, for instance, can make soap irritable to the skin.

During this process, lye is dissolved in water, and the oils are made into a warm liquid state,either by heating a liquid or melting a solid. They are mixed until two stages are fully emulsified.

Bonus Video: The How It’s Made series has a video on the production of soap bars. This further explains the commercial production of soap.

We would like to give special thanks to Ms. La Shonda Tyree of Soap Coach for her input for our blog post.

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