The Bancroft rule in
colloidal chemistry
Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and ...
states: "The
phase in which an
emulsifier
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...
is more
soluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
constitutes the continuous phase." This means that water-soluble surfactants tend to give oil-in-water emulsions and oil-soluble surfactants give water-in-oil emulsions. It is a general rule of thumb, still used, but regarded as inferior to HLD theory (Hydrophilic Lipophilic Difference), which takes many more factors into consideration.
It was named after
Wilder Dwight Bancroft
Wilder Dwight Bancroft (October 1, 1867 – February 7, 1953) was an American physical chemist.
Biography
Born in Middletown, Rhode Island, he was the grandson of historian and statesman George Bancroft and great-grandson of Aaron Bancroft. ...
, an American physical chemist, who proposed the rule in the 1910s.
Technical details
In all of the typical
emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Alth ...
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
Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...
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 rationalized by considering the interfacial tension at the oil-surfactant and water-surfactant interfaces.
See also
*
Azeotrope
An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ...
*
Bancroft point A Bancroft point is the temperature where an azeotrope occurs in a binary system. Although vapor liquid azeotropy is impossible for binary systems which are rigorously described by Raoult's law, for real systems, azeotropy is inevitable at tempera ...
References
{{DEFAULTSORT:Bancroft Rule
Colloidal chemistry