Bancroft rule says: "The phase in which an emulsifier is more soluble constitutes the continuous phase."
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 a 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).
