On reversible emulsions and the role played by electrolytes in determining the equilibrium of aqueous oil systems

The “oil-water” emulsion obtained by shaking equal volumes of olive oil and water with a sufficient amount of n/10 NaOH to render the external aqueous phase alkaline to phenol phthalein, may be readily converted into a “water-oil” emulsion by shaking with a solution containing an amount of CaCl₂ slightly in excess of the chemical equivalent of the NaOH employed in the preparation of the “oil-water” emulsion. The resulting “water-oil” emulsion may be re-converted into an “oil-water” emulsion by shaking with further additions of NaOH until the total amount of that substance in the system slightly exceeds the chemical equivalent of the CaCl₂ previously employed.

When chemically equivalent proportions of Ca and OH are employed in a system containing equal volumes of oil and the aqueous phase, neither type of emulsion appears to predominate. At this critical point the system is in a state of extremely unstable equilibrium. Shaking with a trace of NaOH solution converts it almost instantaneously into an “oil-water” emulsion, while a trace of CaCl₂ exerts the reverse effect, a “water-oil” emulsion being formed.

Magnesium functions in a manner similar to calcium, one ion being equivalent to two of OH; iron and aluminium exert a greater effect, one ion being equivalent to three of OH, at the critical point, provided equal volumes of oil and water have been employed. NaCl exerts no effect on a system of this type. If certain oil-water systems are slowly transformed through the critical point to water-oil systems or vice versa, figures resembling those in karyokinesis may be observed microscopically.

Bancroft 1 concludes that the production of a stable emulsion depends on the formation of a concentration film or membrane at all points of contact between the dispersed and continuous phases, and that the relative solubility of this film in the aqueous and oil phases and the consequent surface tension relations on its two sides determine the nature of the emulsion formed.