Studies at phase interfaces: II. The stabilization of water-in-oil emulsions using oil-soluble emulsifiers

Abstract

A range of oil-soluble emulsifying agents, representing various chemical classes, has been examined for the stabilization of water-in-oil (w/o) emulsions of high water/oil ratio. Some of the physical properties of oil solutions of these emulsifiers have been measured, including their interfacial tensions against water, and the relative rates of coalescence of oil drops in water with bulk oil phase and of water drops in oil with bulk water phase. These properties have provided some information on the type of interfacial film required for w/o emulsification. It is concluded that a strong interfacial film should be formed which prevents the coalescence of water droplets while permitting the coalescence of oil droplets. In almost all systems where stable w/o emulsions are formed, it is possible to postulate some form of molecular interaction, either electrostatic attraction or hydrogen bonding, in the interface. The emulsifier should be hydrophobic in character and have the correct hydrophilic-lipophilic balance (HLB) for the particular solvent concerned. However, the published methods for expressing the HLB value quantitatively do not appear to be suitable for selecting emulsifiers for w/o emulsions. A low interfacial tension between the two phases is desirable, but is not essential if a sufficiently strong interfacial film is formed. Interfacial tension may be important, however, in cases where very rapid emulsification is required, and in these cases the emulsifier should also be adsorbed rapidly at the interface.