SOAPS AND LAUNDRY PRODUCTS:SYNTHETIC DETER GENTS AND SURFACTANTS.

SYNTHETIC DETER GENTS: SURFACTANTS

In the modern world, the laundering of clothes and other types of cleaning processes have been accomplished via the displacement of soap in favor of products called synthetic detergents. Laundry detergents are currently available as liquids or powders and are formulated to perform soil and stain removal and fabric treatment (e.g., bleaching, softening, and conditioning) under varying water chemistry conditions and temperatures. While liquid detergents are recommended for use on oily soils and the pretreatment of soils and stains, powders are apparently quite effective in lifting clay-based soils and ground-in dirt off fabrics. Laundry detergents first came into use because of problems encountered with ordinary soaps, including deterioration on storage, lack of specialized cleaning ability, and lack of complete rinsing out of fabrics after washing. In addition, soaps were unsatisfactory for laundering in hard water. The characteristic of water “hardness” can be attributed to the presence of metal ions (excluding group 1A metals), primarily calcium (Ca2+) and magnesium (Mg2+) ions, which form insoluble precipitates when combined with the fatty acid anions of ordinary soap. An example of such a precipitate is the formation of the very undesirable “bathtub ring” within the bathtub or washbasin. If soap were used as a primary laundering agent, such precipitate residues would also gradually build up as deposits on clothing, causing bad odor and the deterioration of fabric materials. Thus, the first synthetic detergents were developed in an effort to over- come the reaction of soaps with hard water. While the chemical structure of synthetic detergents differs slightly from that of soaps, they are both efficient surfactants.

The chemical process of soap synthesis remained steadily similar throughout the ages, until approximately 1916, when the first synthetic detergent was developed in Germany in response to the shortage of available fats for soap making during World War I. These detergents were basically short-chain alkyl naphthalene sulfonates, made by coupling propyl or butyl alcohols with naphthalene and subsequent sulfonation (addition of sulfur atoms). In the late 1920s and early 1930s, long-chain alcohols were sulfonated and sold as the neutralized sodium salts. House- hold detergent production in the United States began in the early to mid 1930s but did not increase in manufacture until after World War II, when the raw materials for soap manufacture were both scarce and costly. These detergents were long-chain alkyl aryl sulfonates with ben- zene as the aromatic center and the alkyl portion synthesized from kerosene. During World War II, fat and oil supplies were interrupted, and the need for a cleaning agent for use in mineral-rich cold seawater was real- ized within the U.S. military. Further research concerning detergent chemistry was stimulated, and some of the first commercially available synthetic detergents were marketed/used for hand dishwashing and the laundering of fine fabric. Within a few years, cheap synthetic detergents, mostly synthesized from petroleum products, were widely available. During this time, alkyl aryl sulfonates had nearly completely overshad- owed the sales of alcohol sulfate-based laundry detergents. Molecules of synthetic detergents were similar enough to soap to have the same excellent cleansing action but sufficiently different to resist the effects of acidic and hard water.

The ingredients in any commercially available synthetic detergent are contained within eight different groups known as the surfactant system: the builders (both inorganic and organic), fluorescent dyes (optical whit- eners), enzymes, corrosion inhibitors, bleaches, a filler, fragrances, and coloring agents. While builders, bleaches, optical whiteners, and en- zymes will be treated as laundry aids as separate everyday products, the functions of fillers, corrosion inhibitors, fragrances, and coloring agents will be discussed here. One of the major functions of fillers (also called processing agents), primarily sodium sulfate (Na2SO4), was/is to add bulk to the detergent to provide the consumer with a fair sense of volume verses monetary product worth. Fillers also allow granular deter- gents to pour from the packaging box more freely. The amount of filler within a given detergent product may range from 35 percent to 0 per- cent. Corrosion inhibitors prevent the potential deleterious effects of detergent ions that would otherwise quickly rust the steel inside a washing machine. Rusting is an electrochemical process whereby the iron in steel is attacked by negatively charged hydroxyl ions, or detergent ions. These compounds are usually sodium silicates, water-soluble glasses. Fragrances add pleasant odors to the detergent and thus to just-washed fabrics. Coloring agents, primarily blue-toned coloring agents, coat fabrics and pro- vide a blue tint to washed fabrics. Bluing agents contain a blue dye or pigment taken up by fabrics in the wash or rinse cycles; thus, bluing absorbs the yellow part of the light spectrum, counteracting the natural yellowing of many fabrics. This process adds a small amount of brilliance to white fabrics and gives them an appearance of extra cleanliness.

The surfactant system includes major active ingredients such as deter- gents, which form micelles and clean grease off clothing and other items through the same mechanism as soaps. Warm or hot water assists in dis- solving grease and oil within soiled clothing, and modern washing ma- chine agitation (or hand rubbing) helps lift soil out of fabrics. Modern detergent surfactants are synthesized from a variety of petrochemicals (derived from petroleum products) and/or oleochemicals (derived from fats and oils). Similar to the fatty acids used in soap making, petroleum, fats, and oils contain hydrocarbon chains that are hydrophobic but attracted to greasy oils and dirt. The hydrocarbon chains are the foundation of the hydrophobic tail end of the surfactant molecule. Other chemicals, including sulfur trioxide, sulfuric acid, and ethylene oxide, are used to produce the hydrophilic head end of the surfactant molecule. As in soap making, an alkali is used to make detergent surfactants, with sodium and potassium hydroxide being the most common alkalis used.

The solution for the soap precipitation problem in hard water was first realized with the development of alkylbenzene sulfonate (ABS) detergents. ABS detergents were synthesized from propylene (CH2=CHCH3), which is available from petroleum, benzene, and sulfuric acid (H2SO4). Alkyl- benzene is a product of the petroleum industry and is made by the condensation of an a-olefin with benzene. By treating this insoluble material through a process called sulfonation (adding an excess of sulfuric acid), it is converted to the corresponding sulfonic acid. The resulting sulfonic acid (alkylbenzenesulfonic acid [RSO3H]) is then neutralized with a base (e.g., sodium carbonate [NaCO3] or sodium hydroxide [NaOH]), to yield the final branched-chain product (e.g., ABS). The similarities of de- tergent (RSOOO-Na+) and soap (RCOO-Na+) are obvious. Thus, soap molecules have a carbon (C) atom bonded to the oxygen to which the sodium is bonded, and detergents have a sulfur (S) atom in that position. Similar to soap, this ABS synthetic detergent is both hydrophilic and hydrophobic. The detergents produced in this manner are significantly more soluble than soap. Thus, it can stabilize water-oil emulsions, but unlike soaps (fatty acid salts), the sodium can be replaced by calcium or magnesium and the detergent remains in solution, even in hard water (i.e., soap scum is not formed). However, ABS detergents are significantly more stable than soaps and can persist in wastewater systems long after use and discharge.

The increased stability of ABS detergents results from the greater chemical stability of the sulfonate grouping and the raw material petro- leum-based branching characteristic of the long hydrocarbon chain molecules, in sharp contrast to the straight-chain hydrocarbons derived from animal fats. In the natural environment, bacteria did not readily degrade branched ABS detergents; thus, foaming and sudsing began to accumulate within sewage treatment plants, natural waterways (e.g., rivers), and even some reservoirs used as sources of drinking water. By the early 1960s, groundwater supplies were perceived as threatened, and public outcry, along with governmental legislation, persuaded the United States to solve the problem by replacing ABS detergents with biodegradable linear alkylbenzene sulfonate (LAS) detergents. This detergent consists of a long hydrocarbon chain attached to an aromatic or benzene ring attached to a negatively charged sulfonate group. The sulfonate group involves one sulfur atom and three oxygen atoms. The negatively charged head structure allows for the entire surfactant molecule to be easily carried away by water molecules. LAS detergents possess linear unbranched chains of carbon atoms within the hydrocarbon tail of the molecule, often referred to as a linear alkyl group. Microorganisms can thus readily break down LAS molecules by producing enzymes that degrade the molecule, two carbons at one time. Branching of the ABS detergents had inhibited this enzyme degradation reaction. Consequently, the ABS-containing detergents, a major detergent formulation throughout the 1950s, were replaced around 1965 by detergents containing LAS surfactants, which are readily biodegradable (the process of decomposition of an organic [carbon-based] material by naturally occurring microorganisms) and non- polluting. In addition, many liquid detergent formulations contain other efficient, yet more expensive, surfactants called alcohol ether sulfates (AES). AES molecules have a hydrocarbon tail portion derived from an alcohol (or alkylphenol), a polar head portion derived from ethylene oxide, and a sulfate portion.

In domestic synthetic detergents, nonionic surfactants are increasingly used, but anionic surfactants predominate. Anionic detergents, which constitute the great volume of all synthetic powder detergents, are par- ticularly effective at cleaning fabrics that absorb water readily, such as those manufactured from natural fibers (e.g., cotton, wool, and silk). Anionic surfactants (e.g., linear alkylbenzene sulfonate, alcohol ethoxy sulfates, alkyl sulfates) react with hydrocarbons derived from petroleum, fats, or oils to produce new acids similar to fatty acids. A second reaction adds an alkali to the new acids to produce a type of anionic surfactant molecule. Nonionic detergents, many of which have a polar end group that is not ionic and large numbers of oxygen atoms covalently bonded to their hy- drophilic structures, are particularly effective in cleaning synthetic fabrics (e.g., polyester). Most nonionic detergents are used to produce liquid laundry detergents and produce little foaming action. Typical nonionic detergents have a phenolic group [C6H4(OH)], an extremely polar but nonionic group. Nonionic surfactant molecules (e.g., alcohol ethoxylates, alkylphenol ethoxylates, coconut diethanolamide [an alkylolamide]) are produced by first converting the hydrocarbon to an alcohol and then reacting the fatty alcohol with an ethylene oxide. The alkylolamide is prepared by making the fatty acids obtained from coconut oil react with an ethylene oxide derivative called monoethanolamine. These nonionic surfactants are not actually salts and tend to be rather waxy products or liquids. These nonionic surfactants can then be reacted further with sulfur-containing acids to form another type of anionic surfactant mole- cule. While nonionic surfactants typically do not keep dirt particles in suspension as well as anionic surfactants, some nonionic surfactants have the unusual property of being more soluble in cold water than in hot and therefore more suitable for cold water laundering needs. Nonionic surfactants are unaffected by hard water and are actually better than anionic detergents at removing particular soils—for example, they are well suited to remove skin oils from synthetic fibers.