Phase inversion pathways of emulsions stabilized by ethoxylated alkylamine surfactants

• Two phase inversion pathways in ethoxylated alkylamine emulsion are proposed.
• Two pathways have the same threshold and different destinations.
• Phase inversion is induced by the addition of acetic acid, but not exclusively.
• Phase inversion mechanisms rely on changes in the spontaneous curvature.
• Structural transition images help in understanding phase inversion mechanisms.

Composition changes are performed on an invert emulsion stabilized by ethoxylated alkylamine surfactants by adding acetic acid solutions. The addition of acetic acid induces a phase inversion that evolves through the following stages: W/O emulsion, O/W/O dual emulsion, bicontinuous state (studded with lamellar phase), random-shaped O/W emulsion, and normal O/W emulsion. The addition of excess acetic acid will always lead to the separation of pure oil. The speed and direction of the phase inversion can be influenced by the water/oil volume ratio; increasing this ratio facilitates the phase transition or causes it to switch to another direction with the W/O/W emulsion state as the destination. Three critical observations help to elucidate the mechanisms that control the phase inversion: increases in both the hydration radius and pliability of the surfactant layer, which result from the protonation and disorganization of rigid water structures at the interface; an increase in the effective volume fraction of the dispersed phase, which results from the formation of multiple emulsions; and a kinetic phase volume effect, which results in a kinetic phase separation and the nucleation of both oil and water domains.

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