C13 Colloids
This topic focuses on a variety of colloidal systems and stresses their importance in everyday life. Hand cream, salad cream and milk are all emulsions. For example, milk is an emulsion of fat droplets in water. Jelly baby sweets are a gel and muddy river water is a sol. Fog or mist is an example of a colloid called an aerosol.

|Syllabus Core |Page |Notes |
|Know that a colloid consists of one substance finely dispersed in another|p1 & 2 |of this booklet and |
|and know the meaning of the words sol, gel and emulsion |p4 |of the “Unit 11 Colloids Information” document |
|Know that colloidal systems are not | | |
|transparent; | | |
|Appreciate the purpose of emulsifiers | | |
|Supplement | | |
|Understand that colloidal systems are not transparent because they | | |
|scatter light rays | | |
|Understand, in simple terms, the action of emulsifiers | | |

Know what colloids are

Look at the photos on p164-168 of Nuffield Co-ordinated Sciences Chemistry (p 1 – 3 Word doc) and write down some examples of colloids below.
Some examples of colloids:
1. Emulsion paint

2. Cosmetic emulsions

3. Butter

A continuous phase and a disperse phase

Colloids are everywhere and all the different types of colloids have something in common; they consist of a mixture of two substances which cannot mix.

“Every colloid has at least two parts. One part is the continuous phase like the water in milk or the air in mist. The other part is split up into minute particles like the fat in milk or the water droplets in mist. These scattered particles make up the disperse phase.”
Read p164 −168 of Nuffield Co-ordinated Sciences Chemistry and then fill in the gaps in the passage that follows from p165. (page 1 of “Unit 11 Colloids Information”)

“One way or another, many of the things we take for granted involve colloids. Colloids muddle up the simple idea that everything is either a solid, a liquid or a gas . A colloid is made by mixing two things which cannot mix!. This sounds stupid until you look at some examples and think about the meaning of the word “mix“. Thus, the trick to understanding the topic of colloids is to stop and think about how different mixtures are described. It also helps to understand the technical meanings of some of the words used to describe colloids.
“There are eight different types of colloid made by ....................... ......... .............. ..... ......... ....................... ..... ................... .” p168 Nuffield (p 4 of “Unit 11 Colloids Information” – WORD doc).

Fortunately, the syllabus only requires us to memorize only three types:

Know the meaning of the words SOL, GEL and EMULSION.

A sol is a solid dispersed in a liquid. e.g. paint
A gel is a liquid dispersed in a solid. e.g. jelly or butter
An emulsion is one liquid dispersed in another liquid. e.g. salad cream, milk or hand cream

Complete the table below with fairly detailed descriptions of the colloids “emulsion paint”, “jelly” and “milk”. You need to describe the continuous and the disperse phases. You will find the information on pages 2 and 3 of the WORD doc or pages 166 & 167 of the text book, Nuff Co-ord Sc Chem. The first one, a GEL, has been done for you.

|Type of colloid |A Description of what it is made of |
| |This is an example of changing a sol (the jelly crystals dispersed in water) into a gel (as the jelly sets). |
| |“A gelatine gel is made with warm water. Gelatine is a protein. Proteins are natural polymers, and the molecules of a |
| |protein are very large. The large molecules disperse in water to form a sol. As the gelotine -in-water sol cools , the |
| |gelatine molecules are attracted to each other and form a continuous network. In this way a gel is formed in which water |
|A GEL |droplets are trapped in the protein.” |
| |With emulsion paint we have another example of a word which has a different meaning in science from its meaning in |
| |everyday use. In chemistry the word emulsion is only used for colloids in which one liquid is dispersed in another liquid |
|Emulsion Paint |as fine droplets. Salad cream is an emulsion of vegetable oil in vinegar. Milk is an emulsion of fat droplets in water. |
| |In paint the dispersed particles are solid. So paint is not truly an emulsion. Paint is an example of the type of colloid |
| |called a sol. |
| |Photographic “emulsions” are also examples of colloids which are not really emulsions. The light-sensitive layer of a film|
| |consists of fine particles of silver bromide trapped in a gelatine gel. |
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| |Milk is a more complicated colloidal system. Fat droplets spread through a watery liquid. The average diameter of the fat |
|Milk |droplets is 3 to 4 um. Larger fat droplets may cluster together and separate out as a cream layer when milk is left to |
| |stand. |
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Copy Figures 9.11 and 9.12 from p167 Nuffield Co-ordinated Chemistry (p3 Word doc).

When presented with a diagram of a colloid the way to identify the continuous phase from the disperse phase is to place a pencil on the diagram and see if you can go from one side of the diagram to the other without crossing a boundary line; if you can you are in the continuous phase.

Recognising colloids because colloidal systems scatter light

A colloid scatters light because when light rays pass through it, the rays are reflected and refracted every time they meet a boundary between the two phases.

1. Explain why milk looks white. p169 Nuffield Co-ordinated Chemistry (p4,5 Word doc).

Because of the scattering of light rays by colloidal particles of protein and fat.

2. Explain why the sky is blue but sunsets often look red.

Blue light is scattered by small particles much more than red light. When the sun horizon, the light which passes through to someone watching the setting sun looks red because much of the blue light has been scattered.

3. Using the information in Figures 9.17 p170 and 9.20 p171 of Nuffield Co-ordinated Sciences Chemistry, (p 5,6 Word doc) classify the following as either a true solution, a colloid or a suspension:

brine - true solution a foam of egg white - colloid starch solution - colloid milk of magnesia - suspension glucose in water – true solution lime slurry - colloid muddy river water - colloid

Appreciate the importance of emulsifiers
Emulsifiers are important in the human body and also in manufactured products such as food emulsions and creams. The purpose of emulsifiers is to stabilise emulsions, to stop mixtures of immiscible liquids separating into two layers. “Miscible” means that substances mix . and “immiscible” means that substances wont mix

An emulsifier molecule has two ends – a hydrophobic (literally ‘water-hating’) end that is attracted to oil, and a hydrophilic (water-loving) end.

By lowering the surface tension of the droplets within an emulsion, emulsifiers stabilize the liquid droplets and prevent them from coming together and forming two layers.

Copy the rest of the paragraph from p173 Nuffield Co-ordinated Chemistry (p8 Word doc) which begins “When an emulsifier is added to a mixture of oil in water, the molecules make for the boundary between the two liquids so that they can be partly in the water and partly in the oil. This lowers the surface tension between the two liquids and stabilizes the emulsion.

Read 'How do we make colloids?' p171-174 Nuffield Co-ordinated Sciences Chemistry (p6, 7 & 8 Word doc) and then complete the following statements.

1. Small droplets in a colloid tend to join together when they collide because they form a single layer droplet the total surface area is less.

2. The word coalesce means combining together to form smaller particles.

3. Emulsifiers stabilise emulsions and stop them separating. They do this by having a special property that one end is attracted to water while the other end is attracted to oil. Also they increase the thickness of the mixture and this stops the oil droplets coming together.
4. The emulsifier in egg yolk is called lecithin.

5. Another emulsifier used by food manufacturers is glyceryl monostearate (GMS). This is safe to use in foods because it is a compound which is formed naturally in your body when you digest fats.
6. “Many cosmetics are also emulsions.” Give examples of the two types of emulsion creams: oil in water emulsions and water in oil emulsions
7. Soaps and detergents form colloids when they are used in cleaning. Explain. (page 181 Nuff Co-ord Science Chem)

When oil is dispersed in water, the oil wants to separate back into 2 separate layers because of “surface tension forces”. Larger droplets have a . . . . . . . . . . . . surface area than two smaller droplets and so experience a smaller force, so, when two smaller droplets in an emulsion collide, they tend to merge to form a larger droplet with a smaller surface area than the two smaller droplets had. Adding an emulsifier (such as detergent) makes the droplets of oil much more stable in the water. Here the emulsifier molecules surround the oil droplets so that the hydrophobic ends are in the oil and the hydrophilic ends are in the water.

Destroying Colloids

It is important to realize why it is that the globules in some colloids don’t join together and precipitate out (ie. clump together and fall to the bottom of the container). This is because the globules in some colloids are charged. See the diagram below, which helps to explain this idea.

In practice some colloids are useful (mayonnaise, paint etc), whilst others (eg smoke in Hong Kong) are not. It would be handy if we could destroy the less useful colloids by getting the dispersed particles to join together.
To do this, the charge on the dispersed particles would have to be neutralized before they could be forced to join together? In practice it is sometimes possible to neutralize the charge on the dispersed particles.

Lime is often added to clay soils. This makes the clay particles join together to form larger particles, making the soil easier to work with and improving the drainage. This process of making small particles join up to form larger ones is called coagulation.

Aluminium Sulphate is often added at an early stage in the treatment of water during purification to help the suspended clay particles precipitate out during “settling”. The small 3+ charged Al ions attract the negatively charged clay particles in suspension. When the coagulated particles grow too big to remain suspended, they fall to the bottom.

Describe why aluminium chloride is an ingredient in body deodorants (P 176 Nuff Co-ord Chem) (p9 Word doc)

----------------------- continuous phase

droplets of the disperse phase

A colloid consists of one substance finely dispersed in another

Emulsifiers are substances which lower the surface tension at the boundary between two liquids.