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.