Creams: Oily and Aqueous Creams (The Practical)
9 minute read
What you will learn
- Define a cream and describe how it differs from an ointment
- Distinguish oily creams from aqueous creams
- Prepare an aqueous cream by the heat method
- Calculate the quantities of each ingredient
- Dilute a cream correctly
- Pack, label and store a cream
Key points
- A cream is a semi-solid emulsion for the skin, with water as one of its main parts.
- An aqueous cream is oil-in-water. It is not greasy, spreads easily and washes off.
- An oily cream is water-in-oil. It is greasier, and stays on the skin.
- The oil phase and the water phase are heated to about the same temperature, then mixed and stirred until cool.
- Because creams contain water, they need a preservative and have a shorter shelf life than ointments.
- Dilute a cream only with the proper base, in the proportions the formula or the pharmacist states.
- Pack in a wide-mouthed jar or a tube, and label 'For external use only'.
What is a cream?
A cream is a semi-solid emulsion for application to the skin. It contains water, oils or waxes and an emulsifying agent. It is softer and less greasy than an ointment and is usually easy to wash off.
| Cream | Ointment | |
|---|---|---|
| Water | Contains water | Usually none |
| Feel | Smooth, less greasy | Greasy |
| Washes off | Yes (aqueous) | Difficult |
| Preservative needed | Yes | Usually not |
| Shelf life | Shorter | Longer |
Oily and aqueous creams
| Aqueous cream | Oily cream | |
|---|---|---|
| Type | Oil-in-water (O/W) | Water-in-oil (W/O) |
| Feel | Light, not greasy, vanishes into the skin | Heavier, greasy, stays on the skin |
| Washing | Washes off with water | Does not wash off easily |
| Used for | Soothing, moisturising, as a base for other medicines | Very dry skin, protection |
| Example | Aqueous cream | Zinc cream |
Equipment
Water bath or hot plate, evaporating dish or beaker, thermometer, glass rod or spatula, balance, ointment slab or tile, mortar, ointment jars or tubes, and labels. All must be clean and dry.
Making an aqueous cream by the heat method
A typical formula: emulsifying ointment 30 g, preservative 1 g, purified water to 100 g. Use the formula your school provides.
- Calculate the quantities and have them checked.
- Weigh the emulsifying ointment (the oil phase) into a dish and melt it on a water bath.
- Heat the water with the dissolved preservative to the same temperature as the oil phase (about 60 to 70 °C).
- Add the water phase to the oil phase, stirring all the time.
- Stir until the cream is cool and has thickened, to avoid a grainy texture. Do not stop stirring while it is warm.
- Weigh the cream and make up to the final weight with purified water if some has evaporated.
- Pack into a clean jar and label.
Worked example: scaling
Make 200 g of aqueous cream.
- Scale factor = 200 ÷ 100 = 2
- Emulsifying ointment = 30 × 2 = 60 g
- Preservative = 1 × 2 = 2 g
- Purified water = 200 − 60 − 2 = 138 g
Check. 60 + 2 + 138 = 200 g.
Points to watch
- Use a gentle heat. Do not boil.
- Always add the water to the oil phase, not the other way, as directed.
- Keep stirring until cool.
- Use purified or boiled water, because tap water can carry germs.
- Clean equipment is vital. Germs in a cream are hard to see until it has spoiled.
Oily creams
An oily cream such as zinc cream contains a larger proportion of oils, with a smaller part water. It is made by the same idea: the oils are melted, and the water phase is added at the same temperature and stirred until cool. Follow the formula exactly, because the amount of each ingredient decides which type of emulsion forms.
Diluting a cream
A prescriber may ask for a weaker cream than the one in stock. Dilute it only with the proper base, as the formula or the pharmacist says. Using the wrong diluent can break the emulsion or alter the preservative.
strength wanted × weight wanted = strength of stock × weight of stock
Worked example
Make 100 g of a 5% w/w cream from a 10% w/w stock cream.
- 5 × 100 = 10 × x
- x = 500 ÷ 10 = 50 g of the 10% cream
- Base = 100 − 50 = 50 g
Mix on an ointment slab with a spatula until smooth, or in a mortar, by adding the base to the stock cream in portions.
Diluted creams are usually given a short discard date, as a diluted product has a weaker preservative.
Packing, labelling and storage
- Use a wide-mouthed jar, or a collapsible tube, which is cleaner because the cream is squeezed out.
- Fill the jar without air gaps, and smooth the top.
- Label: patient's name, name and strength of the cream, directions, "For external use only", the date, and the discard date.
- Store in a cool place, and out of direct sunlight. Do not freeze.
- Tell the patient: wash the hands before and after use; use a clean spatula or clean fingers; do not put used cream back in the jar.
Faults and what to do
| Fault | Likely cause | What to do |
|---|---|---|
| Grainy cream | The phases were at different temperatures, or stirring stopped early | Make again at the right temperature and stir until cool |
| Oily separated layer | Cracked emulsion | Remake it |
| Water drops on the surface | Not enough emulsifier, or the cream was cold when mixed | Remake it |
| Mould or odour | Contamination, or no preservative | Discard, and review hygiene |
Exam focus
- State the difference between oily and aqueous creams.
- Describe the method for making an aqueous cream.
- Calculate the amount of stock cream and base to make a weaker cream.
- Common mistake: adding cold water to the melted oil phase, which makes the cream grainy or unstable.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.An aqueous cream is a:
2.Why do creams need a preservative?
3.Aqueous cream is made from 30 g emulsifying ointment, 1 g preservative and purified water to 100 g. How much emulsifying ointment is needed for 200 g?
4.How much of a 10% w/w cream is needed to make 100 g of a 5% w/w cream, with a suitable base added?
5.When making an aqueous cream by heating, the water phase should be: