Soap
and Detergent
In the era of globalization, people are increasingly focusing on their
images and health within the advanced technology. The need for efficient
washing of skin and clothes pushed many inventors and scientist to create many
types of substances that facilitated that need. Because modern detergents found
their footing only after the rise of technology and chemistry of 20th century,
our ancestors focused their attention on production of various soaps and
detergent. Soap is one of the major detergent cleaning agents. It is use to
clean your body from bacteria, dirt and grease. There are much type of soap
like bar soap (a solid soap which dissolve in water) and liquid soap (need to
dissolve in water). Detergent comes from the Latin word detergere meaning to
clean; it is defined as a cleansing agent. Therefore, water itself is a
detergent. This essay looks at soap and soapless (or synthetic) detergents. Both
substances we use every day and have a big market commercially, they affect
everyone.
Soaps are sodium or potassium salts of
long-chain fatty acids. The general formula of soap can be written as RCOONa /
RCOOK where R is an alkyl group usually containing 12 or 18 carbon atoms. –R
can be saturated or unsaturated. For example of soap are sodium laurate [CH3
(CH2)10COO-Na+], sodium palmitate [
CH3(CH2)14COO-Na+],
potassium stearate [CH3(CH2)16CO O-K+],
and potassium oleate CH3(CH2)7CH=CH(CH2)7COO-K+].
In the past, soap was made by mixing animal fats with alkaline wood ashes.
Large-scale commercial soap-making occurred in 1791 when a French chemist,
Nicholas Leblane patented a process for making soda ash or sodium carbonate from
common salt. The process yielded large quantities of quality soda ash. Twenty
years later Belgian chemist Emest Solvay developed a process to further reduce
the cost of obtaining soda ash, thereby reducing the cost of soap
manufacturing. Soaps can be made from animal fats and vegetable oils. The
animal fat most commonly used are fats from cows and goats. The vegetable oils
often are palm oils, olive oils and coconut oil. Soaps are prepared by
hydrolysing fats and oils under alkaline condition. The reaction is called
saponification. The saponification involves the boiling fats or oil with
concentrated sodium hydroxide solution or concentrated potassium hydroxide
solution to produce glycerol and the salt of fatty acids which are soaps. The general equation for this reaction is:
Oil
or Fat + Alkali
= Glycerol +
Soap
The fats or oils are hydrolysed first
to form glycerol and fatty acids. The acids then react with an alkali to form
the corresponding potassium or sodium salts.
Any cleaning agent that is not soap is
a detergent. Detergents are usually made from synthetic resources such as
petroleum fractions. They are developed during the Second World War in response
to shortage of animal fats and vegetable oils. Detergents are usually sodium
salts of sulphonic acid. The general formulae for two common detergents are
A sodium alkyl sulphate A sodium alkylbenzene sulphonate
During the preparation of detergents,
a long-chain hydrocarbon obtained from petroleum fraction is converted into
organic acid through a series of steps. The organic acid is then neutralised
with sodium hydroxide solution to produce a neutral salt which is detergent.
Let see one of the example which is preparation of sodium alkyl sulphate.
Step1, formation of an organic acid (Sulphonation), a long-chain alcohol reacts
with concentration sulphuric acid to form alkyl sulphonic acid. Step 2,
neutralisation, the resulting is then converted to sodium salt by a reaction
with sodium hydroxide. For preparation of sodium alkylbenzene sulphonate, a
long-chain alkene reacts with benzene to form alkylbenzene. The alkylbenzene
formed is then reacted with concentrated sulphuric acid to form alkylbenzene
sulphonic acid (Sulphonation). The alkylbenzene sulphonic acid is then converted
to the sodium salt by a reaction with sodium hydroxide (neutralisation).
The cleaning action of both detergent
and soap results from their ability to lower the surface tension of water and
to emulsify oil or grease and to hold them in suspension in water. This ability
is due to the structure of soaps and detergents. In water, sodium soap
dissolves to form soap anions and sodium cations. For example, the following
chemical equation shows the ionisation of sodium palmitate.
CH3 (CH2)14COONa
CH3 (CH2)14COO- + Na+
CH3 (CH2)14COO- + Na+
A soap anion consists of a long
hydrocarbon chain with a carboxylate on one end. The hydrocarbon chain which is
hydrophobic is soluble in oils or grease (water-hating). The ionic part is the
carboxylate group, which is hydrophilic, is soluble in water (water-loving).
In water, detergent dissolves to form
detergent anions and sodium cations. For example, the following chemical
equations show the ionisation of sodium alkyl sulphate and sodium alkylbenzene
sulphonate.
Similarly, the anion part of a
detergent also consists of a hydrophobic part and a hydrophilic part.
Furthermore, I want to explain about
the cleaning action of a soap or detergent on a piece of cloth with a greasy
stain. When soap or detergent is add to water, it reduces the surface tension
of water. This increases the wetting ability of water. Therefore, the surface
of cloth is wetted thoroughly. The hydrophobic part of the soap or detergent
anion is soluble to grease. The hydrophilic part is soluble in water. During
scrubbing, the water is agitated. This help to pull the grease free from the
surface and also break the grease into small droplets. The droplets do not
coagulate and redeposit on the surface of the cloth due to the repulsion
between the negative charges on the droplets. These droplets are suspended in
water forming an emulsion. Rinsing washes away these droplets and leaves the
surface clean.
Although soap is good cleaning agent,
its effectiveness when used in hard water (is the water that has high mineral
content). Hard water contains calcium and magnesium ions. These ions react with
the soap to form an insoluble precipitate know as soap scum.
Formation of a soap scum reduces the
amount of soap available for cleaning, thus causes wastage of soap. Soaps are
only suitable for use in soft water. Soft water does not contain calcium and
magnesium ions. Soaps do not form scum with soft water. The effectiveness of
cleaning action of soap is also reduced when used in acidic water. The hydrogen
ions in acidic water react with soap to form long-chain fatty acids.
CH3 (CH2)16COO-
(aq) [stearate ion] + H+ = CH3 (CH2)16COOH
Long-chain fatty acids are insoluble
in water due to their high reactive molecular masses. This reduces the amount
of soap available for cleaning. The effectiveness of the cleaning action of
soap is thus reduced. Detergent do not form scum with hard water. They form
soluble substances with calcium or magnesium salts. This means detergent can
still perform its cleaning action in hard water. Thus, detergent is more
effective than soap in hard water. Detergents do not form insoluble substances
with hydrogen ions in acidic water. Thus, their cleaning action is not
affected.
As the technology becomes more modern
nowadays, additives are added to a detergent enhance its cleaning efficiency
and to meet the needs of consumers. The additives in detergent and their
respective function are shown in the table below:
Additives
|
Example
|
Function
|
Biological enzyme
|
Amylase, proteases, celluloses,
lipases
|
To remove protein stains such as
blood
|
Whitening agent
|
Sodium perborate
|
To convert stains into colourless
substances
|
Optical whitener
|
Fluorescent dyes
|
To add brightness and whiteness to
white fabrics
|
Builder
|
Sodium tripolyphosphate
|
To enhance the cleaning efficiency
of detergent by softening the water
|
Suspension agent
|
Carboxymethylcellulose
|
To prevent the dirt particles
removed from redepositing onto cleaned fabrics
|
Drying agent
|
Sodium sulphate, sodium silicate
|
To ensure the detergent in powdered
form is always in a dry condition and to enable it be poured easily.
|
Foam control agent
|
silicones
|
To control foaming in detergent.
|
The cleaning action of soap and
synthetic detergents are very similar, they are both surfactants. They act as
emulsifying agents and reduce the surface tension of water allowing it to ‘wet’
better. Each has their own disadvantages and advantages. For example, advantage
of soap is soaps are eco-friendly and biodegradable. Disadvantages of soap are
not suitable in hard water and have weak cleaning action compare to detergent.
On the other hand, advantages of detergent are do not decompose in acidic
medium and do not form scum in hard water. They have stronger cleaning action
than soaps. As detergents are derived from petroleum they save on natural
vegetable oils. Disadvantages of detergent are detergents are resistant to the
action of biological agents and thus are not biodegradable. Their elimination
from municipal wastewaters by the usual treatment is a problem. They have a
tendency to produce stable foams in rivers that extend over several hundred
meter of surfactants used in their preparation. Thus they pose a danger to
aquatic life. They tend to inhibit oxidation of organic substances present in
wastewaters because they form a sort of envelope around them. In conclusion,
our environment has been affected badly because of products made up of soap and
detergent cannot be disposed except through specially designed ways. There are
also some irresponsible users who dump the unwanted product of soap and
detergent as well as scum, into river and even sea as an easier way to dispose
of them. In doing this, the quality of the environments will decline cause by
pollution, especially water pollution. So we need to use and dispose the soaps
and detergents in the correct way to reduce the environment pollution. I think
the production of synthetic detergents is a good thing especially since all the
time improvements are being made to make them environmentally friendly.
Thank you for reading.
References
·
- http://global.britannica.com/EBchecked/topic/550751/soap-and-detergent
- http://whttp://dwb4.unl.edu/Chem/CHEM869A/CHEM869ALinks/www.sdahq.org/sdalatest/html/soapchemistry1.htm
- www.cleaninginstitute.org/clean_living/soaps__detergents_chemistry_2.aspx
- http://www.cleaninginstitute.org/clean_living/soaps__detergent_history.aspx
- http://www.slideshare.net/muhammedthahir129/soap-and-detergents-15243636
- http://berryberryeasy.com/2011/09/spm-chemistry-form-5-notes-chemicals-for-consumers-part-4/
