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Soap

Soap is a surfactant cleaning mixture used for personal or minor cleaning. It usually comes in solid moulded form, termed bars. In the developed world, synthetic detergents have superseded soap as a laundry aid.

Many soaps are mixtures of sodium or potassium salts of fatty acids which can be derived from oils or fats by reacting them with an alkali (such as sodium or potassium hydroxide) at 80°–100 °C in a process known as saponification. The fats are hydrolyzed by the base, yielding glycerol and crude soap. Historically, the alkali used was potash made from the deliberate burning of vegetation such as bracken, or from wood ashes.

Soap most commonly appears in bar form. This particular bar of soap has seen some use and thus has lost its "bar" shape.
Soap most commonly appears in bar form. This particular bar of soap has seen some use and thus has lost its "bar" shape.

Soap is derived from either oils or fats. Sodium Tallowate, a common ingredient in many soaps, is in fact rendered animal fat. Soap that is made of vegetable oils, such as olive oil, are generally termed castile soap.

Purification and finishing

The common process of purifying soap involves removal of sodium chloride, sodium hydroxide, and glycerol. These impurities are removed by boiling the crude soap curds in water and re-precipitating the soap with salt.

Most of the water is then removed from the soap. This was traditionally done on a chill roll which produced the soap flakes commonly used in the 1940s and 1950s. This process was superseded by spray dryers and then by vacuum dryers.

The dry soap (approximately 6-12% moisture content) is then compacted into small pellets. These pellets are now ready for soap finishing. Soap finishing is the process of converting raw soap pellets into salable product, usually bars.

Soap pellets are combined with fragrances and other materials and blended to homogeneity in an amalgamator (mixer). The mass is then discharged from the mixer into a refiner which, by means of an auger, forces the soap through a fine wire screen. From the refiner the soap passes over a roller mill (French milling or hard milling) in a manner similar to calendering paper or plastic or to making chocolate liquor. The soap is then passed through one or more additional refiners to further plasticize the soap mass. Immediately before extrusion it passes through a vacuum chamber to remove any entrapped air. It is then extruded into a long log or blank, cut to convenient lengths, passed through a metal detector and then stamped into shape in refrigerated tools. The pressed bars are packaged in many ways.

Sand or pumice may be added to produce a scouring soap. This process is most common in creating soaps used for human hygiene. The scouring agents serve to remove dead skin cells from the surface being cleaned. This process is called exfoliation. Many newer materials are used for exfoliating soaps which are effective but do not have the sharp edges and poor size distribution of pumice.

Use

Although the word soap continues to be used informally in everyday speech and product labels, in practice nearly all kinds of "soap" in use today are actually synthetic detergents, which are less expensive, more effective, and easier to manufacture. While effort has been made to reduce their negative effect upon the environment, the results have been mixed.

Soaps are useful for cleansing because soap molecules attach readily to both nonpolar molecules (such as grease or oil) and polar molecules (such as water). Although grease will normally adhere to skin or clothing, the soap molecules can attach to it as a "handle" and make it easier to rinse away. Allowing soap to sit on any surface (skin, clothes etc) over time can imbalance the moisture content on it and result in the dissolving of fabrics and dryness of skin.

(water soluble end)
CH3-(CH2)n - COONa
(fatty end)

The hydrocarbon ("fatty") portion dissolves dirt and oils, while the ionic end makes it soluble in water. Therefore, it allows water to remove normally-insoluble matter.

The history and process of soap making

1922 magazine advertisement for Palmolive Soap
1922 magazine advertisement for Palmolive Soap

The earliest known evidence of soap use are Babylonian clay cylinders dating from 2800 BC containing a soap-like substance. A formula for soap consisting of water, alkali and cassia oil was written on a Babylonian clay tablet around 2200 BC.

The Ebers papyrus (Egypt, 1550 BC) indicates that ancient Egyptians bathed regularly and combined animal and vegetable oils with alkaline salts to create a soap-like substance.

A soap factory with bars of scented soap was found in the ruins of Pompeii (79 AD).

Legend has it that soap gets its name from Mount Sapo where ancient Romans sacrificed animals. Rain would send a mix of animal tallow and wood ash down the mountain and into the clay soil on the banks of the Tiber. Eventually, women noticed that it was easier to clean clothes with this "soap".

Historically, soap was made by mixing animal fats with lye. Because of the caustic lye, this was a dangerous procedure (perhaps more dangerous than any present-day home activities) which could result in serious chemical burns or even blindness. Before commercially-produced lye was commonplace, it was produced at home for soap making from the ashes of a wood fire.

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 microorganisms. Manufactured bar soaps first became available in the late nineteenth century, and advertising campaigns in Europe and the United States helped to increase popular awareness of the relationship between cleanliness and health. By the 1950s, soap had gained public acceptance as an instrument of personal hygiene.

Handmade soap

Handmade soap
Handmade soap

Some individuals continue to make soap in the home. The traditional name "soaper", for a soap-maker, is still used by those who make soap as a hobby. The most popular soap-making processes today are the cold process and the melt and pour process. Some soapers also practice other processes, such as the hot process, and make special soaps such as glycerin soap.

Handmade soap differs from industrial soap in that sufficient fat, and usually more, is used to consume the alkali, and in that the glycerin is not removed. Superfatted soap is more skin-friendly than industrial soap. Often emollients such as jojoba oil or shea butter are added at trace, after most of the oils have saponified, so that they remain unreacted in the finished soap.

Disadvantages

Today, fat-based soaps have mostly been superseded by modern detergents. Washing agents do not contain soap for cleaning fabric, but to reduce foaming.

The disadvantages of soap are:

  • Soap deprives the skin of natural, beneficial oils
  • Soap can react mildly basically with fabrics resulting in damage over the long term. This is usually due to excess sodium hydroxide (NaOH, an alkali/base) left from manufacture, but can also be caused by the very slight presence of NaOH from the equilibrium reaction:
    R-CO2-Na + H20 <—> R-CO2- + Na+ + H20 <———> R-COOH (=fatty acid) + NaOH (alkali)
    However, this equilibrium is way over to the left and the fraction of NaOH formed is minuscule.
  • Soap reacts with lime to form an insoluble deposit (soap scum) in "hard water":
    2R-COO-Na+ + Ca2+(HCO3-)2 (calcium bicarbonate) ———> Na+(HCO3)- + Ca(R-COO)2 ,where R stands for an alkyl group.(ppt)


Commercially made bar soaps are formulated to mitigate all of these disadvantages:

  • A wide variety of emollient materials, such as shea or cocoa butters, are substantive to the skin.
  • Poorly finished soaps contain alkali (NaOH) and react mildly basically with skin and fabric; commercial products are finished to neutrality or to a slight acid content to prevent this and be more compatible with the skin's slightly acidic pH.
  • Commercial products use chelating molecules, often EDTA derivatives to bind with any free Ca or Mg molecules and prevent soap scum. These also help reduce fragrance loss, discolouration and rancidity.

See also

References

  • Maine, Sandy (1995). The Soap Book: Simple Herbal Recipes. Interweave Press. ISBN 1883010144.

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