Stability and Structure of Water-in-Oil Emulsions

Water-in-oil (W/O) emulsions consist of discrete water droplets dispersed in a continuous oil phase. They pose a unique challenge to colloid science, as the inherently hydrophilic droplets must be stabilised within a non-polar medium. Thermodynamically unstable, these systems tend towards coalescence, creaming and Ostwald ripening unless kinetically trapped by suitable interfacial or bulk modifications.

Stability arises from the formation of robust interfacial films and the modulation of bulk viscosity. Conventional low-molecular-weight surfactants, such as polyglycerol polyricinoleate, can reduce interfacial tension and form elastic films that resist droplet deformation. Biopolymers and particulate stabilisers can further reinforce this barrier, either by increasing continuous-phase viscosity or creating Pickering layers around each droplet.

Gelling of one or both phases, as with oleogels in the oil or gelation of the aqueous core, adds a three-dimensional network that hinders droplet mobility and coalescence. The microstructure of these networks, droplet size distribution and droplet volume fraction collectively dictate rheological behaviour and long-term stability.

Interest in W/O emulsions spans food science, pharmaceuticals and cosmetics, where they enable low-fat spreads, controlled release of hydrophilic actives and bespoke rheological properties. Advances in interfacial engineering and sustainable particle stabilisation continue to unlock new applications, highlighting the global significance of these complex soft matter systems.

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Stability and Structure of Water-in-Oil Emulsions publication trend

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

Technical terms

Water-in-oil emulsion: A dispersion of water droplets within a continuous oil phase.

Surfactant: An amphiphilic molecule that adsorbs at the oil–water interface, reducing interfacial tension.

Pickering emulsion: An emulsion stabilised by solid particles that adsorb at the droplet interface.

Oleogel: A gelled network within the oil phase formed by structuring agents.

Interfacial tension: The energy per unit area at the interface between two immiscible liquids.

Viscoelasticity: The combined viscous and elastic response of a material under deformation.

Ostwald ripening: The process by which larger droplets grow at the expense of smaller ones via molecular diffusion.

References

1. Water-in-oil emulsions stabilized by surfactants, biopolymers and/or particles: a review. Trends in Food Science & Technology (2020).
2. Water-in-Oleogel Emulsions—From Structure Design to Functionality. Frontiers in Sustainable Food Systems (2021).
3. Pickering Water‐in‐Oil Emulsions Stabilized Solely by Fat Crystals. Advanced Materials Interfaces (2023).
4. Improving the Stability of Water-in-Oil Emulsions with Medium Internal Phase by the Introduction of Gelatin. Foods (2023).
5. Sucrose Esters and Beeswax Synergize to Improve the Stability and Viscoelasticity of Water-in-Oil Emulsions. Foods (2023).