Caustic Chemicals Used in Soap Making

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Guide to chemicals in liquid soap

Average Read Time: 10 Minutes

Soap keeps our world safe. It cleans homes and businesses, offices, and manufacturing plants — versatile, gentle, and effective, it is in many ways the essential product. Without soap, proper sanitation is nearly impossible .

Humans have made basic soaps for millennia — archaeologists have found fragments of soap recipes dating from as early as 2800 BCE . According to one legend, the word "soap" comes from ancient Rome, where animal fat unintentionally mixed with wood ash during religious ceremonies on Mount Sapo. People discovered the resulting paste was an effective cleaning agent, and they called it "sapo" in recognition of its origins.

However, soap-making has evolved since Rome, and modern soap looks much different. What is the process of producing soap, and what are its chemical components? In this guide, we will provide an overview of the typical chemicals involved with liquid soap production, as well as examine the differences between liquid soaps and other everyday cleaning products.

In this article you will learn:

What is the formula for soap

For centuries, humans have known the basic recipe for soap — it is a reaction between fats and a strong base. The exact chemical formula is C17H35COO- plus a metal cation, either Na+ or K+. The final molecule is called sodium stearate and is a type of salt. Depending on the metal cation, soaps are either potassium salts or sodium salts arranged as long-chain carboxylic acids.

Typically, the formation of these chains involves combining potassium hydroxide with an animal or vegetable fat, or sometimes with acetic acid . A soap molecule does two things — it bonds to both water and debris. That is due to its hydrophilic, or "water-loving," and hydrophobic, or "water-fearing," components. A molecule of soap has a hydrophilic anionic "head" and a hydrophobic "tail" made of hydrocarbons. The head of the molecules is attracted and dissolves in water, while the hydrocarbon tail is attracted to dirt and grease, and repelled by water.

Soap is also a surfactant — it reduces the surface tension of water. Water has a strong surface tension , which causes drops to bead on a variety of surfaces ranging from metal to fabric. That slows water's wetting process and inhibits its ability to clean. Because soaps lessen the surface tension of water, it can spread and wet more easily. Also, surfactants loosen and emulsify dirt and debris, dispersing it in water and allowing it to get rinsed away.

Today, the process of making soap most commonly involves reacting an organic acid with an alkaline chemicals like potassium hydroxide or sodium hydroxide. Industrially, the caustic soda base used most often is sodium hydroxide, which is also called lye. The main difference between potassium and sodium soaps is consistency — usually, potassium makes a softer, more water-soluble soap than sodium.

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How soaps get made

Soap-making is a relatively simple process. A standard method involves the saponification of oils and fats, which requires heat. In this method, fats and oils get heated and then reacted against a liquid alkali — this process produces soap, plus excess water and glycerine.

Another common way to make soap is by neutralizing fatty acids with an alkali, most often sodium hydroxide. First, oils and fats get hydrolyzed, or split, using high-pressure steam. This step separates the fats into crude fatty acids and glycerine. Next, the fatty acids get purified through distillation and then neutralized by an alkali, which produces soap and water.

If the alkali is potassium hydroxide, the result is a potassium soap. Potassium soaps are the "soft" liquid soaps that quickly dissolve in water. Alternately, if the alkali is sodium hydroxide, the result is a sodium soap. These are called "hard" soaps and are less water-soluble than soft soaps.

7 most common chemicals in liquid soaps and their functions

Cocamidopropyl betaine (CAPB) is a surfactant — it helps water clean effectively.

CAPB derives from chemicals found in coconut oil and is the result of mixing raw coconut oil with the chemical dimethylaminopropylamine. Classified as an amphoteric surfactant detergent, CAPB can function as either an acid or a base, depending on its chemical surroundings. With a polar head and a hydrocarbon tail, CAPB helps soap break down debris and wash it away in water.

CAPB acts as a thickening agent in many liquid soaps. Manufacturers include CAPB in their liquid soap formulas for its surfactant and foaming properties — in soaps, CAPB creates a rich and thick lather. CAPB also has some antiseptic properties, which makes it a common addition in personal sanitary products.

In liquid soaps, dyes give the soap an appealing color. Like fragrances, the exact ingredients of synthetic dyes depend on the specific manufacturer. Often, they chemically derive from petroleum and coal tar.

The purpose of dyes is purely aesthetic — they make the product visually appealing and have little to no functional value. Because of this, many companies choose synthetically produced dyes and colorants, as opposed to naturally derived compounds, since synthetic dyes are almost always cheaper and more readily available.

The best dyes have long-term color stability and resist fading. Common color choices for liquid soaps are yellow, blue, and green, but the right dye can achieve almost any color.

Many liquid soaps contain some fragrance. The exact chemical vary from manufacturer to manufacturer — currently, the law doesn't require the fragrance industry to disclose the specific chemicals they use in their scents.

In one fragrance, there could be hundreds of ingredients or just a few — more than 3,500 oils and chemicals are approved for use in fragrance products. These add a perfuming element to soap, helping it deodorize as well as clean surfaces.

Synthetically produced fragrance oils are the most popular choices for scented liquid soaps. When compared to other fragrance options like essential oils, synthetic fragrances are inexpensive and relatively easy to produce. That makes them an economical and attractive choice for many companies.

Methylisothiazolinone (MIT) and methylchloroisothiazolinone (CMIT) are common preservatives in many liquid soaps. Both chemicals individually work to inhibit bacteria growth, but they are most often combined.

Typically, MIT and CMIT are ingredients in cleaning and personal care products, and also act as preservatives. Powerful biocides , they eradicate the slime-forming fungi, algae, and bacteria that can develop in many industrial settings, including fuel storage tanks, water cooling systems , paper and pulp mill water systems, and oil extraction systems.

In soap, MIT and CMIT are often excellent for cleaning wood products, as they can control sap stain, mold, and mildew.

In chemistry, pH is the measure of the acidity or alkalinity of a solution or surface. The higher the pH number, the more alkaline a substance is — low numbers indicate acidity. A neutral pH is seven — pH numbers lower than seven are acidic, and numbers higher than seven are alkaline.

The human body has a natural pH of 7.4. By nature, soap is an alkaline substance and will have a high pH balance. However, if a soap will come into contact with humans, it should never have a pH higher than 10 — the closer the soap's pH is to the pH of human skin, the better. If the pH of the soap is too high, it will be irritating and even toxic to humans.

In liquid soaps, some chemical ingredients alter the pH balance of soap. Most often, the pH adjusters in liquid soaps will be citric acid or sodium chloride.

Citric acid is a naturally occurring chemical in citrus fruits and results from the fermentation of carbohydrates. An acid, this product lowers the pH of soaps, making them less alkaline. Citric acid enhances the effectiveness of preservative and antioxidant ingredients in soaps.

Sodium chloride, or salt, reduces the pH of soap solutions. It acts as a stabilizing agent, helping pH levels remain steady. A thickening agent, sodium chloride also has a de-greasing effect, enhancing the cleaning potential of liquid soap.

Water-based soap will need a preservative — if not, the product can become rancid and develop mold and bacteria. Even cold-process soaps made without water will need preservatives if they will contact water, which is true of most cleaning products.

Sodium benzoate is the sodium salt of benzoic acid , used as an anti-corrosive and preserving agent in a wide range of industries. It goes by a variety of names, including benzoate of soda, sobenate, natrium benzoicum, and benzoic acid.

In cleaning, sodium benzoate is beneficial for its antifungal and intrinsic preserving qualities. As a preservative, sodium benzoate extends the shelf life of liquid soap and prevents fungi like yeast and mold from colonizing. Sodium benzoate is often an alternative to parabens in cleaning products such as dishwashing detergent, toilet bowl cleaners, and upholstery cleaners.

Sodium laureth sulfate (SLS) is both a surfactant and emulsifier and contributes a sudsing and foaming element in soap.

Also known as sodium dodecyl sulfate , SLS is highly effective at removing oils and residues. Since it can clean grease from engines and floors, industrial settings often use liquid SLS soaps in high amounts.

For personal care soaps and household products , SLS is more common in less concentrated amounts. In addition to emulsifying oils, SLS suspends dirt and soil in water, allowing it to wash away easily. It reduces the surface tension of water, allowing it to more thoroughly wet and clean surfaces.

How to make liquid soap with caustic soda

Sodium hydroxide, also called caustic soda or lye, is a traditional chemical for soap-making. While potassium hydroxide is more common in liquid soap-making, it is possible to produce liquid soaps using caustic soda.

One of the most commonly used chemicals in the soap industry, sodium hydroxide is a strong base with a broad range of potential applications. Sodium hydroxide is a water-soluble compound that comes in pellets, granules, flakes, or powders. Sodium hydroxide forms through the electrolysis of sodium chloride, and is a powerful alkali. When added to water, sodium hydroxide increases the pH of a substance, which makes it a valuable pH adjuster in acidic formulas.

An inorganic base, sodium hydroxide does not contain any carbon atoms, similar to water. When mixed with water, sodium hydroxide dissociates completely to just hydroxyl and sodium ions. The hydroxyl ions carry a negative charge, and the sodium ions have a positive one. This influx of ions leads to a strong exothermic reaction, which helps hydrolyze fats in the saponification process to form soaps.

Sodium hydroxide is a reagent, or a substance used in a chemical reaction to produce other substances. Caustic soda causes saponification and is an essential ingredient in soap-making. When flakes or beads of sodium hydroxide get added to a liquid, it forms a lye solution. This solution, when mixed with oils or fats, will lead to the chemical reaction called saponification.

Today, most industrial soap-making takes place through a continuous process, which produces a steady stream of soap instead of small batches. Manufacturers first split natural fats into fatty acids and glycerin, typically through a tall, vertical steel column called a hydrolyzer that uses high temperatures to break the fat into its two components. Once separated, the fatty acids get distilled for further purification. Next, the purified fatty acids get mixed with a precisely measured amount of caustic soda. The subsequent saponification creates soap. During this stage, other ingredients may be added to increase shelf life, cleansing power or marketability. But you can also make liquid soap with caustic soda in small batches. You can find many different recipes for making small-batch liquid soap, but the general process remains the same.

4 steps to making liquid soap

You must have a type of fat — the most commonly used fats derive from plants, such as palm kernel oil, coconut oil, and olive oil. You can use one type of oil or a combination of two or three — for example, 70 percent coconut oil and 30 percent olive oil.

To make lye water, mix the caustic soda with water until it dissolves. The reaction of sodium hydroxide and water is exothermic and will produce heat, so use caution during this stage of the process.

Combine the oils with the lye water. Once mixed, allow the soap to rest up to 24 hours.

After the soap has set, slowly add heat and water until the soap is smooth and at a proper
liquid consistency.

By the end of the saponification process, sodium hydroxide is no longer present in the soap — it has been completely dissociated and used in the chemical reaction. Keep in mind that it is essential to weigh your ingredients instead of measuring them. The amount of lye is based on the weight of the oil, not the fluid ounces. Different oils have different densities — one cup of olive oil will not weigh the same as one cup of coconut oil. To ensure you have the right ratio, weigh your ingredients before mixing.

The difference between antibacterial soaps and standard liquid soaps

Antibacterial Soaps

For a soap to be considered antibacterial, it must have active antimicrobial ingredients. More than 75 percent of liquid soaps stocked in stores have at least one antibacterial component — triclosan is the most commonly used ingredient, followed by benzalkonium chloride and alcohol. However, the industry has recently begun a shift away from using triclosan.

Antibacterial soaps are useful in settings where immune systems are at risk, such as health care facilities. Soaps like hand sanitizer that contain high amounts of alcohol are effective alternatives when hot water and soap are not accessible. However, the FDA has reported there is yet no evidence to prove antibacterial soaps are more effective than standard soaps. While antibacterial soaps are specifically designed to eliminate bacteria, they do not discriminate between beneficial and harmful bacteria — they also eliminate healthy bacteria on the skin, which can, in turn, make antibiotics less effective.

Standard Liquid Soaps

Regular liquid soap reduces water's surface tension, which helps lift oils, dirt, and debris from surfaces and washes them away. While regular soaps do not contain added antibacterial ingredients, they are still effective defenses against germs and bacteria.

According to the FDA, standard soaps may be just as effective as antibacterial soaps for eliminating disease-causing bacteria. Unlike antibacterial soaps, conventional liquid soaps do not harm the beneficial bacteria already on the surface of the skin. Standard soaps tend to be cheaper than antibacterial brands. However, standard soaps are not as portable or convenient as antibacterial soaps like hand sanitizers, which can make them less attractive to consumers.

The difference between detergent and liquid soaps


Detergents first became available during World War II as an alternative to soap — during this period, the oils necessary to make soap were in scarce supply, and people needed another type of cleanser. Detergents are synthetic products, and some may have a negative effect on the environment.

However, detergents have one primary advantage over standard liquid soap — detergents do not leave a residue or film after washing, which has made them the cleanser of choice for laundry. Detergents have been engineered to perform well in virtually any temperature or level of water hardness — they are as effective in cold, mineralized water as they are at standard temperatures.

Liquid Soap

While detergents are synthetically engineered, soaps derive from natural chemicals and processes. Although liquid soap is often more environmentally friendly than detergents, it is not effective with every type of water.

Water has varying levels of "hardness," which refers to its mineral content. Water with a high amount of minerals is "hard," while filtered and purified water is "soft." Soap reacts poorly to hard water and will leave behind a film called soap scum . This residue will eventually discolor clothes and can damage surfaces. Soap also needs warm or hot water to be effective.

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