Chapter 24

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%.

Year

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 portion—water hating). Another part of the molecule promotes oil insoluble andwater soluble properties (the hydrophilic portion—water 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

Household Detergents

Commercial Soaps and Detergents

Household Soaps

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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 in

Perspective. 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•<- Hydrophobic end J

Micelles in body of liquid

Substrate

Table 24.1 U.S. Production of Major Household Surfactants

Type

AnionicsLinear alcohol sulfates (AS)Linear alcohol ethoxysulfates (AES)Linear alkylbenzenesulfonates (LAS)

NonionicsNonylphenol ethoxylates (NPE)Alcohol ethoxylates (AE or AEO)Other nonionics

Total

Production(Million Ib)

57890661

4621,200

2443,514

%

2%2519

13347

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

primary amine

"quat"

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.

3.1 Soaps

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.

O

fats Na°H» 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

Year

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 small market for soaps, presently still a significantpercentage. Advantages of soaps include greater biodegradability and lesstoxicity.

3.2 Straight Chain Detergent Intermediates

It is necessary for any soap or detergent to have a high degree of linearityfor it to be biodegradable by bacteria. Many synthetic anionic detergents arebased on straight chain primary alcohols and cc-olefins. New technologyallows these materials to be manufactured from ethylene (using Zieglerpolymerization catalysts) or from linear alkanes (paraffins), followed byconversion into linear alkyl chlorides by chlorination or linear oc-olefins bydehydrogenation, in addition to being formed from naturally occurringstraight chains. These processes will be discussed under LAS detergents, butit is important to realize that synthetic long straight-chain compounds arenow available.

Synthetic Detergents

Soaps

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3.3 Linear Alcohol Sulfates (AS)

Alcohol sulfates (AS) are usually manufactured by the reaction of aprimary alcohol with sulfur trioxide or chlorosulfonic acid followed byneutralization with a base. These are high foam surfactants but they aresensitive to water hardness and higher levels of phosphates are required.This latter requirement has harmed the market for this type of detergent, butthey are 2% of production for the major household surfactant market.Sodium lauryl sulfate (R = Cu) is a constituent of shampoos to takeadvantage of its high-foaming properties.

or triethanolamine

The linear alcohols can be made from other long-chain linear materials,but a new process with a triethylaluminum catalyst allows their formationdirectly from ethylene and oxygen.

3.4 Linear Alcohol Ethoxysulfates (AES)

Alcohol ethoxysulfates (AES) are made by reaction of 3-7 mol ofethylene oxide with a linear C^-Ci4 primary alcohol to give a low molecularweight ethoxylate, followed by the usual sulfonation.

They have high foam for shampoos and are "kind to the skin." They arealso used in light duty products such as dishwashing detergents. It is theleast sensitive of the anionics to water hardness and therefore has benefitedin the trend away from phosphates. They have 25% of the production for themajor household surfactant market.

3.5 Linear Alkylbenzenesulfonates (LAS)

Originally the cheap tetramer of propylene was used as the source of thealkyl group in alkylbenzenesulfonates. This tetramer is not a singlecompound but a mixture. However, they are all highly branched. Forexample:

nonlinear C 12^4

The nonlinear dodecene was then used to alkylate benzene by theFriedel-Crafts procedure. Sulfonation and treatment with caustic completedthe process.

nonlinear

This nonlinear alkylbenzenesulfonate formed the basis for the heavy dutyhousehold washing powders of the 1950s and early 1960s with excellentcleaning ability. But rivers and lakes soon began foaming since enzymespresent in bacteria could not degrade these "hard" detergents because of thehighly branched side chain. They were banned in 1965.

Thus /7-alkanes (Ci0-Ci4) separated from the kerosene fraction ofpetroleum (by urea complexation or absorption with molecular sieves) arenow used as one source of the alkyl group. Chlorination takes placeanywhere along the chain at any secondary carbon. Friedel-Crafts alkylationfollowed by sulfonation and caustic treatment gives a more linearalkylbenzenesulfonate (LAS) which is "soft" or biodegradable. Thechlorination process is now the source of about 40% of the alkyl group usedfor the manufacture of LAS detergent.

linear C\2^26

+ 3, 4, 5, and 6-phenyl isomers C10-C14; 2, 3, 4, 5, 6-isomers; o +p

The other 60% of the alkyl groups for LAS detergents are made throughlinear a-olefins. w-Alkanes can be dehydrogenated to a-olefins, which thencan undergo a Friedel-Crafts reaction with benzene as described above forthe nonlinear olefins. Sulfonation and basification gives the LAS detergent.

Alternatively, linear a-olefins can be made from ethylene using Zieglercatalysts to give the ethylene oligomer with a double-bonded end group.

LAS detergents made from the chlorination route have lower amounts of2-phenyl product. Use of the a-olefins gives greater 2-phenyl content,which in turn changes the surfactant action somewhat. LAS detergents formany years had the highest percentage of the market, but now they own 19%of production for the major household surfactant market.

4. NONIONICS

4.1 Nony!phenol Ethoxylates (NPE)

In nonionics the molecule has a nonpolar hydrophobic portion and amore polar, but not ionic, hydrophilic part capable of hydrogen bonding withwater. For some years the major nonionics were the reaction products ofethylene oxide and nonylphenol and are called nonylphenol ethoxylates

(NPE). Dehydrogenation of w-alkanes from petroleum (C9H2o) is the sourceof the linear nonene. They still have 13% of the production for the majorhousehold surfactant market.

4.2 Alcohol Ethoxylates (AE or AEO)

These are polyoxyethylene derivatives of straight-chain primary orsecondary alcohols with Ci0-Ci8. These linear alcohol ethoxylate nonionics(AE or AEO) are more biodegradable than nonylphenol derivatives and havebetter detergent properties then LAS. They require fewer phosphates andwork well in energy-saving cooler wash water. Nonionics are the fastestgrowing type of surfactant and now the biggest market in terms ofproduction, holding 34% of production for the major household surfactantmarket, having surpassed LAS in the 1990s.

linear

5. AMPHOTERICS

These surfactants carry both a positive and a negative charge in theorganic part of the molecule. They have a long hydrocarbon chain as thehydrophobic tail. They may behave as anionics or cationics depending on

the pH. They are derivatives of amino acids and may have one of twogeneral formulas, where R is linear Ci2-Ci8. Amphoterics are used inshampoos. Near pH = 7 they are less irritating and "milder." They can beused with alkalies for greasy surfaces as well as in acids for rusty surfaces.

6. DETERGENT BUILDERS

The finished household soap or detergent is more than just a surfactant.It is a complex formulation that includes bleaches, fillers, processing aids,fabric softeners, fragrances, optical brighteners, foam stabilizers, soil-suspending agents, corrosion inhibitors, foam regulators, solubilizers,antiredeposition agents, dye fixatives, enzymes for stain removal, andopacifying agents. We will not discuss all of these. Recent work onbleaching agents have shown sodium perborate and sodium percarbonate tobe replacements of choice for chlorine-containing bleaching agents.

One of the most important and controversial materials in a detergent isthe builder, phosphate being a common one. Although the calcium andmagnesium salts of dodecylbenzenesulfonic acid are more soluble than thoseof fatty acids, these ions in solution interfere with the dislodging of dirt fromthe substrate, and the dirt-suspending power is also affected because of theirdouble positive charge. A chemical must be added to the detergent tosequester or complex the ions. These are called builders. Builders areactually an average of 58% by weight of the chemicals found in a detergent,with the surfactants only 36%, and other additives making up the remaining6%.

The first important commercial builder was sodium tripolyphosphate,Na5P3OiQ, first used with Tide® detergent in 1947. Besides sequesteringpolyvalent metal ions, it prevents redeposition of dirt, buffers the solution topH = 9-10, kills bacteria, and controls corrosion and deposits in the lines ofautomatic washers.

In the late 1960s phosphate builders came to be seen as an environmentalproblem. Phosphates pass unchanged through sewage works and into riversand lakes. Since they are plant nutrients they cause blue-green algae to growat a very fast rate on the surface, causing oxygen depletion. This is calledeutrophication. The search for phosphate substitutes began, and about half

sodium sulfate

benzene polycarboxylates

Figure 24.4 Phosphate substitutes.

the states have banned their use. Currently nearly all laundry detergents donot use phosphates. Phosphate use in laundry detergents declined from 2billion Ib in 1970 to almost zero in 1995. Figure 24.4 lists some of thecompounds tried as phosphate substitutes.

Ethylenediaminetetracetic acid (EDTA) is a good sequestering agent butits cost is excessive. Nitrilotriacetate is effective but has been suggested tobe teratogenic and carcinogenic so it is not used in the U.S. Sodium citrateis harmless but does not work well. Benzene polycarboxylates are expensiveand are not biodegradable. Sodium carbonate is successful except in hardwater areas. Commercial use of zeolites and poly-a-hydroxyacrylate hasbegun. Sodium sulfate occurs as a by-product of any sulfate or sulfonatedetergent, but has limited use as a builder, as does sodium silicate.Phosphates have been replaced largely with zeolites and sodium carbonate,

trisodium nitrilotriacetate sodium poly-a-hydroxyacrylate

sodium citrate

sodium silicate sodium aluminosilicate(zeolite)

sodium carbonate

with the aid of newer antideposition aids like acrylic polymers. Only about50% of phosphate came from detergents, with 33% from household wastesand 17% from fertilizer runoff contributing to eutrophication, so we must bewatchful of pollution from other sources. The search for phosphatesubstitutes will probably continue.

Suggested Readings

Chemical and Engineering News, an annual product report entitled "Soapsand Detergents," usually published in a January issue.

Kent, Riegel's Handbook of Industrial Chemistry, pp. 1012-1049.Wittcoff and Reuben, Industrial Organic Chemicals in Perspective. Part

Two: Technology, Formulation, and Use, pp. 182-212.