A Pickering emulsion is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the two phases. This type of emulsion was named after S.U. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903.
If oil and water are mixed and small oil droplets are formed and dispersed throughout the water, eventually the droplets will coalesce to decrease the amount of energy in the system. However, if solid particles are added to the mixture, they will bind to the surface of the interface and prevent the droplets from coalescing, making the emulsion more stable.
Properties such as hydrophobicity, shape, and size of the particle can have an effect on the stability of the emulsion. The particle’s contact angle to the surface of the droplet is a characteristic of the hydrophobicity. If the contact angle of the particle to the interface is low, the particle will be mostly wetted by the droplet and therefore will not be likely to prevent coalescence of the droplets. Particles that are partially hydrophobic (i.e. contact angle of approximately 90°) are better stabilizers because they are partially wettable by both liquids and therefore bind better to the surface of the droplets. The stabilization energy is given by
where r is the particle radius, is the interfacial tension, and is the contact angle.
When the contact angle is approximately 90°, the energy required to stabilize the system is at its minimum. Generally, the phase that preferentially wets the particle will be the continuous phase in the emulsion system.
Additionally, it has been demonstrated that the stability of the Pickering emulsions can be improved by the use of amphiphilic "Janus particles", namely particles that have one hydrophobic and one hydrophilic side, due to the higher adsorption energy of the particles at the liquid-liquid interface. This is evident when observing emulsion stabilization using polyelectrolytes.
Homogenised milk is an example of a Pickering-stabilized emulsion. Casein (protein) units are adsorbed at the surface of milk fat globules and act as a surfactant. The casein replaces the milkfat globule membrane, which is damaged during homogenisation.
It is possible to use latex particles for Pickering stabilization and then fuse these particles to form a permeable shell or capsule, called a colloidosome. Moreover, Pickering emulsion droplets are also suitable templates for micro-encapsulation and the formation of closed, non-permeable capsules. This form of encapsulation can also be applied to water-in-water emulsions (dispersions of phase-separated aqueous polymer solutions), and can also be reversible.
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