According to the Bancroft rule, “the continuous phase is the phase in which an emulsifier is more soluble.” Wilder Dwight Bancroft, an American physical chemist, proposed the rule in the 1910s, and it was named after him.
There are tiny particles (discrete phase) suspended in a liquid in all typical emulsions (continuous phase). Oil is the discrete phase in an oil-in-water emulsion, while water is the continuous phase.
Emulsions are two immiscible liquid phases in a biphasic liquid system. The presence of these phases is such that one phase is dispersed in the other continuous phase. Emulsions are a type of colloids, which are a more general class of two-phase matter systems.
Although the terms colloid and emulsion are sometimes used interchangeably, the emulsion is only used when both phases are liquid. Mayonnaise, milk, lotions, and other products are examples of emulsions.
According to the Bancroft rule, what determines whether an emulsion is 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. Even if a formula contains 60% oil and 40% water, if the emulsifier used is more soluble in water, it will result in an oil-in-water system.
There are some exceptions to Bancroft’s rule, but it’s a good starting point for most systems. The hydrophilic-lipophilic balance (HLB) of a surfactant can be used to determine whether or not it is suitable for the desired emulsion.
- 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).
According to Bancroft’s rule, the emulsifier determines the type of emulsion, and the emulsifier must be soluble in the continuous phase. This empirical finding can be explained by taking into account the interfacial tension at the oil-surfactant and water-surfactant interfaces.
Emulsions can be either “oil in water” or “water in oil.” The properties of the dispersed and continuous phases determine the type of emulsion. The emulsion is known as “oil in water” if the oil phase is dispersed in a continuous aqueous phase. If the aqueous phase is the dispersed phase and the oil phase is the continuous phase, the mixture is referred to as “water in oil.”
The volume fraction of both phases and the type of emulsifier used to emulsify them determine whether an emulsion of oil and water becomes a “water-in-oil” emulsion or a “oil-in-water” emulsion.