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.