Detergents

Summary

Synthetic detergents enable otherwise immiscible materials (water and oil) to form homogeneous dispersions. In addition to facilitating the breakdown and removal of stains or soil from textiles, hard surfaces, and human skin, detergents have found widespread application in food technology, oil-spill cleanup, and other industrial processes, such as the separation of minerals from their ores, the recovery of oil from natural deposits, and the fabrication of ceramic materials from powders. Detergents are used in the manufacture of thousands of products and have applications in the household, personal care, pharmaceutical, agrochemical, oil and mining, and automotive industries, as well as in the processing of paints, paper coatings, inks, and ceramics.

Definition and Basic Principles

Detergents comprise a group of synthetic water-soluble or liquid organic preparations that contain a mix of surfactants, builders, boosters, fillers, and other auxiliary constituents, the formulation of which is specially designed to promote cleansing action or detergency. In the twenty-first century, products were largely mixtures of surfactants, water softeners, optical brighteners and bleach substitutes, stain removers, and enzymes with added fragrance and, in most cases, dyes. They must be formulated with ingredients in the right proportion to provide optimum detergency without damaging the fabrics being washed. In the early 2020s, the global market for laundry detergent was valued at more than $120.7 billion.

Before the advent of synthetic detergents, soaps were used. Soaps are salts of fatty acids made by alkaline hydrolysis of fats and oils. They consist of a long hydrocarbon chain with the carboxylic acid end group bonded to a metal ion. The hydrocarbon end is soluble in fats and oils, and the ionic end (carboxylic acid salt) is soluble in water. This structure gives soap surface activity, allowing it to emulsify or disperse oils and other water-insoluble substances. Because soaps are alkaline, they react with metal ions in hard water and form insoluble precipitates, decreasing their cleaning effectiveness. These precipitates became known as soap scum—the “gunk” that builds up and surrounds the bathtub and causes graying or yellowing in fabrics.

Once synthetic detergents were developed, this problem could be avoided. Detergents are structurally similar to soap and work much the same to emulsify oils and hold dirt in suspension, but they differ in their water-soluble portion in that their calcium, magnesium, and iron forms are more water-soluble and do not precipitate out. This allowed detergents to work well in hard water and, thus, reduce the discoloration of clothes.

Background and History

Soap is the oldest cleaning agent and has been in use for 4,500 years. Ancient Egyptians were known to bathe regularly, and Israelites had laws governing personal cleanliness. Records document that soap-like material was being manufactured as far back as 1500 Before the Common Era (BCE). Before soaps and detergents, clothes were cleansed by beating them on rocks by a stream. Plants such as soapwort and soapbark that contained saponins were known to produce soapy lather and probably served as the first crude detergent. By the 1800s, soap making was widespread throughout Europe and North America, and by the 1900s, its manufacture had grown into an industry. The chemistry of soap manufacturing remained unchanged until World War I, when the German chemical company Henkel developed the first synthetic detergent. By the end of World War I, detergents had grown in popularity as people learned that they did not leave soap scum like their earlier counterparts.

The earliest synthetic detergents were short-chain alkyl naphthalene sulfonates. By the 1930s, sulfonated straight-chain alcohols and long-chain alkyl and aryl sulfonates with benzene were being made. By the end of World War II, alkyl aryl sulfonates dominated the detergent market. In 1946, the use of phosphate compounds in combination with surfactants was a breakthrough in product development and spawned the first of the “built” detergents that would prove to be much better-performing products. Sodium tripolyphosphate (STPP) was the main cleaning agent in many detergents and household cleaners for decades. By 1953, US sales of detergents had surpassed those of soap. In the mid-1960s, it was discovered that lakes and streams were being polluted, and the blame was laid on phosphate compounds; however, the actual cause was found to be branching in their molecular structure, which prevented them from being degraded by bacteria. Detergent manufacturers then switched from commonly used compounds such as propylene tetramer benzene sulphonate to a linear alkyl version. Detergent manufacturers are still grappling with sustainability issues. They are focusing much attention on developing products that are safe for the environment and consumers and using renewable energy in the manufacturing process.

How It Works

Just as forces exist between an ordered and disordered universe, so too do they between soil and cleanliness. People have been conditioned to believe that soil on the surface of an object is unwanted matter, but in reality, soil is being deposited continuously on all surfaces around us, and cleanliness itself is an unnatural, albeit desirable, state. To rid any surface of soil, one must work against nature and have an understanding of the concept of detergency—the act of cleaning soil from a surface (substrate).

The Function of Detergency. The cleaning action of detergents is based on their ability to emulsify or disperse different types of soil and hold it in suspension in water. The workhorse involved in this job is the surfactant, a compound used in all soaps and detergents. This ability comes from the surfactant's molecular structure and surface activity. When a soap or detergent product is added to water containing insoluble materials like dirt, oil, or grease, surfactant molecules adsorb onto the substrate (clothes) and form clusters called micelles that surround the immiscible droplets. The micelle is water soluble and allows the trapped oil droplets to be dispersed throughout the water and rinsed away. While this is a simplified explanation, detergency is a complex set of interrelated functions that relies on the diverse properties of surfactants, their interactions in solution, and their unique ability to disrupt the surface tension of water.

Surface Tension. The internal attraction or association of molecules in a liquid is called surface tension. However simple this may seem, it is a complex phenomenon, and for many students can be hard to grasp. Examining the properties of water and the action of surfactants may dispel any confusion.

Water is polar in nature and very strongly associated, such that the surface tension is high. This is because of its nonsymmetrical structure, in which the double-atom oxygen end is more negative than the single-atom hydrogen end is positive. As a result, water molecules associate so strongly that they are relatively stable, with only a slight tendency to ionize or split into oppositely charged particles. This is why their boiling point and heat of vaporization are very high in comparison to their low molecular weight.

The surface tension of water can be explained by how the molecules associate. Water molecules in the liquid state, such as those in the center of a beaker full of water, are very strongly attracted to their neighboring molecules, and the pull is equal in all directions. The molecules on the surface, however, have no neighboring molecules in the air above; hence, they are directed inward and pulled into the bulk of the liquid, where the attraction is greater. The result is a force applied across the surface, which contracts as the water seeks the minimum surface area per unit of volume. An illustration of this is the fact that one can spin a pail of water around without spilling the contents.

Surfactant compounds are amphiphilic, meaning their backbone contains at least one hydrophilic group attached to a hydrophobic group (called the hydrophobe), usually consisting of an eight to eighteen carbon-hydrocarbon chain. All surfactants possess the common property of lowering surface tension when added to water in small amounts, at which point the surfactant molecules are loosely integrated into the water structure. As they disperse, the hydrophilic portion of the surfactant causes an increased attraction of water molecules at the surface, leaving fewer sides of the molecule oriented toward the bulk of the liquid and lessening the forces of attraction that would otherwise pull them into solution.

Micelle Formation and Critical Micelle Concentration (CMC). As surface active agents, surfactants not only have the ability to lower surface tension but also to form micelles in solution, a unique behavior that is at the core of detergent action. Micelles are aggregate or droplet-like clusters of individual surfactant molecules whose polar ends are on the outside, oriented toward the solvent (usually water), with the nonpolar ends in the middle. The driving force for micelle formation is the reduction of contact between the hydrocarbon chain and water, thereby reducing the free energy of the system. The micelles are in dynamic equilibrium, but the rate of exchange between surfactant molecule and micelle increases exponentially, depending on the structure of each individual surfactant. As surfactant concentration increases, surface tension decreases rapidly, and micelles proliferate and form larger units.

The concentration at which this phenomenon occurs is known as the critical micelle concentration (CMC). The most common technique for measuring this is to plot surface tension against surfactant concentration and determine the breakpoint, after which surface tension remains virtually constant with additional increases in concentration. The corresponding surfactant concentration at this discontinuity point corresponds to the CMC. Every surfactant has its own characteristic CMC at a given temperature and electrolyte concentration.

Applications and Products

The workhorse of detergents is the surfactant, or more commonly, a mix of surfactants. The most important categories are the carboxylates (fatty acid soaps), the sulfates (alkyl sulfates, alkyl ether sulfates, and alkyl aryl sulfonates), and the phosphates.

Laundry Products. The primary purpose of laundry products is the removal of soil from fabrics. As the cleaning agent, the detergent must fulfill three functions: wet the substrate, remove the soil, and prevent it from re-depositing. This usually requires a mix of surfactants. For example, a good wetting agent is not necessarily a good detergent. For best wetting, the surface tension needs only to be lowered a little, but it must be done quickly. That requires surfactants with short alkyl chain lengths of eight carbons and surfactants with an HLB of seven to nine. For best detergency, the surface tension needs to be substantially lowered and that requires surfactants with higher chain lengths of twelve to fourteen carbons and an HLB of thirteen to fifteen. To prevent particles from redepositing, the particles must be stabilized in a solution, and that is done best by nonionic surfactants of the polyethylene oxide type. In general, nonionics are not as effective in removing dirt as anionic surfactants, which is one reason a mixture of anionic and nonionic surfactants is used. However, nonionics are more effective in liquid dirt removal because they lower the oil-water interfacial tension without reducing the oil-substrate tension.

Skin-Cleansing Bars. In the 2020s, most bar soaps being manufactured were called superfatted soap and were made by incomplete saponification, an improved process over the traditional method in which superfatting agents are added during saponification, which prevents all of the oil or fat from being processed. Superfatting increases the moisturizing properties and makes the product less irritating. Transparent soaps are like traditional soap bars but have had glycerin added. Glycerin is a humectant (similar to a moisturizer) and makes the bar transparent and much milder.

Syndet bars are made using synthetic surfactants. Since they are not made by saponification, they are actually not soap. Syndet bars are very mild on the skin and provide moisturizing and other benefits. Dove was the first syndet bar on the market.

Mining and Mineral Processing. Because minerals are rarely found pure in nature, the desired material, called values, needs to be separated from the rocky, unwanted material, called gangue. Detergents are used to extract metals from their ores by a process called froth flotation. The ore is first crushed and then treated with a fatty material (usually an oil), which binds to the particles of the metal (the values) but not to the unwanted gangue. The treated ore is submerged in a water bath containing a detergent, and then air is pumped over the sides. The detergent's foaming action produces bubbles, which pick up the water-repellant-coated particles or values, letting them float to the top, flow over the sides, and be recovered. The gangue stays in the water bath.

Enhanced Oil Recovery (EOR). This process refers to the recovery of oil that is left behind after primary and secondary recovery methods have either been exhausted or have become uneconomical. Enhanced oil recovery is the tertiary recovery phase in which surfactant-polymer (SP) flooding is used. SP flooding is similar to waterflooding, but the water is mixed with a surfactant-polymer compound. The surfactant cleans the oil off the rock, and the polymer spreads the flow through more of the rock. An additional 15 to 25 percent of the original oil in place (OOIP) can be recovered. Before this method is used, a great deal of evaluation and laboratory testing is involved, but it has become a reliable and cost-effective oil recovery method.

Ceramic Dispersions. Ceramic is a nonmetallic inorganic material. Ceramic dispersions are the starting material for many applications. The use of detergents or surfactants enhances the wetting ability of the binder onto the ceramic particles and aids in the dispersion of ceramic powders in liquids. As dispersants, they reduce bulk viscosity of high-solid slurries and maintain stability in finely divided particle dispersions. Bi-block surfactants help agglomeration of the ceramic particles. In wastewater treatment, detergents are used in ceramic dispersions to reduce the amount of flocculents.

Careers and Course Work

Science courses in organic, inorganic, and physical chemistry, biology, and biochemistry, plus courses in calculus, physics, materials science, polymer technology, and analytical chemistry, are typical requirements for students interested in pursuing careers in detergents. Other pertinent courses may include differential equations, instrumental analysis, statistics, thermodynamics, fluid mechanics, process design, quantitative analysis, and instrumental methods. Earning a bachelor of science degree in chemistry or chemical engineering is usually sufficient for entering the field or doing graduate work in a related area.

There are few degree programs in formulation chemistry, but students need to understand the chemistry involved, such as thermodynamics of mixing, phase equilibria, solutions, surface chemistry, colloids, emulsions, and suspensions. Even more important is how their dynamics affect such properties as adhesion, weather resistance, texture, shelf life, biodegradability, and allergenic response.

Students in undergraduate chemistry programs are encouraged to select a specialized degree track but are also being advised to take substantial coursework in more than one of the primary fields of study related to detergent chemistry because product development requires skills drawn from multiple disciplines. Often, a master's degree or doctorate is necessary for research and development.

There are several career paths for students interested in the detergent industry. Manufacturers of laundry detergents, household cleaning products, industrial and institutional (I&I) products, personal-care products, and the ingredients suppliers of all these products are the biggest employers of formulation chemists, technicians, and chemical operators. Career opportunities in research and development, marketing, or sales are also available. Other areas where detergent chemists and technicians are needed include food technology, pharmaceuticals, oil drilling and recovery, mining and mineral processing, and ceramic powder production.

Social Context and Future Prospects

Around the world, sustainability and environmental protection remain the buzzwords for detergents in the twenty-first century. The industry is very dependent on the price and availability of fats and oils since these materials are needed to make the fatty acids and alcohols used in the manufacture of surfactants. Any turmoil in the oil industry impacts how much is paid at the gas pump and is also directly related to the price paid to keep the environment clean.

While the detergents industry, on the whole, has traditionally been relatively recession-proof, conflicts in the Middle East have taken a toll on this vast market. Robert McDonald, chairperson of P&G, alluded to changes in consumer habits, volatility in commodity costs, and increasing complexity of the regulatory environment as rationales for the slowdown. According to manufacturers, the global COVID-19 pandemic saw increased sales of detergents and other cleaners through 2020 and beyond. A spokesperson for Kline and Company, a market research firm reporting on the industry, stated: “The industry's mantra is greener, cleaner, safer,” but goes on to say that the challenge is a double-edged sword, as the industry is battling to hold down costs while trying to produce environmentally sustainable products.

Sustainability and environmental protection are global issues, insights are becoming more astute, and action is being taken. In its annual sustainability report, Henkel reported that, in 2021, the company was working to become carbon neutral and protect biodiversity. They set critical targets for water usage as well. Important emerging markets such as China are no exception in this battle. The Chinese government puts great emphasis on environmental regulations. Analysts say this is a clear message that to expand their sales to other parts of the world, China needs to focus on the green demand and manufacture products that offer innovative and sustainable solutions without compromising on performance.

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