Emulsions of Two Liquid Phases

Whether an emulsion is 0/W or W/0 depends on a number of variables like oihwater ratio, electrolyte concentration, temperature, etc. For most of this centiuy, emulsion chemists have known that surfactants more soluble in water tend to make 0/W emulsions and surfactants more soluble in oil tend to make W/0 emulsions. This is the essence of Bancroft s rule, which states that the continuous phase of an emulsion tends to be the phase in which the emulsifier is preferentially soluble. The word soluble is misleading, however, for two reasons. Firstly, a surfactant may be more soluble in, say, oil than in water in a binary system, but in the ternary system of oil -I- water + surfactant it may partition more into water. A good example of this is with the anionic surfactant Aerosol OT (sodium bis-2-ethylhexylsulfosuccinate) which dissolves in heptane at 25 C up to at least 0.5 m but has a solubility limit in water of only 0.03 M. An emulsion made from equal volumes of water and heptane at 25 °C is 0/W, however. Secondly, no distinction is made between the solubility of monomeric or aggregated surfactant in oil or water. We will see that this is an important omission. 

The first quantitative measure of the balance between the hydrophilic and hydrophobic moieties within a particular surfactant came in 1949 when Griffin introduced the concept of the HLB, or hydrophile-lipophile balance, as a way of predicting emulsion type from surfactant molecular composition. A major 

There have been developments in understanding how the type of emulsion is related to the phase diagram of mixtures of oil + water + surfactant at equilibrium. These have come from the studies with microemulsions and as an example we take the case of a nonionic surfactant of the polyoxyethylene glycol ether type, C.E,.. Let us consider equilibrium systems of heptane and water (equal volumes) containing C12E5. At low [surfactant], monomer distributes between oil and water but heavily in favour of the oil. The partition coefficient defined as (molar concentration in heptane)/(molar concentration in water) increases from 130 at 10 C to —1500 at 50 C. Above a critical surfactant concentration, reached in both phases and designated (typically 5 X 10 m) and c Coji (typically 

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