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Controlling the Emulsion Type Using Adjustable Polyelectrolyte-Surfactant Complexes.

The combination of polyelectrolytes and ionic surfactants in precise proportions presents the possibility of producing a new class of emulsifiers with tunable emulsification properties. We use chitosan along with dioctyl sulfosuccinate sodium, also known as Aerosol-OT (AOT), to demonstrate that emulsion types can be varied, and PIE can be induced via changes in the water phase pH and the molar ratio of surfactant to repeat unit of polyelectrolyte. Confocal microscopy of the emulsions shows that the morphology can be changed from O/W to O/W/O to W/O by varying the surfactant to polyelectrolyte molar ratio at a fixed aqueous phase pH while maintaining droplet sizes in the micrometers to tens of micrometer range. Measurements of the oil (toluene)-water partition coefficient suggest that controlling the emulsion type relies on the ability of the surfactants to partition from the bulk oil to the bulk water phase and to induce polyelectrolyte-surfactant aggregation. We confirm this hypothesis by using different combinations of polyelectrolytes and surfactants. Changes of the water phase pH in situ induces phase inversion only in a particular direction, which suggests that the complexes at the interface are in a kinetically trapped state. Changes of the molar ratio in situ by addition of an oppositely charged surfactant also can induce phase inversion.