Detergent and soap. A detergent is a substance that cleans soiled surfaces. Soap is a type of detergent. But detergent usually refers only to synthetic detergents. Synthetic detergents have a different chemical makeup than soap.
Soap and detergent products are produced in many forms. They may take the shape of bars, flakes, granules (grains), liquids, and tablets. People use soap to wash their bodies. They shampoo their hair and brush their teeth with soaps and detergents. Doctors clean sores and wounds with soap to kill germs that cause infection.
Detergents and soaps have many household and industrial uses. People use these products to wash dishes and laundry, to scrub floors, and to clean windows. Industries use detergents and soaps as cleaners, lubricants, softeners, and polishers. Some motor oils contain detergents. These detergents break down soot, dust, and other particles that can harm engine parts. Tire manufacturers apply soap to hot tires. The soap prevents the tires from sticking to the molds used in vulcanizing (hardening) rubber. In addition, soap is used to polish jewelry and to soften leather.
Detergents and soaps contain a basic cleaning agent called a surfactant or surface active agent. Surfactants consist of molecules that attach themselves to dirt particles in soiled material. The molecules pull these particles out of the material. They hold the particles in the wash water until they are rinsed away.
The chemical industry produces a wide variety of synthetic surfactants. Each has a different chemical makeup. There are three main types of synthetic surfactants. One type becomes positively charged when exposed to water. Nearly all the surfactants used in detergents and soaps are of this type. Another type of surfactant becomes negatively charged. The third type remains neutral.
Most detergents contain a synthetic surfactant along with other chemicals. The other chemicals may improve a detergent’s cleaning ability or make it easier to use. All soaps consist of basically the same kind of surfactant. Detergents and soaps may also contain perfumes, coloring agents, and germicides (germ-killing agents).
Detergents have certain advantages over soaps. For example, detergents have the ability to clean effectively in hard water. Hard water contains certain minerals. Many soaps cannot be used to launder in it. Such soaps react with the minerals. They form a substance called lime soap or soap curd. Lime soap does not dissolve. As a result, it is difficult to remove from fabrics and other surfaces. It also causes “bathtub ring.” Detergents do not leave such deposits. They also penetrate soiled areas better than soap does. In addition, detergents dissolve more readily in cold water.
How detergents and soap work
Detergents and soaps clean soiled material in much the same way. First, the soiled material must be wetted. Then, particles of dirt are removed from the material. The particles are finally suspended (held) in the water until they are rinsed away.
Wetting the material.
The surfactants in detergents and soaps increase the wetting ability of water by lowering its surface tension. Surface tension holds the water molecules together. It also causes water to form drops.
Molecules of surfactant gather at the water’s surface. There, they force the water to expand and spread out. With its surface tension reduced, water penetrates the soiled material more completely. Lowering the surface tension also causes surfactants to form bubbles and suds. Suds do not affect the cleaning ability of the product.
Removing the dirt.
The surfactants in detergents and soaps also help remove dirt. A surfactant has two distinct parts with different characteristics. One part of each surfactant molecule is hydrophilic (attracted to water). The other part is hydrophobic (repelled by water). The hydrophobic parts of surfactant molecules attach themselves to any surface other than water. They do this by means of electrical attraction. Many of them hold on to and surround the particles of dirt in the soiled material. At the same time, the hydrophilic parts pull away from the material and toward the wash water.
The mechanical agitation (motion) of a washing machine, or the movement caused by rubbing by hand, helps break up the dirt. The agitation also helps the hydrophilic parts of the surfactant molecules pull the dirt particles from the material and into the water.
Suspending the particles.
After the dirt particles are in the water, the thin layer of surfactant molecules around the particles keeps them separated. These molecules prevent the dirt from settling on the washed material again. The dirt particles remain suspended in the water until they are rinsed away.
How soap is made
The chief ingredients of soap are fats and chemicals called alkalis. Manufacturers may use animal fats or such vegetable oils as coconut oil and olive oil. Most soapmakers use sodium hydroxide (often called lye or caustic soda) as the alkali. Potassium hydroxide is the alkali in liquid soaps and in some bar soaps. Manufacturers use two chief methods to make soap. They are the kettle method and continuous processing.
The kettle method.
Until the early 1940’s, soap companies made most soap in large kettles. Some soap is still made by this method. Manufacturers use steel tanks that stand three stories tall. The tanks hold more than 100,000 pounds (45,000 kilograms) of ingredients. Steam passes through coils in the tanks. The steam heats the mixture of fats and alkali for several hours.
The heat triggers a chemical reaction called hydrolysis or saponification. This reaction causes a creamy soap to form within the mixture. Salt is added to the soap. The salt causes the mixture to separate into two layers. The soap, called neat soap at this stage, rises to the top. A solution of excess alkali, salt, and a liquid called glycerol remains beneath the layer of soap (see Glycerol).
Other ingredients are added in a huge mixer called a crutcher. They include perfumes, colors, germicides, and builders (substances that help remove dirt). The soap mixture is then hardened into bars or made into flakes or granules.
Continuous processing
makes as much soap in a few hours as can be made in several days by the kettle method. In continuous processing, soap manufacturers use a stainless-steel tube called a hydrolyzer. The tube measures about 3 feet (90 centimeters) in diameter. It is about 80 feet (24 meters) in height. Water under high pressure and heated to a temperature of 500 °F (260 °C) is pumped into the top of the hydrolyzer. At the same time, a machine pumps in hot fat at the bottom. The fat splits into fatty acids and glycerol. The fatty acids rise to the top. They are removed from the hydrolyzer. They are then purified and mixed with alkali to make soap. The soap is then mixed with other ingredients in a crutcher. It is made into bars, flakes, or granules.
Bar soaps
are made for bathing and for laundry use. Manufacturers use several methods to make bars of soap. They make floating soaps by mixing the warm soap solution with air in a machine equipped with cooling coils. The machine cools the soap. It also squeezes the soap out in the form of a long continuous bar.
Another method involves several sets of rollers called mills. The mills mix and squeeze soap flakes to make milled soap. The milling operation produces a hard soap. This soap lathers better than floating soaps.
Modern continuous finishing machinery makes soap bars of better quality than those produced by other methods. A machine sprays hot liquid neat soap into a vacuum chamber. There, excess moisture and impurities are removed from the soap. Then the dried soap is cut into the shape of noodles. The noodles are fed into one or two kneading units. Perfume is added to the soap. The soap comes out of the units in a long bar called a log.
Soap made by any of the above methods is cut into small bars of the desired size, called blanks. A press or stamper forms the bars into various shapes. It also presses the brand name into the finished soap.
Bar soaps used for bathing are usually called toilet soaps. These soaps consist entirely of soap or of a mixture of soap and synthetic surfactants. The synthetic surfactants break up lime soap. They prevent the formation of bathtub ring and other deposits. Deodorant toilet soaps contain a small amount of a germicide.
Granules and flakes.
Almost all soap used for home laundering is produced in the form of granules or flakes. Manufacturers make soap granules by pumping warm soap from a crutcher to the top of a tall drying tower. The soap is sprayed into a stream of hot air. This process dries the soap into bubblelike granules. The granules fall to the bottom of the tower. A filter removes extremely fine particles. Coarse particles are screened out. The filtering and screening leave only granules of about the same size.
Soap flakes are made by pouring soap from a crutcher between two steel rollers. One roller is hot and the other is cold. A thin sheet of soap sticks to the cold roller. As the roller turns, the soap is cut into ribbons. A blade scrapes the soap ribbons off the roller. Then the ribbons enter a dryer. In the dryer, they break or are cut into flakes.
How detergents are made
The manufacture of detergents involves several complicated chemical processes. First, the synthetic surfactant is made in a chemical plant. A variety of substances may be used. They include by-products of petroleum, as well as the same vegetable oils and animal fats used to make soap. For example, many manufacturers use beef fat, called tallow, in the first step of the process. The tallow is made to react chemically with methyl alcohol. The resulting product is treated with hydrogen gas. This process produces hydrogenated tallow alcohol. This liquid is treated first with sulfuric acid and then with an alkali. The resulting product is a synthetic surfactant.
Other ingredients are mixed with the synthetic surfactant in a crutcher. They include bleaches, builders, fabric brighteners, and suds stabilizers. Ingredients called antiredeposition agents help prevent removed dirt from returning to cleaned material. The detergent mixture is then processed into granules, flakes, tablets, or a liquid.
Detergent granules and flakes are produced in much the same way as soap granules and flakes. Manufacturers make detergent tablets by adding special ingredients to detergent granules. They then press the mixture into tablet form. Liquid detergents are made by adding various ingredients to the surfactant. These ingredients keep the detergent a liquid at normal temperatures.
History
Early soap.
No one knows when or where people first made soap. The ancient Romans may have used soap 3,000 years ago. People in France used a rough soap about A.D. 100. By about 700, soapmaking had become a craft in Italy. Spain was a leading soapmaker by 800. Soapmaking began in England about 1200.
In the late 1700’s, Nicolas Leblanc, a French scientist, found that lye could be made from ordinary table salt. Following Leblanc’s discovery, soap began to be made and sold at prices that almost everyone could afford.
Many early settlers in North America made their own soap. They poured hot water over wood ashes to make the alkali potash. Then they boiled the potash with animal fats in iron kettles to make soap. The soap cleaned well. But much of it was harsh and had a bad odor.
The soap industry in North America began in the early 1800’s. Some people collected waste fats from others and made soap in large iron kettles. They poured the soap into large wooden frames for hardening. Then they cut the hardened soap into bars. The bars were sold from door to door. Since the early 1900’s, manufacturers have made big improvements in the mildness, color, fragrance, and cleaning ability of soaps.
The development of detergents.
Fritz Gunther, a German scientist, is usually credited with developing in 1916 the first synthetic surfactant for use in detergents. Industries used his product. However, it was too harsh for household use. In 1933, the first household detergents based on synthetic surfactants were introduced in the United States. The shortage of fats and other chemical raw materials during World War II (1939-1945) slowed the further development of such products. After the war, several soap companies began to produce detergents based on synthetic surfactants. Since then, the detergent industry has developed a variety of detergents for almost every cleaning job.
Before 1965, detergents in sewage sometimes caused surface foam on rivers and streams. Most detergents contained a synthetic surfactant called alkylbenzene sulfonate. It did not break down completely in sewage treatment systems. In 1965, after more than 10 years of research, the detergent industry developed a surfactant called linear alkylbenzene sulfonate (LAS). Bacteria quickly break down LAS molecules. As a result, detergents that contain LAS do not cause foam.
In the early 1970’s, scientists observed that chemicals called phosphates contributed to water pollution. Phosphates were widely used in detergents as builders. When phosphates and other chemicals enter rivers and lakes, they overfertilize simple water organisms called algae. Overfertilization increases the growth of algae. Excessive algae growth causes the oxygen supply in the water to be used up. Fish cannot live in such water, and so they die. Their bodies pollute the rivers and lakes, which also become choked by the algae. See Eutrophication ; Water pollution.
To help solve the problem, several state and local governments banned the sale of detergents that contained phosphates. Manufacturers reduced the amount of phosphates in many detergents. They also developed several phosphate substitutes. This development enabled them to produce phosphate-free detergents. See Phosphate.
See also Antiseptic; Disinfectant.
