ORIGINS OF SOAP

Soap-like materials were already created way back in 2800 BC in Ancient Babylon. The recipe for soap consisted of uḥulu (ashes), cypress (oil) and sesame (seed oil). A formula for soap consisting of water, alkali, and cassia oil was written on a Babylonian clay tablet around 2200 BC.

Eygptians also combined animal and vegetable oils with alkaline salts to create a soap-like substance.

Soap was then later further developed by soap-makers in Naples were members of a guild in the late sixth century, and in the 8th century, soap-making was well-known in Italy and Spain. Soap-making is mentioned both as "women's work" and the produce of "good workmen" alongside other necessities such as the produce of carpenters, blacksmiths, and bakers.

Finer soaps were later produced in Europe in the 16th century, using vegetable oils (such as olive oil) as opposed to animal fats. Many of these soaps are still produced, both industrially and by small scale artisans. 

In modern times, the use of soap has become universal in industrialized nations due to a better understanding of the role of hygiene in reducing the population size of pathogenic micro-organisms.

How Soap Is Produced Today

The industrial production of soap involves continuous processes - involving continuous addition of fat and removal of product. Smaller scale production involves three variations:

1.       the cold process where the reaction takes place substantially at room temperature

2.       the semi-boiled or hot process where the reaction takes place at near boiling point

3.       the fully boiled process where the reactants are boiled at least once and the glycerol recovered.

The cold process and hot process (semi-boiled) are the simplest and typically used by small artisans and hobbyists producing hand made decorative soaps and similar. The glycerine remains in the soap and the reaction continuous for many days after the soap is poured into moulds. 

In the hot process method also, the glycerine is left in but at the high temperature employed, the reaction is practically completed in the kettle, before the soap is poured into moulds. This process is simple and quick and is the one employed in small factories all over the world.

In the fully boiled process on factory scale, the soap is further purified to remove any excess sodium hydroxide, glycerol, and other impurities, colour compounds etc. These components are removed by boiling the crude soap curds in water and then precipitating the soap with salt.

At this stage the soap still contains too much water which has to be removed. This was traditionally done on a chill rolls, which produced the soap flakes commonly used in the 1940s and 1950s. This process was superseded by spray dryers and then by vacuum dryers.

CHEMISTRY OF SOAP

Soap is a necessity that is used by everyone in their everyday lives. Yet has anybody wondered how does soap work? How does it remove the dirt on our skin? Or why does soap turn into something else when it comes into contact with our skin? This mysterious phenomenon can be explained with chemistry.

Soap As A Fatty Acid

In chemistry, soap is a alkai salt of a fatty acid. Soap is mainly used as surfactants for washing, bathing, and cleaning. Soaps for cleansing are obtained by treating vegetable or animal oils and fats with a strongly alkaline solution. Fats and oils are composed of triglycerides: three molecules of fatty acids attached to a single molecule of glycerol. The alkaline solution, often called lye, brings about a chemical reaction known as saponification. In saponification, the fats are first hydrolysed into free fatty acids, which then combined with the alkali to form crude soap. Glycerol, often called glycerine, is liberated and is either left in or washed out and recovered as a useful by-product.

EMULSION OF SOAP

Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.

How Soap Cleans

Soap is formed by molecules with a head which interacts with water (hydrophilic) and a long chain which repels water (hydrophobic).

When soap is added to the water, the hydrophilic heads of its molecules interacts with water molecules via ion-dipole interactions and hydrogen bonding, while the long hydrophobic chains join the oil particles and remain inwards (escaping from the water). In that way, they form circular groups named micelles, with the oily material absorbed inside and trapped. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.

Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:

CH₃(CH₂)₁₆CO₂^-Na⁺ + HCl → CH₃(CH₂)₁₆CO₂H + Na⁺ + Cl^-

These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.

2 CH₃(CH₂)₁₆CO₂^-Na⁺ + Mg²⁺ → [CH₃(CH₂)₁₆CO₂^-]₂Mg²⁺ + 2 Na⁺