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Surfactants, Soaps, and Detergents
1. INTRODUCTION TO THE INDUSTRY
A general area of the chemical industry that manufactures most of thesurfactants, soaps, and detergents is called Soaps, Cleaning Compounds, andToilet Preparations (NAICS 3256), one of the seven major divisions ofChemical Manufacturing (see Fig. 20.1 for this summary). This amounts toover $50 billion and is 14% of all Chemical Manufacturing. We willconcentrate primarily on a subsector of this division, Soaps and OtherDetergents (NAICS 325611), although all subsectors of Soaps, CleaningCompounds, and Toilet Preparations use surface active (surfactant)chemicals, which are further modified into finished products. Over 5 billionIb of surfactants serve all these sectors. In addition to household andindustrial cleaning, oil field applications and personal care products are bigusers of surfactants.
Fig. 24.1 shows the trend in U.S. shipments of Soaps and OtherDetergents (NAICS 325611) along with its further subsections of HouseholdDetergents (3256114), Commercial, Industrial, and Institutional Soaps andDetergents (3256111), and Household Soaps (3256117). Note the rapidincrease for Soaps and Detergents in the 1970s, a slower rate of growth inthe 1980s, and the near constant market in the early 1990s until 1996. Thefollowing important companies have large percentage shares of the U.S.household laundry detergent market: Procter and Gamble (P & G), 43%;Unilever, 21%; Dial, 11%; Huish Detergents, 6%; Church and Dwight, 5%;USA Detergents, 4%; and Colgate-Palmolive, 4%.
Figure 24.1 U.S. shipments of soaps and detergents. (Source: Annual Survey ofManufactures)
Surfactants are chemicals that, when dissolved in water or anothersolvent, orient themselves at the interface between the liquid and a secondsolid, liquid, or gaseous phase and modify the properties of the interface.Surfactants are not only important as the active constituent of soaps anddetergents but are also vital in the stabilization of emulsions, in fabricsoftening, in oil well drilling, etc. Surfactants are the most widely appliedgroup of compounds in Chemical Manufacturing. We will concentrate ontheir use in cleaning, that is, in soaps and detergents.
All surfactants have a common molecular similarity. A portion of themolecule has a long nonpolar chain, frequently a hydrocarbon chain, thatpromotes oil solubility but water insolubility (the hydrophobic portionwater hating). Another part of the molecule promotes oil insoluble andwater soluble properties (the hydrophilic portionwater loving).
Fig. 24.2 summarizes the cleaning action of surfactants. The surfactantlines up at the interface and also forms micelles, or circular clusters ofmolecules. In both cases the hydrophobic end of the molecule gets awayfrom water molecules and the hydrophilic end stays next to the watermolecules (like dissolves like). When grease or dirt come along (primarilyhydrophobic in nature) the surfactants surround it until it is dislodged fromthe substrate. The grease molecules are suspended in the emulsion by thesurfactant until they can be washed away with freshwater.
Soaps & Detergents
Commercial Soaps and Detergents
Figure 24.2 "Solubilizing" effect of surface active agents, (a) Greasy dirt comes intocontact with surfactant solution, (b) Hydrophobic ends of surfactant molecules dissolvein the grease, (c) The surfactant affects the contact angle 0 between the dirt and thesubstrate. If 0< 90, total removal of the grease is impossible, (d) Further agitationdisplaces the greasy dirt as macroscopic particles. These form an emulsion if agitation issufficient. The particles form the center of micelle-like structures. Removal of grease isseldom complete (0 < 90 as in the diagrams on the right rather than the simple"rollback" mechanism on the left). Usually the main body of grease is removed from astrongly adsorbed monomolecular or duplex layer of grease. Agitation is an essential partof the process. (Source: Wittcoff and Reuben, Industrial Organic Chemicals inPerspective. Part Two: Technology, Formulation, and Use, John Wiley & Sons, 1980.Reprinted by permission of John Wiley & Sons, Inc.)
Surface layer of surfactantmolecules
n-*- Hydrophilic end l
Table 24.1 U.S. Production of Major Household Surfactants
AnionicsLinear alcohol sulfates (AS)Linear alcohol ethoxysulfates (AES)Linear alkylbenzenesulfonates (LAS)
NonionicsNonylphenol ethoxylates (NPE)Alcohol ethoxylates (AE or AEO)Other nonionics
100%Source: Chemical Economics Handbook
Surfactants can be divided into four general areas. These will bediscussed separately: cationics, anionics, nonionics, and amphoterics.Major anionics are soaps, linear alcohol sulfates (AS), linear alcoholethoxysulfates (AES), and linear alkylbenzenesulfonates (LAS). Majornonionics are nonylphenol ethoxylates (NPE) and alcohol ethoxylates (AE orAEO). Table 24.1 gives the U.S. production of major surfactants used in thehousehold market, which is over half of the total market. We will brieflydescribe other surfactants, but these five are the most important forhousehold detergents. The 3.5 billion Ib of surfactants make over 10 billionIb of household detergents, since other ingredients are added to thesecomplex formulations. Table 24.1 is a little misleading because it isproduction of surfactant chemicals. Actually the amount of AEO useddirectly is about 500 million Ib, with the rest of the 1,200 million Ib used tomake AES by sulfonation, which then is used as the surfactant. So the AEOfigure in terms of importance as the final surfactant is magnified, though it isa key intermediate in making AES surfactant. We will see how this ispossible when the chemistry is covered.
2. CATIONIC SURFACTANTS
In cationics the long hydrophobic alkyl chain is in the cationic portion ofthe molecule. Another way of saying this is that the organic part is positive.Practically all industrially important cationics are fatty nitrogen compounds
and many are quaternary nitrogen compounds such as tallow fatty acidtrimethylammonium chloride. In the more general structure R1R2R3R4N+X",R1 is a long alkyl chain, the other R's may be alkyl or hydrogen, and X" ishalogen or sulfate.
The long hydrocarbon chain is derived from naturally occurring fats ortriglycerides, that is, triesters of glycerol having long chain acids with aneven number of carbons, being of animal or vegetable origin. A common fatsource for cationics is inedible tallow from meat packing plants. If the fattyacid is desired the ester is hydrolyzed at high temperature and pressure, orwith a catalyst such as zinc oxide or sulfuric and sulfonic acid mixtures.
fat or triglyceride glycerol fatty acid(glycerin)
The fatty acid is then converted into the "quat" by the following sequenceof reactions.
fatty acid amide nitrile
Cationic surfactants are not very good for cleaning because most surfacescarry a negative charge and the cationic portion adsorbs on the surfaceinstead of dissolving the grease. But they do have other important surfactantapplications. They inhibit the growth of bacteria, are corrosion inhibitors,are used in ore flotation processes (separating phosphate ore from silica andpotassium chloride from sodium chloride), and are good fabric softeners andantistatic agents. They also find use in hair conditioners and other personalcare applications.
3. ANIONIC SURFACTANTS
This is by far the most important type of surfactant and will be discussedunder separate subtypes. In anionics the long hydrophobic alkyl chain is inthe anionic part of the molecule. The organic part is negative.
The first type of cleansing agent, used by humankind for centuries, wassoap. Although it has now been supplemented by various syntheticdetergents in advanced countries for laundry and household use, it is stillpreferred for personal hygiene. In less-developed countries it is preferred forlaundry use.
fats NaH glycerol + R-C-Q-Na+A
Soaps are the sodium or potassium salts of certain fatty acids obtainedfrom the hydrolysis of triglycerides. The potassium salts form the "softsoaps" that have become popular recently. The fats used in soapmanufacture come from diverse natural sources. Animal tallows andcoconut oil are the favored sources of the triglycerides, and quite oftenmixtures from different sources are used to vary hardness, water solubility,and cleansing action of the final product. Palm, olive, cottonseed, castor,and tall oil are other sources. The side chains are usually Cn-Ci8 in length.Manufacturing processes are both batch and continuous. Sometimes thetriglyceride is steam-hydrolyzed to the fatty acid without strong caustic andthen in a separate step it is converted into the sodium salt. Either way givesa similar result. Soaps have some disadvantages compared to syntheticdetergents: they are more expensive, they compete with food uses for fatsand oils, and their calcium and magnesium salts formed in hard water are
Figure 24.3 U.S. consumption of soaps vs. total synthetic detergents. (Source:Chemical Economics Handbook)
very insoluble and precipitate onto the clothing being washed. They alsotend to clog automatic washers. They deteriorate on storage and are unstablein acid solutions. This is why in 1940 only 1% of cleaning agents weredetergents; in 1970 they were 85%. Fig. 24.3 shows this historicalreplacement of soaps by detergents from 1940-1970. However, there willalways be that