Detergents and SurfactantsNational 6 Higher Chemistry
Detergents for different cleaning purposes.
A detergent is a substance which helps water clean better
What is a Detergent?What is a Detergent?
Types of detergentsTypes of detergents
There are two types of detergents:
1. Soapy detergents (or soaps — made from fats or oils like butter or palm oil).
2. Soapless detergents (or synthetic detergents — made from petroleum).
Soapless detergents include washing powder, washing-up liquids and shampoos. They are called ‘soapless’ because they contain no soap.
Structures of soaps and soapless Structures of soaps and soapless detergentsdetergents
Detergents are usually sodium (or potassium) salts of long-chain organic acids.
hydrogen atom
carbon atom oxygen atom sodium ion
+-
A detergent anion:(with the sodium ion detached)
hydrophobic tail (oil-soluble part)
hydrophilic head (water-soluble part)
-
Properties of detergentsProperties of detergents
Wetting property of detergentsWetting property of detergentsWater has a surface tension, the result
of H-Bonding between adjacent water molecules
A water skater. It can ‘walk’ on water, which has a high surface tension.
A detergent increases the wetting power of water. Tap water does not wet this piece of cloth easily, but a detergent does.
tap waterdetergent solution
Emulsifying property of detergentsEmulsifying property of detergents
We all know that oil and water do not mix. An oil-water emulsion is unstable.
oil
water
shake
oil
watertiny oil dropletsemulsion
allow it to stand
Shaking a mixture of water and oil (with a little detergent added) and allowing it to stand.
water + a little detergent
oil
shakeallow it to
stand
tiny oil droplets
emulsion
foam
emulsion
foam
tiny oil droplets
detergentanion
hydrophobichydrocarbontail in oil
hydrophilicionic head inwater
oil
water
oil droplet
How detergent anions arrange themselves in an oil-water mixture.
(a) Before the mixture is shaken.
(b) After shaking.
repulsion between oil droplets
How detergent anions arrange themselves in an oil-water mixture.
(c) Negatively charged oil droplets repel each other.
foam
oil/water emulsionstabilized bydetergent
A detergent solution stabilizes an oil/water emulsion.
How detergent removes grease and dirt from a surface.
The hydrophobic tails of detergent anions dissolve in grease.
(a) (b)
A detergent enables water to wet the object thoroughly.
detergentanion
water
grease
dirtparticles
surface
How detergent removes grease and dirt from a surface.
(c) (d)
Water molecules attract the hydrophilic heads of detergent anions, lifting up the grease from the surface.
By stirring, the grease forms tiny droplets, forming an emulsion.
tiny greaseparticles(negativelycharged)
Structure of soap particles Structure of soap particles Soaps are sodium (or potassium) salts of long-chain carboxylic acids. The ionic head of soaps is always a carboxylate group (COO).
CH3(CH2)16 COO Na+
hydrocarbon tail ionic head
hydrogen atom
carbon atom oxygen atom sodium atom
Soap is made by the alkaline hydrolysis (saponification) of animal fats and vegetable oils (e.g. beef fat, palm oil and coconut oil).
The palm oil from these palm trees can be used to make soap.
glyceryl tristearate (an animal fat)
sodium hydroxide
glycerol sodium stearate (a soap)
Soapy detergents (or soaps) are made from _________ or _________ . They are sodium (or potassium) salts of long-chain __________ acids.
fats oilscarboxylic
Cleaning abilities of soaps in hard Cleaning abilities of soaps in hard waterwater
Hard water
Soft water
containing considerable concentrations of calcium and/or magnesium ions
contains no or only very small concentrations of dissolved calcium and/or magnesium ions
Experimental results show that soaps do not work properly in hard water.
This is because soap anions form insoluble substance (called scum) with calcium and/ or magnesium ions.
Ca2+(aq) + 2CH3(CH2)16COO(aq)
(CH3(CH2)16COO)2Ca(s)calcium stearate
(scum)
stearate ion
Mg2+(aq) + 2CH3(CH2)16COO(aq)
(CH3(CH2)16COO)2Mg(s)stearate ion
magnesium stearate
(scum)
Soap cannot be used in strongly
acidic solutions.
CH3(CH2)16COO–(aq) + H+(aq) CH3(CH2)16COOH(s) (from acid) stearic acidstearate ion
The above reactions make soaps useless.
Besides, the scum formed sticks to the materials being washed.
This problem can be solved by using soapless detergents. This is because the ionic head (SO3
) of soapless detergent particles does not form precipitate with either calcium or magnesium ions.
1. Sodium alkylbenzenesulphonate
carbon atom oxygen atom
hydrogen atomsulphur atom
sodium ion
Soapless detergentSoapless detergent
2. Sodium alkylsulphate
carbon atom oxygen atom sodium ion
hydrogen atom sulphur atom
The ionic head of a common soapless detergent anion is usually a sulphonate group (SO3
) or a sulphate group (OSO3
).
Soapless detergents are made from chemicals obtained from petroleum.
conc. H2SO4 NaOH
from petroleum industry
Soapless detergents are derived from ____________ . They are usually sodium salts of long-chain alkylbenzenesulphonate or ___________________.
petroleum
alkylsulphate
The ionic head (SO3) of soapless detergent
particles does not form precipitate with either calcium or magnesium ions in hard water.
Effect of length of hydrocarbon tail Effect of length of hydrocarbon tail on detergent propertieson detergent properties
In general, detergent particles with hydrocarbon chains having 12 to 20 carbon atoms have good detergent properties.
Problems Associated with Non-
Biodegradable Detergents
Detergents used in the early 1950s were non-
biodegradable. This is because the early detergent
particles contained branchedbranched hydrocarbon chains.
Micro-organisms can only digest straight
hydrocarbon chain.
SO3 Na
side chains
CH
CH
3
CH3
CH
CH
3
CH2
CH
CH
3
CH2
CH
CH
3
CH2
SO3 Na
(non-biodegradable)
This river had a persistent thick foam caused by non-biodegradable detergents.
Soapless detergents nowadays are biodegradable.
They contain straight (unbranched) hydrocarbon
chains.
The structure of a biodegradable soapless detergent. (Note the straight hydrocarbon
chain.)
SO3 Na
biodegradable
Problems Associated with Bio-Degradeable
Detergents
Biodegradable detergents
Use up dissolved oxygen in water
Water life will die
Broken down by bacteria in water
Figure 37.26 Rapid growth of algae due to rich phosphates in water.
Fish killed by red tides.
Effect 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.
It is advisable to wear gloves whenever working with detergents.