Bulk, one step generation of single-core double emulsions from polymer-osmose-induced aqueous phase separation in polar oil droplets

Double water-in-oil-in-water (W/O/W) emulsions are aqueous droplet(s) embedded within oil droplets dispersed in a continuous water phase. They are exploited for many applications from cosmetic to food science since both hydrosoluble and liposoluble cargos can be encapsulated within. They are generally prepared using a one-step or a two-step method, phase inversion and also via spontaneous emulsification. Here, we describe a general and simple one-step method based on hydrophilic polymers dispersed in polar oils to generate osmose-induced diffusion of water into oil droplets, forming polymer-rich aqueous droplet within. Polyethylene glycol (PEG), but also other hydrophilic polymers (branched polyethylene imine, bPEI, or polyvinyl pyrrolidone, PVPone) were successfully dispersed in 1-octanol or other polar oils (oleic acid or tributyrin) to produce an O/W emulsion that spontaneously transformed into a W1/O/W2 emulsion, with the inner aqueous droplet (W1) enriched with the hydrophilic polymer (PEG, bPEI or PVPone). Dispersion of hydrosoluble dyes in the oil/polymer mixture in the form of a W/O microemulsion further allowed their encapsulation within the internal W1 aqueous droplet of the double emulsion. Single drop experiments, as well as macroscopic viscosity measurements, confirm that the double emulsion is actually the result of water diffusion, which amplitude can be adjusted by the polymer concentration. Production of high internal phase emulsions (HIPE) was also achieved together with a pH-induced transition from multiple to single core double emulsion. We expect this new method for producing double emulsions to find applications in domains of microencapsulation and materials chemistry.