There is nothing like a refreshing bath after a sweaty workout or a hard day outside. The feeling of freshness and the pleasant odour after the bath is the contribution of the ubiquitous soap. In ancient India, soap nuts were crushed and used to clean, as were the bark of certain trees and specific flowers and leaves. The first use of soap has been traced back to 2800 BC in Mesopotamia. From there this little item later spread into Europe. During the Industrial Revolution, soaps started to be mass-produced. But even until the 19th century, soaps were treated as luxury items and taxed heavily in Europe.
What is soap?
Soap is essentially a sodium (Na) or potassium (K) salt of a fatty acid based on vegetable oil or animal fat. In chemistry parlance, soap is represented by the formula RCOONa or RCOOK, where R is an organic fatty acid chain and C and O are carbon and oxygen atoms, respectively.
For example, a fatty acid based on coconut oil contains lauric acid, which has the formula C₁₂H₂₄O₂. Similarly, a palm-based fatty acid will have palmitic acid, which is represented as C₁₆H₃₂O₂. The corresponding formula of soap with a lauric acid base will be C₁₁H₂₂COONa.
Solid soaps are generally sodium salts while liquid soaps are generally potassium salts, both of fatty acid chains.
How is soap made?
Traditionally, coconut or olive oil has been reacted with caustic soda (NaOH) to produce a crude form of soap. This process of soap-making was quite slow and producing it in large quantities was laborious.
The contemporary process to produce soap en masse is much faster. The process begins by converting the triglycerides in the vegetable oil base to a fatty acid. This process allows the manufacturer the flexibility to use different types of vegetable oils like soya, sunflower or palm, aside from the more conventional coconut or olive oil, to make soap. The fatty acid forms when the vegetable oil is treated with hot water at a very high temperature and pressure:
Triglyceride (vegetable oil) + water = fatty acid + glycerin
The glycerin is refined to remove moisture and other impurities and converted to industrial grade or pharmaceutical grade glycerin, according to demand. The fatty acid is then used to make soap by reacting it with caustic soda (NaOH) in a large vessel:
RCOOH (fatty acid) + NaOH = RCOONa (soap) + H₂O
The soap thus produced is extracted from the mix and dried to remove excess moisture using vacuum drying. The resulting mass is then extruded through a die to produce soap “noodles”. These strings are much thicker than the noodles we eat but are much shorter in length.
At this juncture, a critical element of soap is the total fatty matter (TFM): it is the percentage of natural oils and fat in the mass. The higher the TFM, the better the soap is in terms of its cleaning performance. At this stage, the moisture content in the noodles are controlled depending on the end use. Soap noodles destined for bathing should have less moisture content than those destined for laundering soaps.
The manufacturer moves the soap noodles into a blender, where they are mixed with additional ingredients to achieve the final product. To the fatty matter and moisture already present in the noodles, the manufacturer adds perfume, colour, filler material, and performance enhancers.
Some popular perfumes in India are sandal-wood oil, which is natural, or synthetic alternatives. Similarly, colours can be plant pigments or synthetic options like oxides. Soap fillers are generally talc (that is, magnesium silicate), sodium silicate or certain sulphates.
Surfactants also known as surface active agents, are added to reduce the surface tension of water and allow the soap to spread more easily when bathing. A common surfactant is sodium lauryl sulphate. Depending on the brand, some manufacturers also add antifungal, antibacterial (e.g. triclosan), and other medicinal additives (e.g. tea-tree oil or neem oil) to the soap.
Once the soap formulation is complete, the manufacturer extrudes the blended mixture to produce long soap bars, which are then stamped in a die into the desired shape, size, and weight as individual soap cakes. Finally, they are wrapped in a specifically designed wrapper and packed in cartons for shipment.
The technologies and automation in soap-making have advanced significantly over the years such that automatic production lines can deliver 600-700 soaps per minute (100 gm each) today.
Why do soaps clean?
A soap molecule has two ends: one end attracts water (that is, it’s hydrophilic) and the other end repels water (hydrophobic). Thanks to the presence of surfactants, soap also tends to reduce the surface tension of water, allowing it to spread more evenly.
During a cleaning activity, the hydrophobic end is attracted or embeds itself in grease or dirt while the hydrophilic end stays attached to water. The act of scrubbing and rinsing then dislodges the dirt, which flows out along with the water. The detergents used to wash clothes, utensils, various surfaces, etc. are also liquid soaps in a way — but their formulation consists of a large amount of surfactants along with additives like bleach, fragrances, and dyes.
During World War I, there was a shortage of natural oils and fats to make soaps, which spurred some industrialists to look for alternatives that could be synthesised chemically. Thus, the first commercial soap-like detergents emerged in the mid-1930s.
Depending on the formulation, detergents have the ability to soften hard water making the cleaning action more effective. However, their surfactants are known to be environmentally unfriendly. The use of phosphates has spurred concerns about nutrient pollution in the soil and some sulphonates have been known to linger in the environment for many years. In light of these concerns, chemical engineers are currently developing more biodegradable surfactants and enzymes that can replace phosphates.
Both soaps and detergents are a part of our everyday lives today, so any effort that makes them more ecofriendly should be welcome.
R. Vasudevan has a decade’s experience in the manufacture of soaps and fatty acids.
Published – August 19, 2025 08:30 am IST