Soap

Background

Soap is a combination of animal fat or plant oil and caustic soda. When dissolved in water, it breaks dirt away from surfaces. Through the ages soap has been used to cleanse, to cure skin sores, to dye hair, and as a salve or skin ointment. But today we generally use soap as a cleanser or perfume.

The exact origins of soap are unknown, though Roman sources claim it dates back to at least 600 b.c., when Phoenicians prepared it from goat's tallow and wood ash. Soap was also made by the Celts, ancient inhabitants of Britain. Soap was used widely throughout the Roman empire, primarily as a medicine. Mention of soap as a cleanser does not appear until the second century a.d. By the eighth century, soap was common in France, Italy, and Spain, but it was rarely used in the rest of Europe until as late as the 17th century.

Manufacture of soap began in England around the end of the 12th century. Soap-makers had to pay a heavy tax on all the soap they produced. The tax collector locked the lids on soap boiling pans every night to prevent illegal soap manufacture after hours. Because of the high tax, soap was a luxury item, and it did not come into common use in England until after the tax was repealed in 1853. In the 19th century, soap was affordable and popular throughout Europe.

Early soap manufacturers simply boiled a solution of wood ash and animal fat. A foam substance formed at the top of the pot. When cooled, it hardened into soap. Around 1790, French soapmaker Nicolas Leblanc developed a method of extracting caustic soda (sodium hydroxide) from common table salt (sodium chloride), replacing the wood ash element of soap. The French chemist Eugene-Michel Chevreul put the soap-forming process (called in English saponification) into concrete chemical terms in 1823. In saponification, the animal fat, which is chemically neutral, splits into fatty acids, which react with alkali carbonates to form soap, leaving glycerin as a byproduct. Soap was made with industrial processes by the end of the 19th century, though people in rural areas, such as the pioneers in the western United States, continued to make soap at home.

Raw Materials

Soap requires two major raw materials: fat and alkali. The alkali most commonly used today is sodium hydroxide. Potassium hydroxide can also be used. Potassium-based soap creates a more water-soluble product than sodium-based soap, and so it is called "soft soap." Soft soap, alone or in combination with sodium-based soap, is commonly used in shaving products.

Animal fat in the past was obtained directly from a slaughterhouse. Modern soapmakers use fat that has been processed into fatty acids. This eliminates many impurities, and it produces as a byproduct water instead of glycerin. Many vegetable fats, including olive oil, palm kernel oil, and coconut oil, are also used in soap making.

Additives are used to enhance the color, texture, and scent of soap. Fragrances and perfumes are added to the soap mixture to cover the odor of dirt and to leave behind a fresh-smelling scent. Abrasives to enhance the texture of soap include talc, silica, and marble pumice (volcanic ash). Soap made without dye is a dull grey or brown color, but modern manufacturers color soap to make it more enticing to the consumer.

The Manufacturing
Process

The kettle method of making soap is still used today by small soap manufacturing companies. This process takes from four to eleven days to complete, and the quality of each batch is inconsistent due to the variety of oils used. Around 1940, engineers and scientists developed a more efficient manufacturing process, called the continuous process. This procedure is employed by large soap manufacturing companies all around the world today. Exactly as the name states, in the continuous process soap is produced continuously, rather than one batch at a time. Technicians have more control of the production in the continuous process, and the steps are much quicker than in the kettle method—it takes only about six hours to complete a batch of soap.

The Kettle Process

Boiling

• 1 Fats and alkali are melted in a kettle, which is a steel tank that can stand three stories high and hold several thousand pounds of material. Steam coils within the kettle heat the batch and bring it to a boil. After boiling, the mass thickens as the fat reacts with the alkali, producing soap and glycerin.

Salting

• 2 The soap and glycerin must now be separated. The mixture is treated with salt, causing the soap to rise to the top and the glycerin to settle to the bottom. The glycerin is extracted from the bottom of the kettle.

Strong change

• 3 To remove the small amounts of fat that have not saponified, a strong caustic solution is added to the kettle. This step in the process is called "strong change." The mass is brought to a boil again, and the last of the fat turns to soap. The batch may be given another salt treatment at this time, or the manufacturer may proceed to the next step.

Pitching

• 4 The next step is called "pitching." The soap in the kettle is boiled again with added water. The mass eventually separates into two layers. The top layer is called "neat soap," which is about 70% soap and 30% water. The lower layer, called "nigre," contains most of the impurities in the soap such as dirt and salt, as well as most of the water. The neat soap is taken off the top. The soap is then cooled. The finishing process is the same as for soap made by the continuous process.

The Continuous Process

Splitting

• 1 The first step of the continuous process splits natural fat into fatty acids and glycerin. The equipment used is a vertical stainless steel column with the diameter of a barrel called a hydrolizer. It may be as tall as 80 feet (24 m). Pumps and meters attached to the column allow precise measurements and control of the process. Molten fat is pumped into one end of the column, while at the other end water at high temperature (266°F [130°C]) and pressure is introduced. This splits the fat into its two components. The fatty acid and glycerin are pumped out continuously as more fat and water enter. The fatty acids are then distilled for purification.

Mixing

• 2 The purified fatty acids are next mixed with a precise amount of alkali to form soap. Other ingredients such as abrasives and fragrance are also mixed in. The hot liquid soap may be then whipped to incorporate air.

Cooling and finishing

• 3 The soap may be poured into molds and allowed to harden into a large slab. It may also be cooled in a special freezer. The slab is cut into smaller pieces of bar size, which are then stamped and wrapped. The entire continuous process, from splitting to finishing, can be accomplished in several hours.

Milling

• 4 Most toiletry soap undergoes additional processing called milling. The milled bar lathers up better and has a finer consistency than non-milled soap. The cooled soap is fed through several sets of heavy rollers (mills), which crush and knead it. Perfumes can best be incorporated at this time because their volatile oils do not evaporate in the cold mixture. After the soap emerges from the mills, it is pressed into a smooth cylinder and extruded. The extruded soap is cut into bar size, stamped and wrapped.

Byproducts

Glycerin is a very useful byproduct of soap manufacture. It is used to make hand lotion, drugs, and nitroglycerin, the main component of explosives such as dynamite.

Where To Learn More

Books

Cavitch, Susan M. The Natural Soap Book: Making Herbal and Vegetable-Based Soaps. Storey Communications, 1995.

Maine, Sandy. The Soap Book: Simple Herbal Recipes. Interweave Press, 1995.

Spitz, Luis, ed. Soap Technologies in the 1990s. American Oil Chemists Society, 1990.

Other

About Soap. Procter & Gamble, 1990. (513) 983-1100.

—Sheila Dow

Soap

Soaps are cleaning agents that are usually made by reacting alkali (e.g., sodium hydroxide) with naturally occurring fat or fatty acids. The reaction produces sodium salts of these fatty acids, which improve the cleaning process by making water better able to lift away greasy stains from skin, hair, clothes, and just about anything else. As a substance that has helped clean bodies as well as possessions, soap has been remarkably useful.

History of Soap

The discovery of soap predates recorded history, going back perhaps as far as six thousand years. Excavations of ancient Babylon uncovered cylinders with inscriptions for making soap around 2800 b.c.e. Later records from ancient Egypt (c. 1500 b.c.e.) describe how animal and vegetable oils were combined with alkaline salts to make soap.

According to Roman legend, soap got its name from Mount Sapo, where animals were sacrificed. Rain would wash the fat from the sacrificed animals along with alkaline wooden ashes from the sacrificial fires into the Tiber River, where people found the mixture helped clean clothes. This recipe for making soap was relatively unchanged for centuries, with American colonists collecting and cooking down animal tallow (rendered fat) and then mixing it with an alkali potash solution obtained from the accumulated hardwood ashes of their winter fires. Similarly, Europeans made something known as castile soap using olive oil. Only since the mid-nineteenth century has the process become commercialized and soap become widely available at the local market.

Chemistry of Soap

The basic structure of all soaps is essentially the same, consisting of a long hydrophobic (water-fearing) hydrocarbon "tail" and a hydrophilic (waterloving) anionic "head":

CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂COO⁻ or CH₃(CH₂)_nCOO⁻

The length of the hydrocarbon chain ("n") varies with the type of fat or oil but is usually quite long. The anionic charge on the carboxylate head is usually balanced by either a positively charged potassium (K⁺) or sodium (Na⁺) cation. In making soap, triglycerides in fat or oils are heated in the presence of a strong alkali base such as sodium hydroxide, producing three molecules of soap for every molecule of glycerol. This process is called saponification and is illustrated in Figure 1.

Like synthetic detergents, soaps are "surface active" substances (surfactants ) and as such make water better at cleaning surfaces. Water, although a good general solvent, is unfortunately also a substance with a very high surface tension. Because of this, water molecules generally prefer to stay together rather than to wet other surfaces. Surfactants work by reducing the surface tension of water, allowing the water molecules to better wet the surface and thus increase water's ability to dissolve dirty, oily stains.

In studying how soap works, it is useful to consider a general rule of nature: "like dissolves like." The nonpolar hydrophobic tails of soap are lipophilic ("oil-loving") and so will embed into the grease and oils that help dirt and stains adhere to surfaces. The hydrophilic heads, however, remain surrounded by the water molecules to which they are attracted. As more and more soap molecules embed into a greasy stain, they eventually surround and isolate little particles of the grease and form structures called micelles that are lifted into solution. In a micelle, the tails of the soap molecules are oriented toward and into the grease, while the heads face outward into the water, resulting in an emulsion of soapy grease particles suspended in the water.

With agitation, the micelles are dispersed into the water and removed from the previously dirty surface. In essence, soap molecules partially dissolve the greasy stain to form the emulsion that is kept suspended in water until it can be rinsed away (see Figure 2).

As good as soaps are, they are not perfect. For example, they do not work well in hard water containing calcium and magnesium ions, because the calcium and magnesium salts of soap are insoluble; they tend to bind to the calcium and magnesium ions, eventually precipitating and falling out of solution. In doing so, soaps actually dirty the surfaces they were designed to clean. Thus soaps have been largely replaced in modern cleaning solutions by synthetic detergents that have a sulfonate (R-SO₃⁻) group instead of the carboxylate head (R-COO⁻). Sulfonate detergents tend not to precipitate with calcium or magnesium ions and are generally more soluble in water.

Uses of Soap

Although the popularity of soap has declined due to superior detergents, one of the major uses of animal tallow is still for making soap, just as it was in years past. Beyond its cleaning ability, soap has been used in other applications. For example, certain soaps can be mixed with gasoline to produce gelatinous napalm, a substance that combusts more slowly than pure gasoline when ignited or exploded in warfare. Soaps are also used in "canned heat," a commercialized mixture of soap and alcohol that can be ignited and used to cook foods or provide warmth. Overall, soap is a remarkably useful substance, just as it has been for thousands of years.

David A. Dobberpuhl

Bibliography

Brady, James E.; Russell, Joel W.; and Holum, John R. (2000). Chemistry: Matter and Its Changes, 3rd edition. New York: Wiley.

Internet Resources

"The History and Chemistry of Soaps and Detergents." The Soap and Detergent Association. Available from the SDA Kids Corner at <http://www.sdahq.org/>.

soap, a cleansing agent. It cleanses by lowering the surface tension of water, by emulsifying grease, and by absorbing dirt into the foam.

Ancient peoples are believed to have employed wood ashes and water for washing and to have relieved the resulting irritation with grease or oil. In the 1st cent. AD, Pliny described a soap of tallow and wood ashes used by Germanic tribes to brighten their hair. A soap factory and bars of scented soap were excavated at Pompeii. Soap fell into disuse after the fall of Rome but was revived in Italy probably in the 8th cent. and reached France c.1200; Marseilles became noted as a soapmaking center. Although soap was known in England in the 14th cent., the first English patent to a soapmaker was issued in the 17th cent. The industry was handicapped in England from 1712 to 1853 by a heavy tax on soap. In the American colonies soap factories appeared at an early date, and many housewives made soap from waste fats and lye (obtained by leaching wood ashes).

The manufacture of soap was stimulated by Chevreul's discovery of oleic and stearic acids in the early 19th cent. and by Leblanc's method (1791) of preparing soda from salt. Chemically, soaps are metallic salts of fatty acids. The manufacture of soap is based on a chemical reaction (saponification) in which an alkali acts upon a fat to form a metal salt (soap) and an alcohol (glycerol). A number of methods may be employed to make soap, but all are based on the same principle of operation. Fats and oils (often blended) are heated in a large vessel, then enough alkali to react with all the fat is stirred in. Salt is added, and the soap then forms a light curd that floats to the surface. Glycerol, a valuable byproduct, can be distilled from the liquid residue.

To produce a purer soap, the curds are washed with salt solution, water is later added, and the solution is allowed to settle; the upper of the two layers thus formed is the pure soap, called settled soap. It is thoroughly churned, poured into huge frames, cut with wires, shaped, and stamped. Hard-milled soap is run over chilled rollers and is scraped off as chips which are rolled into ribbons, cut, and shaped. Soap is marketed also as chips, flakes, and beads and in powdered form. Soap powders, as distinguished from powdered soap, contain builders that assist in rough cleaning. Soaps differ according to the lathering properties of the fat or oils and according to the alkali employed. When sodium hydroxide is used as the alkali, hard soaps are formed; potassium hydroxide yields soft soaps.

Aluminum, calcium, magnesium, lead, or other metals are used in place of sodium or potassium for soaps used in industry as paint driers, ointments, and lubricating greases and in waterproofing. Fillers are added to many soaps to increase lathering, cleansing, and water-softening properties; the sodium salt of rosin is commonly used in yellow laundry soap to increase lathering. Soap substitutes include saponin-containing plants such as soapwort and shagbark and the modern soapless detergents (usually sulfonated alcohols), which may be used in hard water and even in saltwater without forming curds.

soap / sōp/ • n. 1. a substance used with water for washing and cleaning, made of a compound of natural oils or fats with sodium hydroxide or another strong alkali, and typically having perfume and coloring added: a bar of soap. 2. inf. a soap opera: the soaps are at the top of the ratings. • v. [tr.] wash with soap: she soaped her face. PHRASES: no soap inf. used to convey that there is no chance of something happening or occurring: They needed a writer with some enthusiasm. No soap.DERIVATIVES: soap·less adj.

soap Cleansing agent made of salts of fatty acids, used to remove dirt and grease. Common soaps are produced by heating fats and oils with an alkali, such as sodium hydroxide or potassium hydroxide. Soap consists of long-chain molecules; one end of the chain attaches to grease while the other end dissolves in the water, causing the grease to loosen and form a floating scum. See also detergent

soap informal term for a soap opera, a television or radio drama serial dealing typically with daily events in the lives of the same group of characters, so named (in the 1930s) because such serials were originally sponsored in the US by soap manufacturers.

soap sb. OE. sāpe = (M)LG. sēpe, MDu. seepe (Du. zeep), OHG. seipha (G. seife) :- WGmc. *saipō.
Hence vb. XVI.

soap •aslope, cope, dope, elope, grope, hope, interlope, lope, mope, nope, ope, pope, rope, scope, slope, soap, taupe, tope, trope •myope • telescope • periscope •stereoscope • bioscope • stroboscope •kaleidoscope • CinemaScope •gyroscope • microscope • horoscope •stethoscope • antelope • envelope •zoetrope • skipping-rope • tightrope •towrope • heliotrope • lycanthrope •philanthrope • thaumatrope •misanthrope •isotope, radioisotope

SOAP (səʊp) Med. subjective, objective, analysis, plan (method of compiling patients' records)