The Chemistry Of Soap Making

30th December 2020

Blog

Woman taking soap bars out of plastic form

The chemistry of soap making is an ancient science. In fact, soap is one of the earliest inventions of humanity. It’s almost as old as civilisation, with its earliest recorded evidence being traced all the way back to ancient Babylon 4,800 years ago – though its invention probably dates back much farther than this. Even more astounding is  that the basic ingredients of soap haven’t changed over the millenia – so how is it made exactly?

How Is Soap Made?

Soaps are produced both industrially as commercial goods and locally as artisan products. In either case, the basic ingredients and the process are the same: soaps are made from an alkaline substance, namely lye (also known as sodium hydroxide), oil, and a choice of fragrance.

However, the specific types of ingredients vary from soap to soap. For example, there are many different kinds of oils used in soap production, including tallows from beef or mutton fats, palm oil, coconut oil, olive oil, laurel oil, and canola oil. 

Even though it’s an ancient practice, the steps taken to make soap haven’t radically changed over the millennia. What has changed are the scale and technology. Soap can now be manufactured at a huge scale because of our advancements in manufacturing and equipment. Paddle mixers are often used in large-scale soap production, as well as rotating mills, refining and extruding plodders, and industrial soap presses. 

However, this equipment is only needed for industrial applications – you can actually make soap at home using items that can easily be bought from the supermarket or even derived from raw materials you can salvage for free. Here are the generic steps and fundamental principles of soap making:

  • Step 1 – Measuring: Choose your ingredients and carefully measure out the proportions. Many recipes for soaps require a 40% lye concentration dissolved in water. The proportion of oil with the lye solution may vary depending on the type of oil. For example, coconut oil can be up to 33% of the lye solution-oil mixture, while only 5% of grapeseed oil is recommended for soapmaking
  • Step 2 – Cooking: The dissolved lye and oil are boiled and mixed while being stirred until a certain consistency is reached. Whether small-scale or large-scale, this is the most dangerous step: aside from the high temperature, this mixture is also corrosive. This is because lye, also known as sodium hydroxide, is a corrosive substance that can cause chemical burns if it comes into contact with skin – so make sure you handle this mixture carefully! 
  • Step 3 – Removing excess alkali: Depending on the volume of the mixture and the intended result, it may take between several hours and two days of cooking to molecularly bind the alkali to the oil. When a certain saturation point is reached in the cooking process, you should stop and continue to remove any excess alkali. This is because alkalis can deeply penetrate skin tissue, meaning that too much alkali in soap can be very drying or irritating to the skin. 
  • Step 4 – Adding other ingredients and stirring: Once the desired saponification level is achieved, you can add other optional ingredients, like essential oils, colours and fragrances. Continue stirring everything until it’s thoroughly mixed together. 
  • Step 5 – Pouring into moulds: While the soapy mixture is still hot, you can begin pouring it into moulds. These can either be silicone moulds, which are available in a range of different designs, or even something as simple as a wooden box mould – if you decide to use the latter, though, make sure to line it with parchment paper!
  • Step 6 – Cooling: Allow the soap mixture to cool and set for a few hours. If you’re using a silicone mould, this may take a bit longer than normal because the soap won’t come into contact with any air. 
  • Step 7 – Trimming: Remove the soap from the mould and trim any excess or deformed parts. At this stage, the soaps are ready to be used, so it’s just a case of making them look the way you want!
  • Step 8 – Packaging: This step is optional if you’re only making soap for personal use. If you’re intending to sell the soap, however, then you need to package it, not only to make it look presentable but also to ensure that it’s protected.
Handmade soap in silicone mould
Silicone moulds are popularly used in soapmaking

Chemicals In Soap

While the specific chemistry of soap making may vary from product to product, soaps cannot work without two basic ingredients: oil and alkali.

Soaps are chemically classified as salts of fatty acids because of the presence of an ionic, or polar head, and a nonpolar glyceride tail. The head is composed of positively charged sodium ions and negatively charged oxygen ions, while the tail is a fatty chain.

The molecular structure of soap makes it excellent for cleaning grease and destroying the cell membranes of microbes. Depending on the strength, soaps are widely used in domestic applications such as bathing, washing clothes, and washing the dishes. On a large industrial scale, soaps are used for a variety of applications, such as in thickeners, lubricant components, and as precursors or chemical reagents to some catalysts.

Liquid soaps have slightly different ingredients because their liquid state must be maintained at room temperature, and too many bubbles must be prevented from forming. Here are some of the common ingredients of liquid soaps:

  • Sodium benzoate and benzoic acid
  • Sodium laureth sulfate
  • Methylisothiazolinone and methylchloroisothiazolinone
  • Cocamidopropyl betaine
  • Fragrance, either from essential oils of flowers or from synthetic sources
  • pH adjusters
Ingredients for soapmaking
The two main ingredients in soap are oil, like coconut oil, and lye, a.k.a. sodium hydroxide

What Is The Chemical Name For Soap?

Just like other substances, such as sugar, the chemical name of a particular type of soap depends on its composition. Here are some of the most common types and the respective chemical names of soaps:

  • Sodium tallowate: This is made using sodium hydroxide and tallow, i.e. beef fat or mutton fat
  • Sodium palmate: This is made using sodium hydroxide and palm oil as the main ingredients
  • Sodium palm kernelate: This is made with sodium hydroxide and palm kernel oil
  • Sodium cocoate: This is made using sodium hydroxide and coconut oil 

What Is The Chemical Formula For Soap?

There’s actually no single chemical formula for soap: it varies depending on the ingredients used. Although there are other ingredients that can be added to soaps, such as colours and fragrances, there are only two functional ingredients of soaps – of course, we’re talking about oil and alkali again. 

The formula of soap indicates the chemical composition of the oily tail and the ionic head we mentioned earlier. For example, most metallic soaps, which are water-insoluble and used in the coatings industry, are prepared by neutralising purified fatty acids. So, a generic balanced chemical equation for metallic soap using something like calcium oxide is:

2 RCO2H + CaO → (RCO2)2Ca + H2O

Ultimately, the complexity of soap formulas depends on the type of oil being used. Here are two examples of chemical formulas for the most common types of soaps, which have shorter chains of fatty acids:

  • Sodium laureate: C₁₂H₂₃O₂Na
  • Sodium palmate: C16H31NaO2

Are There Harmful Chemicals In Soap?

The most harmful chemical in soap is the alkaline ingredient lye, also known as sodium hydroxide or caustic soda. This alkali substance is corrosive and poisonous when ingested. However, when chemically combined with fatty acids in oils, it gets neutralised, meaning that it’s entirely safe to use. 

Sodium hydroxide vector illustration
In soapmaking, sodium hydroxide gets neutralised, making it non-corrosive and non-toxic

Soaps are chemically known as the salts of fatty acids. The toxicity and corrosiveness of soaps are low, but still potentially harmful, especially if you have sensitive skin. The chemistry of soapmaking uses other potentially toxic ingredients depending on the strength and purpose. Some liquid soap products, for instance, contain bleaching chemicals like chlorine.

However, these are usually only present in products like dishwasher detergents, where the chemicals are needed to kill bacteria. Even in these formulations, though, the concentration of potentially harmful ingredients is kept very low so that it’s safe for use.

Disclaimer

All content published on the ReAgent.ie blog is for information only. The blog, its authors, and affiliates cannot be held responsible for any accident, injury or damage caused in part or directly from using the information provided. Additionally, we do not recommend using any chemical without reading the Material Safety Data Sheet (MSDS), which can be obtained from the manufacturer. You should also follow any safety advice and precautions listed on the product label. If you have health and safety related questions, visit HSE.gov.uk.