the cleaning action of soap and detergent...

1. The cleansing action of soap or detergent depends on their chemical bonding and

structures.

a) The ionic ‘head’ (negatively charged) is soluble in water (hydrophilic) but

insoluble in oily layer.

b) The long hydrocarbon ‘tail’ (neutral) is insoluble in water (hydrophobic) but

soluble in oily layer.

2. Oil cannot be washed away from clothing with water because oil (a covalent
molecult0 is insoluble in water.


3. Lifting greasy dirt from the surface cloth. When soap or detergent is added to the
dirty surface of a piece of cloth covered with a layer of oil or grease.

a) The negatively charged ‘head’ (hydrophilic) of soap ions or detergent ions

dissolves in water.

b) The hydrocarbon ‘tail’ (hydrophobic) of soap or detergent ions dissolves in

the layer of grease.

4. I the water is agitated slightly, the grease begins to be lifted off the surface. This
cause by the forces of attraction between the water molecules and the negatively
charged heads.

5. On further agitation during washing, the greasy dirt is lifted from the surface.


6. Emulsifying dirt in water

a) Soaps and detergents can act as emulsifying agents to emulsify oils and

grease.

b) The process of emulsification breaks large drops of grease into smaller

droplets that floats in water. The greasy droplets repel on another because

they carry the same charge. As a result, the grease is suspended in the

solution.

c) When the cloth is rinsed with the water, the droplet will be carried away.

d) The cleaning process become more efficient in the water containing the

soap or detergent solution is stirred

the additives in detergent and their respective function

1. Modern detergents used for washing clothes usually contains a few types of
additives to :
                                          a) Increase their cleaning power.

b) Make them attractive and saleable.

2. Only about 20% of the substances in a detergent are cle4ansing agents (sodium
alkyl sulphate or sodium alkylbenzene sulphonate). The other substances are
additives. The examples of addictives and their functions are described as follows:


3. Builders : Sodium tripolyphosphate (Na5P3O10)

a) Sodium tripolyphospathe is usd to soften hard water. In the presence of

sodium tripolyphosphate, Ca2+

ions and Mg2+ ions are removed.

b) Sodium tripolyphosphate increases the pH value of water. In this way,

muddy dirt can be removed.

4. Whitening / bleaching agents : sodium perborate

a) Bleaches (bleaching agents) remove coloured stains by oxidation process.

When coloured stanis are oxidized, the colour will disappear.

b) The whitening (bleaching) agents commonly used in detergent are sodium

perborate (NaH2BO43H2O). Sodium perborate decomposes in hot water to

release oxygen (an oxidising agent) which is responsible for the whitening

(bleaching) action.

c) Unlike chlorine, oxygen does not bleach the colour of dyes are not

damaging to fabrics. When properly used, the perborate bleaches make

fabrics whiter than chlorine bleaches and the colourful dyes of the fabrics

do not fade when dirty stains are removed.

d) Besides sodium perborate, sodium hypochlorite, (NaCIO) can also be used

as bleaches in detergents. The IUPAC name of sodium hypochlorite is

sodium chlorate (I).

e) Sodium hypochlorite releases chlorine that bleaches with dirty stains.

However, high concentrations of chlorine can be quite damaging to fabrics.

These bleaches do not work well on synthetic fabrics (polyster fabrics),

often causing a yellowing rather than the desire whitening. Also chlorine

causes the dyes on fabrics to fade.

5. Biological enzymes : Amylase, lipase, and protease

a) Protein stains such blood, milk, and tomato sauce cannot be removed by

the ordinary detergents because these types of stains are insoluble in

water.

b) Biological enzymes in detergents can break down fat and protein

molecules in food stains. The fatty acids, glycerol and amino acids

produced are soluble in water and are removed during washing.

6. Brighteners

a) Figure below shows the action of brighteners. The brighteners absorb the

invisible ultra-violet and re-radiate it as blue light.

b) Brighteners make fabrics appear whiter and brighter because the blue light

can hide any yellowing on the fabrics. Blue light added to the yellow light
reflected on old fabrics make them look white.

7. Drying agents ; Sodium sulphate and sodium silicate

Anhydrous sodium sulphate and sodium silicate (Na2S2O3) are used as drying
agents to ensure that the detergent in powdered firm is always in a dry condition.

8. Stabilisers

a) The functions of stabilizers is to prevents the formation of foam.
b) In an automatic washing machine, excessive foam can stop the pump
working. So, washing powders for automatic washing machine are made
using detergents that are good at removing and emulsifying grease, but do
not produced foam.

9. Perfumes

Perfumes are added to make clothes smell fresh and clean.

the effectiveness of the cleansing action of soap and detergent

Advantages of soaps
1. Soaps are effective cleansing agents in soft water, that is water does not contain
Mg2+ and Ca2+ ions.


2. Soaps do not cause pollution problems to the environment. This is because soaps
are made from chemical found in animals and plants. This means that soaps are
biodegradable, that is they can be composed by the action of bacteria.


Disadvantages of soaps
1. Soaps are ineffective in hard water, that is, water that contains magnesium and
calcium salts.


2. In hard water, soaps will react with Mg2+ and thus, soaps do not lather in hard
water.


3. Scum is grey solid that is insoluble in water. It consists of magnesium stearate and
calcium stearate.


4. Soaps are not also effective in acidic water, for example rainwater containing
dissolves acids. H+ ions from acids will react with soap ions to produce carboxylic
acids molecular size that are insoluble in water.


5. Stearic acids and other carboxylic acids do not act as cleansing agents because
they exist mainly as molecules and do not anionic hydrophilic ends (’head’) that
dissolves in water.


Advantages of detergents
1. Detergents are cleansing agents that are effective in soft water as well as hard
water. This is because detergents do not form scum with Mg+ and Ca2+ ions found
in hard water.


2. The detergents ions (R –O – SO3- and R – SO3- ) react with Mg+ and Ca2+ ions in hard
water. However, the magnesium salts and calcium salts which are formed are
soluble in water. Hence, the scum is not formed and the detergents are still active
in hard water and lathers easily.


3. Detergents are synthetic cleansing agents. This means that the structure of the
hydrocarbon chain can be modified to produce detergents with specific properties.
Nowadays, different types of detergents have been synthesised for specific uses
such as shampoos and dish cleaner.


4. Furthermore, detergents are also effective in acidic water because H+ ion is acidic
water do not combined with detergents ions.


Disadvantages of detergents
1. Most detergents have branched hydrocarbon chains and are non-biodegradable,
that is, they cannot decomposed by bacteria. As a result, non-biodegradable
detergents cause water pollution.


2. Phosphates in detergents act as fertilizers and promote the growth of water plants
and algae. When the plants die and decay, they will used up the oxygen dissolves
in water. This will decrease the oxygen content in water and kill fishes and other
aquatic lives.


3. Detergents produce a lot of foam in water. The layer of foam that covers the water
surface will prevents oxygen from dissolving in water. This condition will cause
fish and other aquatic life ti die from oxygen starvation.


4. Additives such as sodium hydrochlorite (bleaching agents) releases chlorine gas
in water that is acidic. Chlorine gas is highly toxic and kills aquatic life.

                                                                ~ detergent~