Bancroft rule
The Bancroft rule states: "The phase in which an emulsifier is more soluble constitutes the continuous phase."
It was named after Wilder Dwight Bancroft, an American physical chemist.
In all of your typical emulsions, you have tiny particles (discrete phase) suspended in a liquid (continuous phase). In an oil-in-water emulsion, oil is your discrete phase, while water is the continuous phase.
What the Bancroft rule states is that contrary to common sense, what makes an emulsion oil-in-water or water-in-oil is not the relative percentages of oil or water, but which phase the emulsifier is more soluble in. So even though you might have a formula that's 60% oil and 40% water, if the emulsifier you chose is more soluble in water, you will have an oil-in-water system.
There are some exceptions to Bancroft's rule, but it's a very useful rule of thumb for most systems.
Once you understand the rule, you can then use the Hydrophilic-lipophilic balance (or HLB) of a surfactant in order to determine whether it's a good choice for your desired emulsion or not.
- In Oil in Water emulsions – use emulsifying agents that are more soluble in water than in oil (High HLB surfactants).
- In Water in Oil emulsions – use emulsifying agents that are more soluble in oil than in water (Low HLB surfactants).
Bancroft's rule tells us the type of emulsion is dictated by the emulsifier and that the emulsifier should be soluble in the continuous phase. This empirical observation can be rationalised by considering the interfacial tension at the oil-surfactant and water-surfactant interfaces.
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