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Sodium hydroxide in soap: Is it safe for the skin?

Updated: Jul 1, 2023

Sodium hydroxide, commonly known as lye, is an essential ingredient in soapmaking. When making soap, saponification occurs; the sodium hydroxide reacts completely with the oils or fats, and the resulting soap contains no remaining sodium hydroxide. Properly formulated and cured soap made with sodium hydroxide is considered safe for use on the skin.


What is saponification?

During saponification, the sodium hydroxide breaks down the triglyceride molecules in oils or fats. Triglycerides consist of three fatty acid chains attached to a glycerol molecule. The hydroxide ions (OH-) from sodium hydroxide react with the fatty acids, releasing glycerol and forming sodium salts of the fatty acids, known as soap molecules.


The following simplified equation can represent the reaction:

Sodium Hydroxide + Triglycerides (Oils/Fats) -> Glycerol + Sodium Fatty Acid Salts (Soap)


The soap molecules formed during saponification are composed of a hydrophilic (water-loving) head, which is the sodium salt of the fatty acid, and a hydrophobic (water-repelling) tail, which is the fatty acid chain. This unique molecular structure allows soap to interact with both water and oils/fats, making it effective for cleansing.


As the saponification process proceeds, the sodium hydroxide is completely consumed, and the resulting soap no longer contains unreacted lye. This process ensures that the soap is safe for use on the skin.


Can real soap be made without lye?

No, real soap cannot be made without lye. Lye, either sodium hydroxide or potassium hydroxide, is essential in the soapmaking process. Some alternative soapmaking methods claim to create "lye-free" or "no-lye" soap, but this is inaccurate. These methods typically involve using pre-made soap bases or melt-and-pour soap, which have already undergone the saponification process with lye. Therefore, even in these cases, lye has been used in the initial soapmaking process.


What about a cleansing bar?

A cleansing bar refers to a type of soap-like product that is used for cleansing and washing the body or face. It is commonly found in the form of a solid bar, although there are variations, such as liquid cleansing bars or hybrid bar-liquid formulations. They typically contain surfactants, detergents, or soap molecules that have the ability to interact with both water and oil-based substances, facilitating the cleansing action. (see the article: What's the Big Deal About Hand-Crafted Soap? ) You notice I said they might include soap molecules. Soap molecules are made through the process of saponification see "What is saponification?" above.


Difference between a cleansing bar vs homemade soap?


The main differences between a cleansing bar and homemade soap lie in their formulation, manufacturing process, and potentially the ingredients used. Here are some points of distinction:


1. Formulation: A cleansing bar is a commercially produced soap-like product. It may contain a combination of synthetic detergents, surfactants, and other cleansing agents, along with additives such as fragrances, preservatives, and colorants. Homemade soap, on the other hand, typically involves a simpler formulation consisting of oils or fats, water, and lye (sodium hydroxide or potassium hydroxide) for solid soap or a liquid soap base made from oils, water, and potassium hydroxide for liquid soap.


2. Manufacturing process: Commercial cleansing bars are usually mass-produced using automated processes. They often undergo additional steps, such as milling and extrusion, to achieve a specific shape, texture, and appearance. Homemade soap is traditionally made through the cold process (see article: Cold Process Method) or hot process methods, which involve mixing lye with oils or fats, allowing for saponification, and curing the soap over a period of weeks to harden and develop its desired characteristics.


3. Ingredients: Cleansing bars can contain a wide range of ingredients, including synthetic additives, fragrances, and preservatives, which may not be present in homemade soap. Homemade soap typically uses natural oils or fats, which may be organic, along with natural colorants, essential oils for fragrance, and other natural additives such as herbs, clays, or botanical extracts. (see article: Oils and Butters Used in Handmade Soap) However, it's important to note that homemade soap recipes can also include synthetic additives or fragrance oils, depending on personal preference.


4. Customization: Homemade soap offers greater flexibility for customization. Soapmakers can select specific oils or fats, adjust superfatting levels (the amount of excess fats remaining in the soap), incorporate natural additives, and experiment with various scents and designs. This allows for a more personalized and tailored soap product. Cleansing bars, being mass-produced, may offer fewer options for customization.


5. Quality control: While commercial cleansing bars undergo quality control processes to ensure consistency and safety, homemade soap production may involve variations in quality depending on the skills and expertise of the soapmaker.


In conclusion, sodium hydroxide (lye) is an essential component in the soapmaking process, and it is transformed into soap molecules, making the resulting soap safe for use on the skin. Real soap cannot be made without lye. Cleansing bars are a combination of synthetic detergents, surfactants, and other additives. Homemade soap, in contrast, involves a simpler formulation using natural oils or fats, water, and lye and allows for greater customization and personalization. Homemade soap offers more control over ingredients and customization, while commercial cleansing bars undergo standardized production methods and quality control processes.


Choosing between a cleansing bar and homemade soap depends on individual preferences, ingredient preferences, and customization options.


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