# Soaps, detergents and agrochemicals: Synthetic detergents: surfactants, formulation and manufacture

Source page: https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p3/

[Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 9: Soaps, detergents and agrochemicals](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/) Part 3 of 5

## Synthetic detergents: surfactants, formulation and manufacture

Soap has one serious weakness: hard water. Synthetic detergents fixed it and took most of the laundry market. A modern detergent is less a chemical than a recipe, with surfactants doing the cleaning and many other ingredients softening water, removing stains and making the powder flow. This part covers the surfactant families, the recipe, the plants and the economics.

### 3.1 Surfactants come in four families, and the synthetics do not form scum in hard water

A surfactant, short for surface-active agent, has a water-hating tail and a water-loving head; Kent calls this an amphipathic structure. In water the tails cluster into micelles, heads outward, and oily soil dissolves in their cores. Kent classifies surfactants by the charge on the head.

Anionic surfactants have a negative head. They include soap, linear alkylbenzene sulphonate (LAS), alcohol sulphates (AS), alcohol ether sulphates (AES), alpha olefin sulphonates and methyl ester sulphonates. Nonionic surfactants carry no charge; the head is a chain of ethylene oxide units, as in alcohol ethoxylates (AE), or a sugar, as in alkyl polyglycosides. Shreve notes they clean better at low temperatures and foam little, which suits drum washers. Cationic surfactants have a positive head, usually a quaternary ammonium group. They clean poorly but are antistatic and germicidal, so they go into fabric softeners and disinfectants. Amphoterics such as betaines carry a pH-dependent charge and are mild but costly, so they go into shampoos.

Why did detergents displace soap? Shreve gives the reason: soap forms insoluble calcium and magnesium salts with hard water, which precipitate as scum and cut foaming and cleaning, while the hardness salts of detergents stay dissolved or dispersed. Kent adds that synthetics were first developed after the First World War, when edible fats were scarce. Kent's US figures show the speed: 2,306 million lb of soap and 30 million lb of synthetics in 1940; by 1960, 1,230 million lb of soap and 3,940 million lb of synthetics, 76% of the total.

Soap is not dead. In Kent's 2003 world table it is still 19.8 of 40.1 billion lb of surfactants, almost half, because in many countries it remains the main product for washing both people and clothes. LAS is the largest synthetic, at 6.4 billion lb.

Data The four surfactant families

| Family | Head group | Examples | Strengths | Typical uses |
|---|---|---|---|---|
| Anionic | Negative: sulphonate, sulphate, carboxylate | Soap, LAS, AS, AES, AOS, MES | Strong cleaning, good foam, low cost | Laundry powders and liquids, dishwashing, shampoo |
| Nonionic | No charge: ethylene oxide chain or sugar | Alcohol ethoxylates, alkylphenol ethoxylates, alkyl polyglycosides | Oily soil, low temperatures, low foam | Laundry liquids, low-foam powders, agrochemical emulsifiers |
| Cationic | Positive: quaternary nitrogen | Benzalkonium chloride, cetyl trimethyl ammonium chloride, esterquats | Antistatic, germicidal; weak cleaning | Fabric softeners, disinfectants |
| Amphoteric | Charge depends on pH | Cocoamidopropyl betaine, amine oxides | Mild to skin, stabilises foam; costly | Shampoos, body wash, dishwashing liquids |

 Source: Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents, pdf pp. 1452, 1457-1464; Shreve (1984) Ch 29 pdf pp. 550-552  Case

#### From hard ABS to soft LAS

The first big synthetic was branched alkylbenzene sulphonate, made from propylene tetramer. It cleaned well but bacteria broke it down slowly, and Dryden notes that detergents foaming in sewage plants made biodegradability a priority. Kent records that by 1965 most of the industry had switched to Linear alkylbenzene, with biodegradation above 95% in sewage plants. The straight paraffins come from kerosene using molecular sieves that admit straight chains but not branched ones (Dryden, Shreve).

- Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1451-1458
- Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 547-552
- Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf pp. 306-307

### 3.2 A laundry detergent is a recipe in which the surfactant is a minority

Surfactants are rarely more than about a fifth to a third of a laundry powder. Kent's typical spray-dried recipes, in the table, have 0-20% LAS and 0-20% alcohol ethoxylate; the rest is builders, fillers and small doses of performance additives.

Builders soften the water. Calcium and magnesium in hard water weaken anionic surfactants, so builders lock the ions away. Sodium tripolyphosphate (STPP) does this by sequestration, forming a soluble complex with calcium. Shreve calls proper phosphate building the key that made the detergent industry possible: it also keeps soil in suspension and works with the surfactant, so less surfactant is needed. STPP was 20-35% of a phosphate powder (Kent) and 30-50% in Shreve's and Dryden's formulations.

Then came the phosphate problem. In the 1960s phosphate from detergents was linked to algal growth, or eutrophication, in lakes, and US states banned it. The first substitute, NTA, was dropped in 1970 over toxicity fears later judged unfounded for laundry use. The lasting replacements are zeolite A, a crystalline aluminosilicate that swaps its sodium for calcium inside its pores, and soda ash, which precipitates calcium as carbonate. Zeolite removes little magnesium, so the two are used together. In 1980 phosphates were still half the US builder market and zeolites 12% (Shreve).

The minor ingredients do the visible work. Enzymes digest particular stains: proteases cut protein, lipases fat, amylases starch, and cellulases trim fibre fuzz to keep colours bright (see Industrial enzymes). Peroxygen bleaches oxidise coloured stains. Optical brighteners, at only 0.05-0.2%, are fluorescent dyes that absorb ultraviolet light and re-emit blue-white light, masking yellowing. Sodium carboxymethylcellulose and polyacrylates stop soil settling back on the cloth. Sodium sulphate, 10-30% of many powders, does little in the wash: it is filler that controls density and flow.

Data A spray-dried laundry powder: ingredients and functions

| Ingredient | Phosphate built (wt%) | Phosphate free (wt%) | Function |
|---|---|---|---|
| Sodium tripolyphosphate | 20-35 | 0 | Builder: sequesters calcium and magnesium, suspends soil |
| Zeolite A | 0 | 0-20 | Builder: exchanges sodium for calcium |
| Sodium carbonate | 5-10 | 5-10 | Builder and alkali: precipitates hardness |
| Sodium LAS | 0-15 | 0-20 | Anionic surfactant: main cleaning agent |
| Alcohol ethoxylate | 0-15 | 0-20 | Nonionic surfactant: oily soil, low temperature |
| Sodium AES | 0-5 | 0-5 | Anionic surfactant, hard-water tolerant |
| Sodium silicate | 3-6 | 3-6 | Corrosion protection, buffering |
| Sodium sulphate | 10-30 | 10-30 | Filler: density and flow control |
| Sodium CMC | 0 | 0.1-1 | Stops soil redepositing |
| Enzymes | 0.1-1 | 0.1-1 | Digest protein, fat and starch stains |
| Optical brightener | 0.1-0.3 | 0.1-0.3 | Fluorescent whitening |
| Bleach | 0-5 | 0-5 | Oxidises coloured stains |
| Water | 3-8 | 3-8 | Residual moisture |

 Source: Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents, pdf pp. 1464-1466 (Table 36.6); Shreve (1984) Ch 29 pdf pp. 558-559  In the data

#### Reading the recipe as a bill of materials

Shreve's heavy-duty granule needs, per tonne, 75 kg each of alkylbenzene and tallow fatty alcohol, 150 kg of oleum, 200 kg of caustic solution, 500 kg of STPP, 125 kg of silicate, 30 kg of additives and 500 kg of water, most of it evaporated. The organic raw materials are only 150 kg. Half the finished weight is phosphate, so phosphate bans reshaped formulation and cost.

- Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1464-1466
- Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 549-550, 558-560
- Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf p. 307

### 3.3 Powders are spray dried or agglomerated; liquids are simply mixed

A detergent factory has two halves: making the active and making the product. Shreve's Procter & Gamble flowsheet shows both. Alkylbenzene is sulphonated with oleum at about 55 C in a "dominant bath", a loop that recirculates at least 20 times the throughput through a cooler, because the reaction is fast (about 96% complete in under a minute) and strongly exothermic, and overheating darkens the product. Fatty alcohol is sulphated in a second loop and the acids neutralised with caustic soda, releasing six to eight times as much heat again. Kent notes that modern plants prefer sulphur trioxide diluted to 4-8% in dry air, in falling-film reactors: the capital is higher, but the product is 95-98% active against about 90% from oleum, with far less spent acid. The SO3 is often made on site from sulphur, as in a Sulphuric acid plant.

Spray drying turns a wet slurry into hollow beads. Surfactant paste, builders and most additives are mixed in a crutcher at 64-72% solids, pumped at 300-1,200 psig and sprayed from nozzles near the top of a tower 40-200 ft high and 10-35 ft wide. Hot gas at 250-270 C enters near the bottom and rises against the falling droplets, leaving at 110-140 C. Spray drying gives density control (0.20-0.80 g/cm3), solubility and high output, but Kent names its drawbacks: high capital and energy use. Heat-sensitive enzymes, bleach and perfume are added afterwards, as 2-15% of the formula.

Agglomeration builds granules by spraying liquid surfactant or silicate onto dry powders in a mixer, then conditioning and screening. It needs less capital and energy, and its denser product cuts packaging cost, but it cannot carry as much surfactant.

Liquids need only mixing vessels or in-line mixers; they are 50-60% water. Kent reports liquids at 60-70% of the North American laundry market despite a higher cost per wash.

Process flowchart Heavy-duty detergent granules by sulphonation and spray drying Inputs **Linear alkylbenzene**75 kg per t product **Tallow fatty alcohol**75 kg **Oleum**150 kg **Caustic soda solution**200 kg **Sodium tripolyphosphate**500 kg **Sodium silicate**125 kg **Minor additives**30 kg **Water**500 kg Process steps 1 **Sulphonator**

Alkylbenzene + oleum at about 55 C; recirculation at least 20:1; 96% conversion in under a minute
 2 **Sulphater**

Fatty alcohol + more oleum at 50-55 C
 3 **Neutraliser**

NaOH; removes six to eight times the heat of sulphonation
 4 **Crutcher**

Surfactant slurry + STPP + additives; STPP hydrates and thickens the paste
 5 **Spray tower**

24 m tall; high-pressure nozzles; hot air rises countercurrent
 6 **Air lift and cyclone**

Cools granules from 115 C and separates fines
 7 **Screening, perfuming, packing**

Heat-sensitive ingredients added after drying
 Outputs **Detergent granules**1 t **Water vapour**most of the water fed

A 1980s phosphate formula; current plants often use air/SO3 sulphonation and zeolite or soda ash builders.
 Source: Shreve (1984), Ch 29 Soap and Detergents, pdf pp. 559-562  Worked example

#### How much water does the tower evaporate?

Take a slurry at 70% solids, inside Kent's range, dried to a powder with 5% moisture. One tonne of powder holds 950 kg of solids, which arrived with 950 x 30 / 70 = 407 kg of water. The powder keeps 50 kg, so the tower evaporates about 357 kg of water per tonne. At 64% solids it must evaporate 950 x 36 / 64 - 50 = 484 kg, a third more. Hence Kent calls slurry preparation the key factor in spray drying.

- Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 559-562
- Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1458-1461, 1466-1470

### 3.4 The money is in raw materials and brands, not in the plant

Washing products show a sharp split between cheap ingredients and valuable packs. Shreve notes that energy in soap making matters little next to raw materials, packaging and distribution. Dryden states that oils and fats account for a large share of the cost of toilet and laundry soap. For detergents, Kent observes that surfactant raw material prices rise with the number of processing steps, so a petrochemical LAS and a coconut-based alcohol sulphate compete on cost per unit of cleaning, not on origin.

Supply is a mix of make and buy. Shreve reports that the big soap and detergent companies make their own LAS, fatty alcohol sulphates and fatty acids, in hundreds of millions of pounds, but buy oleum, Caustic soda, phosphates and most additives.

Scale works differently in the two halves. Sulphonation and spray drying reward large plants: a tower making up to 80,000 lb an hour is a big fixed investment that must run full. Agglomeration and liquids need little capital, so small producers can enter. Indian experience shows both. Dryden lists a market in the mid-1990s led by Hindustan Lever, Tata Oil Mills, Procter & Gamble, Godrej and Colgate-Palmolive, alongside local players such as Nirma. Detergents made up Rs 2,000 crore, 67%, of turnover above Rs 3,000 crore. Detergents reached India only in 1956, yet India had become the second largest detergent market after the United States, about 1.71 million tonnes worth Rs 30 billion and growing 10% a year, with a large unorganised sector selling low-cost washing products.

The pattern for an investor follows from these facts. The cost floor is set by raw materials whose prices are made elsewhere, in palm plantations and petrochemical complexes. The premium is earned by brands and distribution. The plant in between rarely confers advantage unless scale or a better process cuts the cost of the active.

Data Demand for washing products in India (thousand tonnes)

| Year | Toilet soap | Laundry soap | Synthetic detergents | Total |
|---|---|---|---|---|
| 1990 | 330 | 800 | 1,175 | 2,305 |
| 1994 | 450 | 930 | 1,735 | 3,115 |
| 1998 | 600 | 1,120 | 2,470 | 4,190 |
| 2000 (projected) | 690 | 1,180 | 2,925 | 4,795 |
| 2005 (projected) | 975 | 1,425 | 4,275 | 6,675 |

Chemical Weekly Annual (1993) as quoted by Dryden; later years are projections made in the early 1990s.
 Source: Dryden (1997), IIIB Soaps and Detergents; Glycerin, pdf p. 315  In the data

#### India: the growth was in synthetics

In Chemical Weekly figures quoted by Dryden, Indian demand for washing products in 1990 was 2,305 thousand tonnes, of which synthetic detergents were 1,175, or 51%. The projection for 2000 was 4,795 thousand tonnes, with detergents at 2,925, or 61%. Demand for alkyl benzene sulphonate surfactant was about 140,000 tonnes in 1990, rising to a projected 310,000 in 2000.

- Shreve (1984), Ch 29 Soap and Detergents · pdf pp. 547, 550, 565
- Kent (2012), Ch 36 Soap, Fatty Acids, and Synthetic Detergents · pdf pp. 1453, 1467-1470
- Dryden (1997), IIIB Soaps and Detergents; Glycerin · pdf pp. 307-308, 314-316

### See it in the table
[H₂O Water](https://www.materialinsights.org/commodities/water/)[Mg Magnesium](https://www.materialinsights.org/commodities/magnesium/)[Sda Soda ash](https://www.materialinsights.org/commodities/soda_ash/)[Zeo Zeolites](https://www.materialinsights.org/commodities/zeolites/)[Su Sugar](https://www.materialinsights.org/commodities/sugar/)[H2SO4 Sulphuric acid](https://www.materialinsights.org/chemicals/sulphuric_acid/)[NaOH Caustic soda](https://www.materialinsights.org/chemicals/caustic_soda/)[C2H4 Ethylene](https://www.materialinsights.org/chemicals/ethylene/)[C3H6 Propylene](https://www.materialinsights.org/chemicals/propylene/)[Bz Benzene](https://www.materialinsights.org/chemicals/benzene/)[EO Ethylene oxide](https://www.materialinsights.org/chemicals/ethylene_oxide/)[LAB Linear alkylbenzene](https://www.materialinsights.org/chemicals/lab/)[Dye Dyes](https://www.materialinsights.org/chemicals/dyes/)[Srf Surfactants](https://www.materialinsights.org/chemicals/surfactants/)

### Check your understanding

Why does soap lose cleaning power in hard water while LAS largely does not?
 Shreve: soap's calcium and magnesium salts precipitate as scum and reduce foaming and cleaning; detergent hardness salts stay dissolved or dispersed. LAS is anionic.

A powder maker must remove STPP. Which pair of builders most directly replaces it?
 Kent: soda ash, or soda ash with zeolite, replaced STPP in granular laundry products. Zeolite exchanges calcium; soda ash precipitates it and handles magnesium. Sulphate and chloride are fillers.

What is the main economic drawback of spray drying compared with agglomeration?
 Kent lists capital and energy as spray drying's disadvantages. Agglomeration costs less and gives a denser product that cuts packaging cost, but carries less surfactant.

[← Part 2: Making soap and recovering glycerine](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p2/)[Mark read · Part 4: Agrochemicals: classes, modes of action and discovery →](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p4/)

## Other sections

- [Module overview](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/index.md)
- [Fats and oils: the renewable raw material](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p1/index.md)
- [Making soap and recovering glycerine](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p2/index.md)
- [Agrochemicals: classes, modes of action and discovery](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p4/index.md)
- [Making, formulating and selling pesticides](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/p5/index.md)
- [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/soaps-detergents-agrochemicals/review/index.md)