Detergents and Surfactants National 6 Higher Chemistry

What is a Detergent? Detergents for different cleaning purposes. A detergent is a substance which helps water clean better

Types 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 detergents Detergents are usually sodium (or potassium) salts of long-chain organic acids. hydrogen atom carbon atom oxygen atom sodium ion + -

(with the sodium ion detached) A detergent anion: (with the sodium ion detached) hydrophobic tail (oil-soluble part) hydrophilic head (water-soluble part) -

Properties of detergents Wetting property of detergents Water 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.

tap water detergent solution A detergent increases the wetting power of water. Tap water does not wet this piece of cloth easily, but a detergent does.

Emulsifying property of detergents We all know that oil and water do not mix. An oil-water emulsion is unstable. oil water tiny oil droplets emulsion oil water allow it to stand shake

allow it to stand shake oil emulsion foam tiny oil droplets water + a little detergent oil tiny oil droplets emulsion foam allow it to stand shake Shaking a mixture of water and oil (with a little detergent added) and allowing it to stand.

(a) Before the mixture is shaken. (b) After shaking. detergent anion hydrophobic hydrocarbon tail in oil hydrophilic ionic head in water oil oil droplet (a) Before the mixture is shaken. (b) After shaking. How detergent anions arrange themselves in an oil-water mixture.

repulsion between oil droplets (c) Negatively charged oil droplets repel each other. How detergent anions arrange themselves in an oil-water mixture.

foam oil/water emulsion stabilized by detergent 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 detergent enables water to wet the object thoroughly. detergent anion water grease dirt particles surface (a) (b) How detergent removes grease and dirt from a surface.

How detergent removes grease and dirt from a surface. 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 grease particles (negatively charged) (c) (d) How detergent removes grease and dirt from a surface.

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 oils carboxylic

Cleaning abilities of soaps in hard water containing considerable concentrations of calcium and/or magnesium ions Soft water 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 acid stearate 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.

Soapless detergent 1. Sodium alkylbenzenesulphonate oxygen atom carbon atom oxygen atom hydrogen atom sulphur atom sodium ion

2. Sodium alkylsulphate sodium ion oxygen atom carbon atom hydrogen atom sulphur atom

Soapless detergents are made from chemicals obtained from petroleum. 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 ____________ 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 on 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 branched hydrocarbon chains. Micro-organisms can only digest straight hydrocarbon chain.

CH 3 2 SO Na (non-biodegradable) SO 3 Na side chains

This river had a persistent thick foam caused by non-biodegradable detergents.  

Soapless detergents nowadays are biodegradable Soapless detergents nowadays are biodegradable. They contain straight (unbranched) hydrocarbon chains. biodegradable The structure of a biodegradable soapless detergent. (Note the straight hydrocarbon chain.)

Biodegradable detergents Use up dissolved oxygen in water Problems Associated with Bio-Degradeable Detergents Biodegradable detergents Broken down by bacteria in water Use up dissolved oxygen in water Water life will die

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.