Soaps and laundry products: personal cleansing products and bar soap

Soaps and Laundry Products

For a substance to play a role in cleaning, it must remove items such as dirt and grease from various materials (e.g., clothes, carpets, surfaces, animal skin tissue, etc.). Dirt and grime usually adhere to skin, clothing, and other types of surfaces because they combine with greases and oils (e.g., body oils, cooking fats, lubricating greases, etc.) that act similarly to glues. Because oil is not hydrophilic and thus not soluble in water, attempting to wash dirt and grime from surfaces using water alone results in little success. Soaps and detergents usually found in a typical modern consumer home can be grouped into four categories: personal cleaning/hygiene, laundry, dishwashing, and household cleaning. In all types of cleaning, the soil is transferred from the dirty object into a sepa- rate phase, either liquid (e.g., water) or solid, and then removed from the object. Essential to personal and public health, soaps and detergents contribute to good personal hygiene, reduce the presence of disease- causing microorganisms, extend the value and usefulness of clothing and household surfaces and furnishings, and maintain environments of well- being, all through their ability to loosen and remove soil from various synthetic and natural surfaces.

To understand the physical process involved with effective cleaning, it is helpful to understand basic soap and detergent chemistry. One of the most common liquids used for cleaning is water. Water (H2O) has a property called “surface tension.” Within/throughout the bulk of the liquid water, the total force of attraction exerted by any one molecule of water on all its surrounding neighbors is dispersed spherically, in all directions. However, at the surface, where there are only occasional mole- cules of gases in the atmosphere and the very few water molecules that have escaped via evaporation, the water molecules are only surrounded by other water molecules below and to the side. A tension is created as the water molecules at the surface are pulled into the main body/bulk of the liquid water. It is this strong attraction of the surface water molecules for each other and for the water molecules immediately below them that results in the cohesive property of the surface known as surface tension. Surface tension causes water to “bead up” on surfaces (e.g., human skin, glass, cloth fabric, furniture), which slows the overall wetting of the sur- face and thereby inhibits the cleaning process. For the cleaning process to be successful, the surface tension must be reduced so that water can evenly overspread, wet, and saturate surfaces. Organic (carbon-based) chemicals that decrease surface tension are termed “surface active agents” or, simply, “surfactants.” When added to water, the hydrocarbon chains of surfactants form the uppermost layer in the liquid. While these hydrocarbon chains pull together and create surface tension, they attract one another with what are called van der Waals forces, not hydro- gen bonds, and thus create a surface tension only about 30 percent that of water molecules. Overall, the presence of surfactants within a body of water significantly reduces the surface tension normally exhibited by the water.

Surfactants are also involved with other aspects of successful cleaning, including loosening, emulsifying (dispersing one liquid into another im- miscible liquid, which prevents oily soils from resettling on the surface of water), and holding soil in suspension until it can be removed and/or washed away. The arrangement of soap micelles within water is called a stable emulsion. Unlike a regular mixture of oil and water, this arrange- ment does not separate when it sits unattended. Thus, surfactants may act as emulsifiers, substances that help to form and stabilize emulsions. This means that, although oil, which attracts greasy dirt, does not actually mix with the water, soap can suspend the oil and greasy dirt in such a way that it can be removed. Therefore, the cleaning power of many surfactants results from the enhanced ability of the water to wet the normally hydrophobic (non-water-soluble) surface, penetrate into fibers more freely, and lift off the dirt. For this reason, surfactants are often referred to as “wetting agents” because they help water actually wet surfaces. Many surfactants provide an alkaline environment within the cleaning process, which can be essential in removing acid-based soils and grime.

Surfactants are classified by their ionic (electrically charged particles) properties in water. Surfactant molecules can be described as resembling a tadpole (immature frog) because they contain a fairly long fatty acid tail (hydrophobic or water insoluble) and a small, often electrically charged head (hydrophilic or water soluble). The long hydrocarbon (CH2 groups) tails of surfactants are soluble in hydrophobic substances such as oil, and the hydrophilic heads of surfactants containing carboxyl or sulfonate groups are soluble in water. Water is polar: the H2O molecules have an attraction for other polar substances, such as common table salt (NaCl). When salt is added to water, the salt molecule ions are attracted to, and become surrounded by, the water molecules. This occurrence is known as solubility. Oil, however, is a nonpolar substance and therefore will only dis- solve in other nonpolar substances. Because nonpolar substances cannot form hydrogen bonds with water molecules, they do not bind well with water and are essentially insoluble in water. Thus, there is an actual chem- ical foundation behind the phenomenon that oil and water do not mix! Surfactants promote an environment in which oil can seemingly dissolve in water by bridging the oil-water interface. There are four possible combinations of surfactants: (1) anionic, in which a negative charge is concentrated in the hydrophilic head region; (2) nonionic, which does not have a specific charge; (3) cationic, in which the hydrophilic head region carries a positive charge; and (4) amphoteric, which carries both a positive charge and a negative charge in the same molecule. Amphoteric sur- factants act as either an anion (negative charge) or cation (positive charge), depending on the pH of the solution in which they are used. Many cleaning products contain two or more types of surfactants to optimize their intended cleaning use.

Detergents are excellent examples of surfactants. The word “deter- gent” comes from the Latin detergere, meaning to clean or to wipe off. Thus, a detergent is any chemical substance that cleans, particularly if the detergent removes nonpolar substance such as greasy oils, fats, and waxes from skin, food, and plants. Typical modern detergents include soaps and synthetic detergents. As the water surface becomes saturated with detergent surfactant molecules and additional detergent is added, the ex- cess detergent molecules will become crowded out of the surface. When the concentration of detergent molecules in the water reaches a certain value, called the “critical micellar concentration,” the excess detergent molecules initiate the process of shielding their hydrophobic tails from the water molecules by clustering into micelles within the main body/ bulk of the liquid water. In general, the term “micelle” refers to very small (submicroscopic) spheres or globules of a particular substance dis- tributed throughout another substance, usually a liquid. These communal micelle molecules contain roughly forty to one hundred molecules. Within the bulk of the liquid water, the hydrophilic heads of the deter- gent molecules form the surface of the spheres, while the hydrophobic tails point inward, forming a group of hydrophobic hydrocarbon chains shielded from the surrounding water molecules. Thus, the detergent molecules are well dispersed in the water, present as a colloid, but are not actually dissolved. The inside of the micelle is nonpolar and therefore tends to collect oily soil molecules. Greasy oils and dirt within the water dissolve in the hydrophobic tail portion in the center of the micelle and are broken down into tiny droplets and dispersed within the aqueous so- lution. The micelles float freely within the body of water and collect and hold onto any oil molecules they encounter. The oily dirt is then washed away, possibly down the drain, as a result of the interaction/attraction of the hydrophilic head portion with the water molecules on the surface of the micelle spheres. Polar soil molecules, including salts from perspiration and ground dirt, simply dissolve in the water, where they become ions that are carried away in the shells of water molecules.

Soap, other detergents, and most dirt are biodegradable. For example, when a detergent/dirt/water suspension is released into the environment via a drainage system, it can be converted into either biomass or carbon dioxide and the associated remaining water returned to the natural environment.

PE RS ONAL CLEANSING PRODUCTS and  BAR SOAP

Soap is the oldest surfactant; it is thought to have been in use for more than 4,500 years. The origins of personal cleanliness quite possibly date back to prehistoric times. Early people cooking their meats over fires may have noticed that after a rainstorm, the strange foam around the remains of the fire and its ashes caused their cooking pots and hands to become cleaner than was ordinarily expected. In early societies that developed near waterways, a soaplike substance is thought to have been extracted from plants such as soapwort, soap root, soap bark, yucca, horsetail, fuchsia leaves, bouncing bet, and agave, all of which tend to flourish on riverbanks or near lakes. It is recorded that Babylonians were making soap around 2800 BC. Evidence of such soap making was made apparent after a soaplike material was found in clay cylinders during an excavation of ancient Babylon. Inscriptions on the cylinders indicated that fats were boiled with ashes, which is a known method of soap making. Evidence also indicates that soap making was known to the Phoenicians around 600 BC. While the evidential purpose of this early “soap” was unclear, it is thought that these early references to soap and soap making indicate the use of soap in the cleaning of textile fibers (e.g., cotton and wool) in preparation for weaving cloth, and later as hair-styling products or as a medicament on wounds. Evidence also indicates that Egyptians bathed on a regular basis, and the Ebers Papyrus (a medical document dated approximately 1500 BC) describes a soaplike material synthesized from combining animal and vegetable oils with alkaline salts, which was used for both washing and the treatment of skin diseases. Also at this time, Moses provided the Israelites with detailed laws concerning personal cleanliness and health, and biblical accounts indicate that the Israelites possibly knew that hair gel was produced by combining oil and ashes. While the early Greeks bathed for aesthetic reasons, they chose to clean their bodies with blocks of clay, sand, pumice, and ashes rather than with soap. A metal implement known as a “strigil” was used to scrape off oils and ashes used to anoint bodies, and body dirt apparently was removed with this scraping process.

In ancient Rome, oils, unguents, plant essences, and cosmetics were apparently used in heavy quantities, but there is no reference to soaps and their use as cleaning agents. While the Romans were known for their practice and use of public baths, personal cleaning involved rubbing their bodies with olive oil and sand and using a strigil to scrape the oil, sand, dirt, grease, and dead skin cells off their bodies. However, the name “soap” is thought to have originated, according to an ancient Roman legend, from Mount Sapo, where animals were sacrificed. Rain poured down this mountain, through a mixture of melted animal fats, or tallow, and wood ashes into the clay soil along the Tiber River below. Women washing clothes at the river apparently noticed that clothing exposed to the soapy mixture of saponified acids (fats) and alkali (caustic ashes) within the river water become cleaner very quickly with little effort. Sa- ponification, the chemical term for the “soap-making” reaction, bears the name of this mountain in Rome. The first reliable evidence of soap making is found in the historical accounts of ancient Rome. The Roman historian, Pliny the Elder, described the synthesis of soap from goat tal- low and caustic wood ashes and also indicated that common salt was added to harden soap. The Romans knew, long before the actual chemi- cal process was completely understood, that heating goat fat with ex- tracts of wood ashes, which contain alkaline (basic) products (e.g., potassium hydroxide [KOH] and potassium carbonate [K2CO3]), produces soap. The first reaction formed potassium hydroxide, which causes the breakdown of the fat triglycerides into the component parts, glycerine and fatty acids. In the process, the fatty acid is neutralized by the strong alkali and ends up in the salt form. The Romans also used lye (sodium hydroxide [NaOH]), a stronger base than ash extracts, and more effective in changing fats into actual soap. The word “lye” is apparently related to soap and the process of soap making through an extensive path of linguistics, including words from Latin, Greek, Old English, Old Irish, and other languages, meaning lather, wash, bathe, and even ashes. In AD 79, the city of Pompeii, Italy, was destroyed after the eruption of the volcano named Mount Vesuvius. Interestingly, excavation of Pompeii revealed an entire soap-making factory, complete with finished bars of soap preserved in the hardened lava. Soap used as a personal cleansing technique had become popular during the later centuries of the Roman Empire.

By the second century AD, the Greek physician named Galen was rec- ommending soap for both medicinal and bathing/cleansing purposes. Although the first Roman baths had been built in approximately 312 BC, supplied with water from their extensive aqueducts, the fall of the Roman Empire in AD 467 resulted in the decline in bathing habits in Western Europe. There was little soap making performed or use of soap for clean- ing in the European Dark Ages. While there were apparently public bath- houses, called stews, where patrons were provided with bars of soap for personal cleansing during the European Middle Ages, it was later during medieval times when bathing fell out of custom. Many historians suspect that the lack of personal cleanliness and related unsanitary conditions substantially contributed to the outbreak of the great disease plagues of the Middle Ages, especially the occurrence of the Black Death of the fourteenth century. Remaining stews were closed, as authorities suspected they promoted the spread of disease. It is well known that during the Renaissance, people preferred to cover their bodies with heavy scents rather than try to maintain body cleanliness. However, daily bathing was a common custom in Japan, and weekly hot spring bathing was popular in Iceland during the Middle Ages.

The Celtic peoples are also thought to have discovered soap making. Many historians believe that, possibly because of increased contact with the Celts by the Romans, soap was used by the Celtic people for personal washing. It is also possible that the Celts and the Romans independently discovered saponification. In the Byzantine Empire, the cultural remains of the Roman Empire in the eastern Mediterranean region, in the expanding Arab nations, and in the regions conquered by the Vikings, soap was manufactured and used. The ancient Germans and Gauls are also credited with discovering a substance called soap, made of tallow and ashes, that they used to dye their hair a reddish color. Soap making was an established craft in Europe by the end of the seventh century. Soap making was revived in Italy and Spain, beginning in approximately the eighth century. By the thirteenth century, the city of Marseilles, France, emerged as a prominent soap-making center for the European markets. During the fourteenth century, England also formally initiated saponification techniques. At this time, saponification usually involved combining vegetable and animal oils with ashes of plants, along with pleasant fragrances. While soaps produced in southern Europe, Italy, Spain, and southern ports of France were synthesized from high-quality olive oils, soaps produced in England and northern France were produced from lower-quality animal fats (e.g., tallow, the fat from cattle), and soaps produced in northern Europe often resorted to fish oils. Southern European countries enjoyed a lively trade of exporting fine soaps, as these regions all boasted a rich supply of olive oil as well as barilla, a fleshy plant whose ashes were used to make the lye formula required for saponification at that time. Soap-making guilds and trade associations recognized the need to regulate the lucrative soap-making process, and soaps were taxed as luxury items in England and France throughout the seventeenth, eighteenth, and nineteenth centuries. The habit of bathing came back into fashion and the consumption of soap increased tremendously in the nineteenth century. Abolishment of some European soap taxation eased the transition of soap from a luxury item to a common personal hygiene household commodity.

While the first European settlers to New England carried soap with them on the ships chartered by the Massachusetts Bay Company and later manufactured soap themselves from the by-products (e.g., boiling of wood ash lye, cooking grease and animal fats together) of their home- steading activities, commercial soap making in the American colonies began in 1608 with the arrival of several Polish and German soap makers on the second ship from England to reach Jamestown, Virginia. Professional soap makers who traded and sold soap, often called soapboilers, would collect large amounts of waste fats from households in exchange for small amounts of presynthesized soap. Since tallow was also the main ingredient for candles, many soapboilers produced both soap and candles. A major step toward large-scale commercial soap making occurred in 1791, when a French chemist, Nicholas Leblanc, patented a process for making soda ash (lye), or sodium carbonate, from a brine solution of common salt. Soda ash is the alkali obtained from ashes that is combined with various fats to form soap. The Leblanc process was an easy, inexpensive technique that yielded large quantities of good-quality soda ash at the industrial level. During the 1800s, other French and Belgian chemists contributed to the advancement of soap technology by discovering the chemical nature and relationship of fats, glycerine and fatty acids, and by inventing the ammonia process, which uses common table salt (sodium chloride [NaCl]) to make soda ash inexpensively and in increased quantities for quality high-volume soap production. Thus, during Victorian times, soap came of age when saponification turned from part craft and folklore item to a fully developed industry. Soap making was one of America’s fastest-growing industries by 1850, with the rise of companies such as Colgate & Company, Palmolive, Lever Brothers (an English company), and Procter & Gamble. The broad availability of soap allowed for the product to change from a luxury item, frequently synthesized as a household art, to an everyday necessity item with widespread use.

Soaps actually make up a very narrow class of detergents. The term “soap” is restricted to the sodium (or, infrequently, potassium) salts of long-chain carboxylic acids. As indicated above, natural soaps, the so- dium or potassium salts of fatty acids, were originally synthesized via sa- ponification, a process of boiling lard or other animal fats (and later vegetable fats) together with lye or potash (potassium hydroxide). The fats and oils thereby undergo hydrolysis, yielding crude soap and a by- product called glycerol. An example of such a crude soap is sodium octadecanoate (C17H35COONa). Soaps are anionic (negatively charged) surfactants produced from the hydrolysis of fats in a chemical reaction called saponification. In other words, soaps are water-soluble sodium or potas- sium salts of fatty acids synthesized from fats and oils (or their fatty acids) by chemically treating them with a strong alkaline substance (base). Each soap molecule has a long hydrocarbon chain (CH2 groups that terminate in a CH3 group), often referred to as a “tail,” that is linked by a covalent bond to a carboxyl group (CO2H), often called a “head.” The anion of the carboxyl group is balanced by the positive charge of a sodium or po- tassium cation. In water, the sodium or potassium ions float freely, leav- ing the head region with a negative charge. It has an oxygen end, which is polar, and a fatty acid end, which is nonpolar. Thus, part of the mole- cule contains a polar hydrophilic (water-loving) structure; the ionic head region is attracted toward water molecules but shuns hydrocarbons and other oil-based substances. The long hydrocarbon tail of the molecule is a nonpolar hydrophobic (water-fearing) structure; it shuns water but mixes easily with greasy, oil-based substances that repel the hydrophilic head portion. The hydrocarbon chains of various soap molecules within a body of water are attracted to each other by dispersion forces and cluster together to form micelles. Within these micelles, the carboxylate head groups form a negatively charged spherical surface, with the hydrocarbon chains inside the sphere attracting oily dirt. Because the outer portion of the micelles are negatively charged, the soap micelles repel each other and remain dispersed in water. Grease and dirt are attracted to the nonpolar hydrocarbon portion of the micelles, are subsequently “caught” inside the micelles, and then can be rinsed away.

Soaps are detergents that both lower the surface tension of water, which allows water to penetrate a dirty substance and then suspend the grease droplets within the micelles, preventing them from redepositing on the substance being cleaned, and allow the greasy dirt to be washed away. Since the anionic carboxylate groups of the soap molecules provide a cover of negative electrical charge on the surface of each micelle, soaps are considered anionic detergents.

The fats and oils used in soap making originate from animal or plant sources. Each fat or oil is composed of a distinctive mixture of several tri- glycerides. Triglycerides, the principal organic compounds of animal and vegetable oils, are composed of three fatty acid molecules attached to one molecule of glycerine. Fatty acids are the components of fats and oils that are used to synthesize soap. They are weak acids composed of two parts: a carboxylic acid group and an attached hydrocarbon chain. Because of the combination of both hydrophilic and hydrophobic properties in a soap molecule, it is necessary to obtain a balance between the two properties to achieve the greatest benefit. For example, the best benefit seems to be derived from the reference of the C-14 fatty acid molecule within the hydrocarbon chain. As the hydrocarbon chain is lengthened, the solubility in water decreases. Above C-18, the solubility becomes too low to be of any practical use. When the hydrocarbon chain is shortened, the ability to suspend oil droplets is significantly decreased. Sodium salts of fatty acids containing ten to eighteen carbons make the best soaps. There are many types of triglycerides, with each type possessing a particular combination of fatty acids. Coconut oil is considered quite suitable be- cause of the unusually high content of lauric, myristic, and palmitic acids. Tallow, fat from beef and lamb, is predominantly composed of palmitic, stearic, and oleic acids. Castile soap is made primarily from olive oil, which can be less irritating to the skin and prized by some people for that reason. The alkalis used in soap making were originally obtained from the ashes of plants but are now available as commercially synthesized prod- ucts. An alkali is a soluble salt of an alkali metal such as potassium or so- dium, but the term can also refer to a substance that is a base, which reacts with and neutralizes an acid to form a salt. The common alkalis used in soap making are sodium hydroxide, called caustic soda, and potassium hydroxide, also called caustic potash.

Saponification, the process of soap making, occurs when fatty acid- containing fats and oils are mixed with a strong alkali. This method in- volves heating fats and oils and subsequently reacting them with a liquid alkali to produce soap and water plus glycerine. In the industrial manu- facture of soap, tallow (fat from beef or lamb) or vegetable fat (e.g., co- conut oil, olive oil, etc.) is heated in the presence of sodium hydroxide. Once the saponification reaction is completed, a salt (e.g., sodium chlo- ride) is added to precipitate the soap from solution. The water layer is then removed from the top of the mixture and the glycerol by-product is recovered, often using vacuum distillation. Impurities within the crude soap (e.g., sodium chloride, sodium hydroxide, and glycerol) are subsequently removed by repeating the boiling and salt-precipitation processes. Another major soap-making process involves the neutralization of fatty acids with an alkali. Fats and oils are hydrolyzed with a high-pressure steam to yield crude fatty acids and glycerine. Hydrolysis (the decompo- sition of a substance, or its conversion to other substances, through the action of water) of the triglyceride generates both glycerol, which remains dissolved in the water, and the salts of the various acids that make up the triglyceride. The fatty acids are then purified by distillation and neu- tralized with an alkali to produce soap and water. After the splitting of the fats and oils, the sodium or potassium portion of the alkali joins with the fatty acid molecules. The salts of the fatty acids actually congeal at the surface as the mixture cools, and these acids constitute the soap. Histori- cally, both the glycerol and the soap contained unreacted alkali, which destroyed skin tissues with each use, thus substantiating the deleterious effects caused by improperly homemade “lye soap.” When the alkali added is sodium hydroxide, sodium-based soap is formed. Such sodium soaps are “hard” soaps. Glycerine is also considered a valuable by-product and can be recovered by chemical treatment, followed by evaporation and refin- ing. Refined glycerine is a primary ingredient of many soaps available over the counter and as an important industrial material used in such items as foods, cosmetics, and drugs.

Bar soaps are formulated for cleaning the hands, face, and body. Tra- ditional bar soaps are made from fats and oils (which are largely esters) or their fatty acids reacted with inorganic water-based bases. They have a general formula, RCOO-Na+, where R is a long hydrocarbon chain, CH3(CH2)10-16. The saponification reaction is summarized as follows: an ester (fat) and a base (caustic soda, sodium hydroxide) react together to yield the salt of a fatty acid (soap) and an alcohol (e.g., glycerol). This is simply the reverse of an esterification reaction. The main sources of fats include beef and lamb (mutton) tallow and white palm, coconut, and palm kernel oils. These raw materials are pretreated to remove impurities and to achieve the color, odor, and performance features desired (e.g., sudsing ability) in the finished soap bar product. The hardness of soap depends on the fats from which it was made. Saturated fats (fats that re- main fairly solid at room temperature, such as those derived primarily from animal fats) produce the hard bar soaps. Sodium palmitate, the so- dium salt of palmitic acid, is a typical soap. Beef tallow yields principally sodium stearate [CH3(CH2)16COO-Na+], one of the more common soaps. Palm oil yields sodium palmitate [CH3(CH2)14COO-Na+], a component in many expensive soaps. Standard soaps comprise approximately 80 per- cent tallow and 20 percent coconut oil, with added chemical sudsing agents. The total amount of water permissible is usually 17 percent. The characteristic of good lathering/sudsing ability is attributable to the fact that as surfactants, soap molecules tend to align along the surface of the water, with the hydrocarbon chains directed toward the surface and the saltlike ends directed into the solution exposed to the water. The water surface is thus weakened and promotes foaming. The density of soap can be decreased by the incorporation of air to allow for floating. Additives within modern soaps include creams (emollients), perfumes, preservatives, antioxidants, deodorants, abrasives (e.g., pumice, silica), and colorings (e.g., titanium dioxide in the case of white soaps). Soaps termed “combo bars” usually are formed from a combination of actual soap and synthetic surfactants. Specialty bars include transparent/translucent soaps, luxury soaps, and medicated soaps.

Although soaps are excellent cleansers, their effectiveness is limited when used in hard water. Hardness in water is attributable to the pres- ence of various mineral salts, including calcium (Ca), iron (Fe), magne- sium (Mg), and manganese (Mn). The mineral salts react with soap to form insoluble salts. This collective insoluble precipitate is often referred to as “soap film” or “soap scum.” Hard water wastes soap because a good portion of the soap that would otherwise be used for cleaning is consumed in precipitate formation as it reacts with the mineral ions of hard water. In addition, as salts of weak acids, soaps are converted by mineral acids into free fatty acids, and these fatty acids are less soluble than the sodium or potassium salts. Thus, soaps are also ineffective in acidic water. The insoluble salts form bathtub rings, leave films on skin, and gray/roughen bathroom tiles with repeated washings. However, soap is an excellent cleanser in soft water, it is relatively nontoxic, it is de- rived from renewable resources (animal fats and vegetable oils), and is biodegradable.