Chapter 30 Detergents 30.1 Introducing detergents 30.2 Structure of detergents 30.3 Properties of detergents 30.4 Making detergents 30.5 Washing powder 30.6 Comparing soaps and soapless detergents 30.7 Problems associated with use of detergents CONTENTS OF CHAPTER 30

30.1 INTRODUCING DETERGENTS We use detergents every day, in one form or another. Like plastics and alkanols, most detergents are made from petroleum products. 30.1 INTRODUCING DETERGENTS

toothpaste shampoo soap body lotion face cleaning lotion washing powder dish-washing liquid car wash Figure 30.1 Detergents for different cleaning jobs. 30.1 INTRODUCING DETERGENTS

WHAT IS A DETERGENT? TYPES OF DETERGENTS A DETERGENT is a substance which helps water to clean things better. TYPES OF DETERGENTS There are two types of detergents:  Soapless detergents (or synthetic detergents)  Soapy detergents (or soaps) 30.1 INTRODUCING DETERGENTS

Soapy detergents include bath soaps, laundry soaps and liquid soaps. Figure 30.2 Soapless detergents include washing powders, washing-up liquids, shampoos and hair conditioners. They are called ‘soapless’ because they contain no soap. Figure 30.3 Soapy detergents include bath soaps, laundry soaps and liquid soaps. 30.1 INTRODUCING DETERGENTS

30.2 STRUCTURE OF DETERGENTS GENERAL STRUCTURE OF DETERGENT PARTICLES Detergents are usually sodium (or potassium) salts of long-chain organic acids. The detergent anion consists of two parts: (1) An ionic group (the ‘head’) (2) A hydrocarbon chain (the ‘tail’) 30.2 STRUCTURE OF DETERGENTS

Figure 30.4 General structure of a detergent anion. Detergent anions therefore attract to both water and oil. This dual nature explains two important properties of detergents — the wetting property and the emulsifying property. 30.2 STRUCTURE OF DETERGENTS

Structure of soapless detergent particles Two common soapless detergents are:  Sodium alkylbenzene sulphonate 30.2 STRUCTURE OF DETERGENTS

 Sodium alkyl sulphate 30.2 STRUCTURE OF DETERGENTS

Structure of soap particles Soaps are sodium (or potassium) salts of long-chain alkanoic acids. The ionic head of soaps is always a carboxylate group (–COO-). A common soap is sodium stearate: 30.2 STRUCTURE OF DETERGENTS

A30.1 30.2 STRUCTURE OF DETERGENTS

30.3 PROPERTIES OF DETERGENTS AS A WETTING AGENT Water has a high surface tension. Figure 30.6 A pond skater. It can ‘walk’ on water, which has a high surface tension. 30.3 PROPERTIES OF DETERGENTS

A detergent reduces the surface tension of water A detergent reduces the surface tension of water. As a result of this, water spreads over the surface and wets it more easily. A detergent thus acts as a wetting agent. 30.3 PROPERTIES OF DETERGENTS

detergent solution tap water Figure 30.7 A detergent increases the wetting power of water. Tap water does not wet this piece of cloth easily, but a detergent solution does. 30.3 PROPERTIES OF DETERGENTS

AS AN EMULSIFYING AGENT Oil and water do not mix. An oil-water emulsion is unstable. On standing, the tiny oil droplets rapidly join together and grow larger to form a separate oily layer again. 30.3 PROPERTIES OF DETERGENTS

oil water Figure 30.8 Oil and water do not mix. 30.3 PROPERTIES OF DETERGENTS

Shaking a mixture of water and oil and allowing it to stand. Figure 30.9 Shaking a mixture of water and oil and allowing it to stand. 30.3 PROPERTIES OF DETERGENTS

An oil/water emulsion is unstable. 30.3 PROPERTIES OF DETERGENTS

Figure 30.10 Shaking a mixture of water and oil (with a little detergent added) and allowing it to stand. 30.3 PROPERTIES OF DETERGENTS

An oil-water emulsion is stabilized by a detergent. Figure 30.11 How detergent anions arrange themselves in an oil-water mixture: (b) after shaking (c) negatively charged oil droplets repel each other. (a) before the mixture is shaken 30.3 PROPERTIES OF DETERGENTS

An oil/water emulsion is stabilized by a detergent. 30.3 PROPERTIES OF DETERGENTS

To test properties of a detergent. 30.3 PROPERTIES OF DETERGENTS

CLEANSING ACTION OF DETERGENTS foam oil/water emulsion stabilized by detergent Figure 30.12 A detergent solution stabilizes an oil/water emulsion. 30.3 PROPERTIES OF DETERGENTS

30.3 PROPERTIES OF DETERGENTS

DETERGENTS are cleansing agents DETERGENTS are cleansing agents. They are surfactants (surface active agents). They work by reducing the surface tension of water, enabling it to wet things more effectively, and by emulsifying grease. In general, ionic groups joined to hydrocarbon chains having 12 to 20 carbon atoms have good detergent properties. 30.3 PROPERTIES OF DETERGENTS

A30.2 (a) C, F and G. Detergents are sodium or potassium salts of long-chain organic acids (usually with number of carbon atoms between 12 and 20). A is an ester. B is an alkanoic acid. D has too few carbon atoms, while E has too many, H is a magnesium salt of alkanoic acid, so they do not possess good detergent properties. (b) C and F. (G is a soapless detergent as its anionic group is — OSO3, not — COO-). 30.3 PROPERTIES OF DETERGENTS

30.4 MAKING DETERGENTS MAKING SOAPLESS DETERGENTS Soapless detergents are manufactured from hydrocarbons obtained from petroleum. 30.4 MAKING DETERGENTS

MAKING SOAPS Soaps are made from animal fats (e.g. beef and mutton fat) or vegetable oils (e.g. palm oil and coconut oil). Figure 30.14 The palm oil from these palm trees can be used to make soap. 30.4 MAKING DETERGENTS

Fats and oils are naturally occurring triesters Fats and oils are naturally occurring triesters. In general, the formula of fats and oils can be represented as: 30.4 MAKING DETERGENTS

(a) General formula of fats and oils. (b) Model of a fat/oil molecule. Figure 30.15 (a) General formula of fats and oils. (b) Model of a fat/oil molecule. 30.4 MAKING DETERGENTS

Saponification There are two basic processes in making soaps:  Salting-out of soap Saponification Fat is hydrolysed (broken down by water) in alkaline solution to give a soap. The process is called saponification. fat / oil + sodium hydroxide  glycerol + soap 30.4 MAKING DETERGENTS

A30.3 (a) No. Paraffin oil is a mixture of hydrocarbons, not esters. (b) No. Only alkalis can saponify fats and oils. Salting-out of soap After saponification is complete, much of the soap dissolves in the solution. To get the soap out , add a concentrated sodium chloride solution. This process is called salting-out of soap: conc. NaCl(aq) Soap(aq) soap(s) 30.4 MAKING DETERGENTS

To prepare a soap. 30.4 MAKING DETERGENTS

(1) Wear safety spectacles. (2) Cover the beaker with a watch glass (to prevent solution from spurting out of the beaker). (3) Heat the beaker gently. 30.4 MAKING DETERGENTS

30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS COMPARING CLEANING ABILITIES IN SOFT WATER AND HARD WATER Water may be soft or hard. Soft water contains no or only very small concentrations of dissolved calcium and/or magnesium ions. Hard water contains appreciable concentrations of calcium and/or magnesium ions. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

To compare action of soap and soapless detergent in soft water and hard water. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

Experiment results show that soaps form a lather easily in soft water; they hardly form any lather in hard water. Instead, they give a sticky insoluble substance called scum. On the other hand, soapless detergents form a lather easily in both soft water and hard water. A30.5 (a) (i) Yes (ii) No. (b) (i) Yes (ii) Yes. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

ADVANTAGES AND LIMITATIONS OF SOAPS Advantages Soap has several advantages as a detergent: (1) It cleans very well in soft water. (2) It is non-toxic to water life. (3) It is biodegradable (i.e. can be broken down by bacteria). Therefore it will not cause foaming in sewage works and rivers. (4) It is only mildly alkaline (with pH between 8 and 9). Thus it seldom causes skin allergy. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

Limitations Soap has the following limitations: (1) Soap is only slightly soluble in water. Thus it is seldom used in washing machines. (2) It does not work well in hard water. Taking sodium stearate as an example of soap, Ca2+(aq) + 2CH3(CH2)16COO–(aq)  (CH3(CH2)16COO)2Ca(s) Mg2+(aq) + 2CH3(CH2)16COO–(aq)  (CH3(CH2)16COO)2Mg(s) stearate ion (scum) 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

(a) (b) Figure 30.19 (a) Soap in soft water — lather is formed. (b) Soap in hard water — no lather is formed. Note the scum formed. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

CH3(CH2)16COO–(aq) + H+(aq)  CH3(CH2)16COOH(s) (3) Soap cannot be used in strongly acidic solutions. Taking sodium stearate as example, CH3(CH2)16COO–(aq) + H+(aq)  CH3(CH2)16COOH(s) stearate ion (from acid) stearic acid (with no detergent properties) 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

Figure 30.20 Soap in strongly acidic solution — insoluble alkanoic acid is precipitated out. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

SOLVING THE SOAP PROBLEM (a) Hard water. Sea water contains magnesium salts in addition to sodium chloride. (b) No. Sea water is hard water. The calcium and/or magnesium ions present would react with soaps to form scum. SOLVING THE SOAP PROBLEM (1) Remove the hardness and acidity of water. An effective water softener is washing soda, sodium carbonate-10-water (Na2CO3 •10H2O). 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

ADVANTAGES OF SOAPLESS DETERGENTS Ca2+(aq) + CO32–(aq)  CaCO3(s) Mg2+(aq) + CO32–(aq)  MgCO3(s) Phosphates are also used as softeners. (2) Use soapless detergents instead of soaps. ADVANTAGES OF SOAPLESS DETERGENTS (1) Soapless detergents do not have the limitations of soaps. They do not form scum with hard water. (2) Soapless detergents are made from petroleum products, not from fats and oils. (3) Soapless detergents can be ‘tailor-made’ to suit a particular cleaning problem. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

Figure 30.22 Specially designed soapless detergents each suited for a particular purpose. 30.6 COMPARING SOAPS AND SOAPLESS DETERGENTS

30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS PROBLEMS ASSOCIATED WITH NON-BIODEGRADABLE DETERGENTS Detergents used in the early 1950s were non-biodegradable. This is because the early detergent particles contained branched hydrocarbon chains. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Figure 30.23 The structure of an early soapless detergent. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Figure 30.24 This river had a persistent thick foam caused by non-biodegradable detergents. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Soapless detergents nowadays are biodegradable Soapless detergents nowadays are biodegradable. They contain straight (unbranched) hydrocarbon chains. Figure 30.25 The structure of a biodegradable soapless detergent. (Note the straight hydrocarbon chain.) 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Hydrocarbon chains in soaps are straight (i.e. unbranched). 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

PROBLEMS ASSOCIATED WITH BIODEGRADABLE DETERGENTS Even biodegradable detergents affect rivers and lakes. Firstly, because they are biodegradable, bacteria would use up dissolved oxygen in water. Secondly, commercial detergents usually contain phosphate additives. Phosphates are plant nutrients, causing rapid growth of algae in rivers and seas. This is believed to be one of the causes for ‘red tides’ in Hong Kong waters. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Figure 30.26 Detergents may not be harmful to the environment, but the additives may. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Figure 30.27 Rapid growth of algae due to rich phosphates in water. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Fish killed by red tides. Figure 30.29 Fish killed by red tides. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

EEFECT OF DETERGENTS ON SKIN Most detergent solutions have pH values between 5 and 9. Detergents with pH values outside this range may cause skin allergy. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

Figure 30.31 Detergents with too high or too low a pH are harmful to the skin. 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

A30.8 30.7 PROBLEMS ASSOCIATED WITH USE OF DETERGENTS

SUMMARY 1. A detergent is a substance which helps water to clean things better. A detergent has cleaning properties because it can act as a wetting agent and an emulsifying agent. 2. There are two types of detergents:  Soapless detergents — made from petroleum products  Soaps — made from animal fats or plant oils 3. Detergents are usually sodium (or potassium) salts of long-chain organic acids. SUMMARY

4. (a) General structure of a detergent anion: (b) General structure of a soapless detergent anion: SUMMARY

(c) General structure of a soap anion: 5. Soaps can be made by reacting animal fats or plant oils with sodium hydroxide. Two processes are involved:  Saponification fat / oil + sodium hydroxide  glycerol + soap  Salting-out of soap SUMMARY

8. Problems associated with the use of detergents: 6. Soft water contains no or only very low concentrations of dissolved calcium and/or magnesium ions. Hard water contains appreciable concentrations of calcium and/or magnesium ions. 7. Soaps work well in soft water but not in hard water. Soapless detergents work well in both soft water and hard water. 8. Problems associated with the use of detergents:  Detergents may cause skin allergy.  Detergents go down the drain into the sewage system and eventually to rivers or seas. Bacteria in water use up oxygen during the decomposition of these detergents. This would kill water life. SUMMARY

. Many detergents contain phosphate additives  Many detergents contain phosphate additives. The phosphates are nutrients for algae. This may lead to ‘red tides’ and death of water life. SUMMARY