Bancroft rule
Encyclopedia
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 the typical emulsion
s, there are tiny particles (discrete phase) suspended in a liquid (continuous phase). In an oil-in-water emulsion, oil is the 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 there may be a formula that's 60% oil and 40% water, if the emulsifier chosen is more soluble in water, it will create 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.
The Hydrophilic-lipophilic balance
(HLB) of a surfactant
can be used in order to determine whether it's a good choice for the desired emulsion or not.
Bancroft's rule suggests that 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.
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
in which an emulsifier is more soluble constitutes the continuous phase."
It was named after Wilder Dwight Bancroft
Wilder Dwight Bancroft
Wilder Dwight Bancroft was an American physical chemist.Born in Middletown, Rhode Island, he received a B.A. from Harvard University in 1888, a Ph.D. from University of Leipzig in 1892, and honorary SCDs from Lafayette College and Cambridge University...
, an American physical chemist.
In all of the typical emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
s, there are tiny particles (discrete phase) suspended in a liquid (continuous phase). In an oil-in-water emulsion, oil is the 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 there may be a formula that's 60% oil and 40% water, if the emulsifier chosen is more soluble in water, it will create 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.
The Hydrophilic-lipophilic balance
Hydrophilic-lipophilic balance
The Hydrophilic-lipophilic balance of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule, as described by Griffin in 1949 and 1954...
(HLB) of a surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
can be used in order to determine whether it's a good choice for the 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 suggests that 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.