1 Chapter 30Detergents 30.1Introducing detergents 30.2Structure of detergents 30.3Properties of...

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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

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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.


3toothpaste

shampoo

soap body lotion face cleaning lotion

washing powder dish-washing liquid car wash

Figure 30.1 Detergents for different cleaning jobs.


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WHAT IS A DETERGENT?

TYPES OF DETERGENTS

There are two types of detergents:

Soapless detergents (or synthetic detergents)

Soapy detergents (or soaps)

A DETERGENT is a substance which helps water to clean things

better.


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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.

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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’)

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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.


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Structure of soapless detergent particles

Two common soapless detergents are:

Sodium alkylbenzene sulphonate


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Sodium alkyl sulphate


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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:


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A30.1


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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.

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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.


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tap water

detergent solution

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.


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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.


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water

oil

Figure 30.8

Oil and water do not mix.


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Figure 30.9

Shaking a mixture of water and oil and allowing it to stand.


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An oil/water emulsion is unstable.

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Figure 30.10

Shaking a mixture of water and oil (with a little detergent added) and allowing it to stand.


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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

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An oil/water emulsion is stabilized by a detergent.

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To test properties of a detergent.

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CLEANSING ACTION OF DETERGENTS

foam

oil/water emulsion stabilized by detergent

Figure 30.12

A detergent solution stabilizes an

oil/water emulsion.


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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.


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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).


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30.4 MAKING DETERGENTS


MAKING SOAPLESS DETERGENTS

Soapless detergents are manufactured from hydrocarbons

obtained from petroleum.

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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.


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Fats and oils are naturally occurring triesters. In general, the

formula of fats and oils can be represented as:


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Figure 30.15

(a) General formula of

fats and oils.

(b) Model of a fat/oil

molecule.


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There are two basic processes in making soaps:

Saponification

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


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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)


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To prepare a soap.

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A30.4

(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.


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30.6 COMPARING SOAPS AND SOAPLESS 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.

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To compare action of soap and soapless detergent in soft water

and hard water.

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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.


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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.


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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)


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(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.

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(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)


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Figure 30.20

Soap in strongly acidic solution — insoluble alkanoic acid is precipitated out.


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A30.6

(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).


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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.


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Figure 30.22

Specially designed soapless detergents each suited for a particular purpose.


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30.7 PROBLEMS ASSOCIATED WITH USE OF 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.

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Figure 30.23 The structure of an early soapless detergent.


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Figure 30.24

This river had a persistent thick foam caused by non-biodegradable detergents.


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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.)


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A30.7

Hydrocarbon chains in soaps are straight (i.e. unbranched).


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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.


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Figure 30.26

Detergents may not be harmful to the environment, but the additives may.


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Figure 30.27 Rapid growth of algae due to rich phosphates in water.


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Figure 30.29

Fish killed by red tides.


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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.


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Figure 30.31 Detergents with too high or too low a pH are harmful to the skin.


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A30.8


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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

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SUMMARY

4. (a) General structure of a detergent anion:

(b) General structure of a soapless detergent anion:

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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

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SUMMARY

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.

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SUMMARY

Many detergents contain phosphate additives. The

phosphates are nutrients for algae. This may lead to ‘red

tides’ and death of water life.

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