A typical oil-in-water cream contains oil droplets surrounded by an aqueous phase. The boundary around each droplet is the interface. What happens there helps explain why emulsifying oil and water takes more than stirring them together.
Which liquid surrounds the droplets?
The continuous phase is the liquid surrounding the dispersed droplets. In an oil-in-water emulsion, water is continuous; in a water-in-oil emulsion, oil is continuous. Those names describe how the liquid phases are arranged. IUPAC’s emulsion definition also recognises structures involving liquid crystals.
Each line around a droplet marks where the liquid phases meet. The diagram uses a line to show that boundary; it does not depict a solid shell.
What an emulsifier contributes
Creating droplets creates more contact area between oil and water. An emulsifier helps an emulsion form or slows processes that lead to droplets gathering or merging. Aggregation means droplets gather; in coalescence, they merge into larger droplets. See IUPAC’s definition of an emulsifier.
Looking smooth is only the first observation
| Question | Information to examine |
|---|---|
| What is in the cream? | Formula and raw-material composition |
| How is it arranged? | Phase identity and appropriate structural observations |
| Does it change during storage? | Results from a product-specific stability study |
If a layer appears or the texture changes, compare the observations before and after the change. Check the formula and processing history as part of the investigation.
For the next step, read about planning a stability study.
Further reading
- IUPAC Gold Book: emulsion — terminology and phase notation.
