Stabilization Mechanisms in Water-in-Water Emulsions

Water-in-water emulsions arise from the phase separation of aqueous solutions of incompatible polymers or biopolymers, yielding droplets of one aqueous phase dispersed within another. The ultralow interfacial tensions, often in the micro- to nano-Newton per metre range, present a challenge to droplet stability and necessitate bespoke mechanisms to prevent coalescence and creaming. Stabilization strategies fall broadly into steric, electrostatic and pickering categories. Steric repulsion can be provided by adsorbed polymers or block copolymers that extend into both phases, forming a physical barrier to droplet approach. Electrostatic contributions arise from charged macromolecules or polyelectrolytes that accumulate at interfaces, generating an energy barrier to droplet fusion. Particle-driven Pickering stabilization exploits colloidal inorganic platelets, protein microgels or nanofibrils that adsorb irreversibly at the interface and anchor droplets. Beyond static barriers, stimuli-responsive materials such as thermoresponsive or pH-sensitive polymers enable reversible control over droplet stability, offering on-demand assembly and disassembly. Recent advances have also harnessed interfacial crosslinking of natural proteins to form self-standing capsules, and have explored photocatalytic particles to combine stability with functional activity. These approaches expand the utility of water-in-water emulsions in biocompatible reactors, targeted delivery systems and structured foods, where solvent-free, mild conditions and selective partitioning are crucial.

Research from Nature Portfolio

One foundational study introduced protein nanofibrils as interfacial scaffolds that template the formation of two-dimensional networks around aqueous droplets. The nanofibrils crosslink to form robust colloidosome-like shells, termed fibrillosomes, which remain stable even when detached from an interface. Control over shell thickness is achieved by multilayer deposition of fibrils, enabling precise tuning of permeability and mechanical strength. This approach demonstrates how natural protein assemblies can overcome ultralow interfacial tension to yield self-standing, all-aqueous microcapsules without recourse to oil phases or harsh crosslinkers.

Stabilization Mechanisms in Water-in-Water Emulsions publication trend

The graph below shows the total number of articles in stabilization mechanisms in water-in-water emulsions across all publications each year (not limited to Nature Index journals).

Technical terms

Aqueous two-phase system (ATPS): A mixture of two incompatible water-soluble polymers that separates into distinct aqueous phases, each enriched in one polymer.

Pickering emulsion: An emulsion stabilised by solid particles that adsorb irreversibly at the interface, providing physical barriers against coalescence.

Protein nanofibril: Filamentous aggregates of proteins that can self-assemble into nanometre-scale fibres capable of interfacial network formation.

Microgel particle: Soft, swollen polymeric particles that can adsorb to interfaces and deform to cover droplets, imparting steric and viscoelastic stability.

Block copolymer: A polymer consisting of two or more chemically distinct blocks which can segregate to interfaces and provide steric or responsive stabilisation.

References

1. Fabrication of fibrillosomes from droplets stabilized by protein nanofibrils at all-aqueous interfaces. Nature Communications (2016).
2. Stabilisation of water-in-water emulsions by montmorillonite platelets. Journal of Colloid and Interface Science (2017).
3. Tribology and rheology of water-in-water emulsions stabilized by whey protein microgels. Food Hydrocolloids (2023).
4. pH sensitive water-in-water emulsions based on the pullulan and poly( N , N -dimethylacrylamide) aqueous two-phase system. Polymer Chemistry (2022).
5. Graphitic Carbon Nitride Stabilized Water‐in‐Water Emulsions. Macromolecular Rapid Communications (2020).