Soap

This article is about a common cleaning mixture. For other uses of the word Soap, see Soap (disambiguation).

Soap is a surfactant cleaning mixture used for personal or minor cleaning. It usually comes in solid moulded form. Synthetic detergents have superseded soap as a laundry aid.

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.

Table of contents

1 Purification and finishing

Purification and finishing

Reaction contains 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.

Sand or pumice may be added to produce a scouring soap.

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.

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.

(water soluble end)

CH3-(CH2)n - COONa

(fatty part)

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

The ancient world was generally innocent of soap; the Romans built baths, but did not use soap in them. According to Pliny the Elder, soap was invented by the ancient Gauls. They did not use it for washing, though; they used it as a pomade to keep their hair shiny.

Historically, soap was made in the home 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 eliminating disease vectors such as germs. 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

Disadvantages

Nowadays soap has 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 oils
Soap reacts basically, which results in damage to the fabric:
R-CO₂-Na --H₂0--> R-CO₂^- + Na⁺ + H₂0 ----> R-COOH (=fatty acid) + OH^- (basically)
Soap reacts with lime to insoluble deposit:
2R-COO^- + Ca²⁺ (lime) ----> Ca(R-COO)₂