The cleansing action of soap and detergent
The cleansing action for soaps and
detergent results from their ability;
i.
to lower the surface tension of
water,
ii.
to emulsify oil or grease /
dirt, and
iii.
to hold them in suspension in
water.
This ability is due to the structure of
soaps and detergent
Structure of soaps
Soap consists of sodium/potassium cations and soap anions.
When a sodium/potassium soap is dissolve in
water, it will form soap anions and sodium ion, Na+ or potassium ion, K+
Sodium soap à soap anions
+ sodium cations [Na+]
Potassium soap à soap
anions + potassium cations [K+]
What is soap anions?
From the diagram above shows that the structure
of soap anions consists of,
i.
hydrocarbon chains known as hydrophobic part
ii.
carboxylate/ionic group known
as hydrophilic part
Structure of
detergent
Detergent
of sodium cations and detergent
anions.
When a detergent is dissolve in water, it
will form detergent anions and sodium ion, Na+.
What is detergent anions?
i)
Sodium alkyl sulphate detergent
From the diagram above shows that the
structure of alkyl sulphate anions
consists of,
i. hydrocarbon/alkyl chains
known as hydrophobic part
ii. sulphate/ionic group known
as hydrophilic part
i)
Sodium alkyl benzene sulphonate detergent
From the diagram above shows that the
structure of alkyl benzene sulphonate
anions consists of,
i.
hydrocarbon/alkyl benzene
chains known as hydrophobic part
ii.
sulphonate/ionic group known as
hydrophilic part
Remember
this;
In water, soap and detergent dissolves to
form soap anions and detergent anions. The anions will carry out the cleansing
action, because these anions consists of hydrophobic and hydrophilic part.
Why?
- Hydrophobic is water-hating part, is soluble in oil/grease/dirt, but
insoluble in water.
- Hydrophilic is water-loving
part, is soluble in water but
insoluble in oil/grease/dirt.
- Both of the properties of anions, makes soap and detergent
act as an effective cleansing agent.
How soap and detergent carry out the cleansing action?
-
The cloth is dipped in a soap or detergent solutions.
- Soap
and detergent reduces the surface tension of water.
- This increase the wetting ability of
water, therefore the surface of
the cloth is wetted thoroughly.
-
Hydrophilic part dissolves in water.
-
Hydrophobic part dissolves in dirt/grease.
- Mechanical
agitation during scrubbing helps pull the dirt/grease
free,
and break the grease into small part.
-
The droplets do not coagulate and redeposit on the surface
of the
cloth
because the repulsion between the negative charges on their
surface.
-
The droplets is suspended in the water forming an emulsion.
- Rinsing
washes away these droplets and leaves the surface clean.
The
effectiveness action of soap and detergent
Advantages of soap
1.
Soap are effective cleaning agent in soft water.
- Soft water did not contains Mg2+ ions
and Ca2+ ions.
- Example:
rain water, tap water
2.
Soap does not polluted the
environment.
- Soap is biodegradable (can be decomposed by
bacteria)
because it was made from substance that
found in animal
and plant.
3.
Soap will not harmed the aquatic life.
- Soap will not change pH value of river and
pool because
soap is salt and weak alkali.
Disadvantage of soap
1. Soap is not suitable for use in hard water.
-
Hard water contains Mg2+
ions and Ca2+ ions.
-
Example: Sea water, river water
Why not suitable?
-
Mg2+ and Ca2+
ions react with the soap ions to form an insoluble precipitate known as soap scum.
-
Soap scum reduces the amount of
soap available for cleaning, thus wasting soap.
-
Soap scum does not rinse away
easily.
-
Soap scum can form deposits
on clothes causing them to be grey or yellow in colour.
2. Soap did not effective in
acidic water.
- Soap ion react with ion H+
to produce carboxylic acid
molecule.
Advantage of detergent
- Detergent is effective in hard and soft water.
-
Detergent do not form scum with
hard water.
-
Detergent form soluble
substances with calcium or magnesium ions.
- Detergent effective in acidic water
- Detergent
ion did not combine with H+ .
- Detergent is synthetic cleansing agent.
- Structure
of the hydrocarbon chain can be modified to
produce
detergents with specific properties.
Disadvantage of detergent
- Detergent can cause pollution to environment
-
Detergents not biodegradable,
so that it cannot be
decomposed by bacteria
- Detergent harmed to aquatic life
- Detergent
contains phosphate fertilizer will increases the
growth of seaweed and algae in pond or
river. Lots of
soluble oxygen in water will be use up by
bacteria to
decompose seaweed and algae when the plant
died.
The content of oxygen in water decreases,
thus can kill
aquatic life.
- Detergent produce a lot of foam.
- Foam
can cover the water surface and prevent the
oxygen dissolves into
water, thus will kill aquatic life.
Additives in detergent
The main substance in detergent is sodium alkyl benzene sulphonate and sodium alkyl sulphate. Example of
detergent is shampoo, clothes washing powder and dishes washing liquid.
Various type of additive is also added
to detergent to enhance the cleaning efficiency
and to meet the needs of consumers.
additives
|
Function
|
Whitening agent (sodium perborate)
|
React as bleach agent to vanished the
colour stain but did not fade the colour of the clothes.
|
Biological
enzymes (amilase, lipase, protease, selulase)
|
React to substance that in organic properties
like blood, food and water. Enzymes dissolve and change it to substance that
can dissolve in water.
|
Fluorescent agent
(Blancophor)
|
Make the clothes more white, shiny and
bright.
|
Buildup agent
(sodium tripoliphosphate)
|
Soften hard water, expel ion Mg2+
dan Ca2+ . increase the pH value
|
Drying agent
(sodium sulphate,
Sodium silicate)
|
Ensured that the detergent is always in a
dry condition.
|
Stabilizers
|
To prevent the formation of foam
|
Perfumes
|
To make clothes smell fresh and clean.
|
Antiseptic substance
|
To kill all microorganism that cause
disease and smell.
|
