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, thus causing the emulsion to be 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", 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.
Recently, using latex particles for Pickering stabilization and then fusing these particles to form a permeable shell, a new form of capsule, called a colloidosome, has been developed. According to the definition of Dinsmore, a colloidosome is a selectively permeable capsule that is composed of colloidal particles. Moreover, Pickering emulsion droplets are suitable templates for micro-encapsulation and the formation of closed/non-permeable capsules as well.
Recently, it was shown that dispersions consisting of phase-separated aqueous polymer solutions can be stabilised using Pickering stabilisation.
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