Formation of water-in-water emulsions and microgels in nonionic surfactant + gelatin aqueous mixtures

Hypothesis: Water-in-water (W/W) emulsions can be obtained when two water-soluble components are mutually immiscible. The scientific literature on W/W emulsions focuses on polymer–polymer mixtures, with only a few reports on polymer-salt systems, and no documented cases involving polymer-surfactant mixtures. Our hypothesis was that by lowering the cloud temperature of a surfactant through the addition of a polymer, phase segregation into two immiscible aqueous solutions could enable the formation of W/W emulsions. Experiments: The system composed of an ethoxylated triglyceride surfactant (Kolliphor ELP) and gelatin was selected. The effect of gelatin on the surfactant's cloud temperature was assessed. Water-in-water (W/W) emulsions were prepared by stirring biphasic mixtures at temperatures above gelatin's gelation point. Gelatin microgels were formed by cooling down gelatin-in-Kolliphor W/W emulsions. Mucin particles were incorporated to improve the colloidal stability of these emulsions and genipin, a natural reagent, was added to produce crosslinked microgels. Findings: The addition of gelatin lowered the cloud temperature of the surfactant, resulting in the formation of two aqueous phases: a surfactant-rich solution and a gelatin-rich solution, which enabled the preparation of W/W emulsions. Chemically crosslinked microgels were successfully obtained by cooling down gelatin-in-surfactant emulsions, crosslinking with genipin and stabilized with mucin microparticles. To the best of our knowledge, this is the first report of W/W emulsions based on polymer-surfactant mixtures, unlike most W/W emulsions reported in the literature, which rely on immiscibility between two polymers in aqueous solutions.