Emulsions: The Practical
9 minute read
What you will learn
- Define an emulsion and name its two phases
- Distinguish oil-in-water from water-in-oil emulsions
- Name common emulsifying agents
- Make an emulsion by the dry gum and wet gum methods
- Calculate the primary emulsion quantities
- Recognise creaming, cracking and how to avoid them
Key points
- An emulsion is a mixture of two liquids that do not mix, with one spread as tiny droplets in the other, held there by an emulsifying agent.
- In an oil-in-water (O/W) emulsion, oil droplets are in water. Oral emulsions are of this type.
- In a water-in-oil (W/O) emulsion, water droplets are in oil. These are for external use.
- Acacia is the usual emulsifying agent for oral emulsions. Tragacanth helps to thicken and stabilise.
- For a fixed oil, a primary emulsion is made with oil, water and gum in the ratio 4 : 2 : 1.
- Creaming is a reversible layer that forms on standing. Cracking is a permanent breakdown.
- Label 'Shake the bottle well before use' and add a discard date.
What is an emulsion?
An emulsion is a mixture of two liquids that do not mix, such as oil and water. One liquid is spread through the other as tiny droplets. A third ingredient, the emulsifying agent, forms a film around the droplets and stops them joining together.
| Part | Name | Meaning |
|---|---|---|
| The droplets | Internal (dispersed) phase | The liquid in small drops |
| The liquid around the droplets | External (continuous) phase | The liquid that surrounds them |
Types
| Type | Droplets | Outer liquid | Use | Feel |
|---|---|---|---|---|
| Oil-in-water (O/W) | Oil | Water | Oral emulsions, and many creams | Not greasy, washes off with water |
| Water-in-oil (W/O) | Water | Oil | Some external products | Greasy, stays on the skin |
Simple tests for the type
- Dilution test: an emulsion mixes with its outer liquid. An O/W emulsion mixes with water. A W/O emulsion does not.
- Dye test: a water-soluble dye colours the outer phase of an O/W emulsion evenly.
Emulsifying agents
| Agent | Notes |
|---|---|
| Acacia (gum arabic) | The usual agent for oral emulsions. Makes O/W |
| Tragacanth | Thickens and stabilises. Used with acacia |
| Methylcellulose | Thickens the water, and can act as an emulsifier |
| Soaps, such as those from wool fat or fatty acids | Used in external creams and lotions |
| Synthetic agents (Tweens and Spans) | Used in many creams and lotions |
The amount and type of agent are taken from the formula. Do not change them.
Making an oral emulsion
For a fixed oil such as arachis oil or castor oil, a primary emulsion is made first, in the ratio:
oil : water : gum = 4 : 2 : 1
The primary emulsion is then diluted with the rest of the water to the final volume.
Worked example
Make a primary emulsion for 30 mL of fixed oil.
- 4 parts = 30 mL, so 1 part = 30 ÷ 4 = 7.5
- Gum = 1 part = 7.5 g
- Water = 2 parts = 2 × 7.5 = 15 mL
Mineral oils and volatile oils use slightly different ratios, so follow the formula, for example 3 : 2 : 1 for liquid paraffin.
Dry gum method (Continental method)
- Put the gum in a dry mortar.
- Add the oil and mix lightly with the pestle.
- Add the water all at once, and stir briskly and in one direction until a thick, white, creamy mixture forms. You hear a characteristic clicking or crackling sound.
- Stir for about a minute more to complete the primary emulsion.
- Dilute with water, a little at a time, stirring.
- Add the soluble ingredients and flavour, then make up to the volume.
- Transfer to the bottle and label.
Wet gum method (English method)
- In the mortar, make a thick mucilage by mixing the gum with the water.
- Add the oil in small amounts, stirring constantly, until the primary emulsion forms.
- Dilute and make up to volume as above.
The dry gum method is quicker. The wet gum method is useful when the gum is lumpy or when you need more control.
Points to watch
- Use a dry, wide, clean mortar, not a glass one with a smooth surface, because a rough surface helps.
- Add the water all at once in the dry gum method, because it must reach the right concentration to form the primary emulsion.
- Stir in one direction, quickly.
- Add alcohol, strong salts or acids only after the emulsion is made, since they can break it.
- Add flavouring and preservative after dilution.
Instability
| Problem | What happens | Can it be reversed? |
|---|---|---|
| Creaming | A layer of oil or cream gathers at the top or bottom | Yes, by shaking |
| Cracking (breaking) | The oil and water separate and will not mix again | No, so remake it |
| Phase inversion | O/W changes to W/O or the other way, often with too little emulsifier or too much heat | No |
Causes: too little emulsifying agent, wrong ratio of oil, water and gum, too much heat or cold, strong salts or alcohol, germs, and not enough mixing.
Labelling and storage
- "Shake the bottle well before use."
- A discard date, because emulsions contain water and can grow germs. Use a preservative where the formula says.
- Store in a cool place. Do not freeze, as freezing breaks the emulsion.
- Supply a 5 mL spoon or an oral syringe.
Faults and what to do
| Fault | Likely cause | What to do |
|---|---|---|
| The primary emulsion is thin and oily | The ratio was wrong, or too much water was added | Remake it |
| Oil layer that will not shake in | Cracked | Remake it |
| Lumps of gum | The gum was not dry, or was added slowly | Powder the gum, mix with oil first |
| Air bubbles | Too much beating | Stir more gently, and leave to settle |
Exam focus
- Define emulsion, and state the difference between O/W and W/O.
- Describe the dry gum method and the wet gum method.
- Calculate the amounts of oil, water and gum for a primary emulsion in the 4 : 2 : 1 ratio.
- Common mistake: adding the water to the oil and gum in a large quantity at once, so the primary emulsion never forms.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.An oral oil emulsion is usually of which type?
2.Which is a common emulsifying agent for oral emulsions?
3.A primary emulsion for 30 mL of a fixed oil uses the ratio oil : water : gum = 4 : 2 : 1. How much gum is needed?
4.A layer of oil forms at the top of an emulsion on standing, but mixes in again when the bottle is shaken. This is:
5.In the dry gum method, the gum is first mixed with: