Pickering emulsion

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 its discoverer, S.U. Pickering, who first described the phenomenon in 1907. ["Emulsions" S.U. Pickering, J. Chem. Soc. 91 (1907) 2001]

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 :$Delta\; E\; =\; pi\; r^2gamma\_\{OW\}(1-|cos\{\; heta\_\{OW|)^2$where r is the particle radius, $gamma\_\{OW\}$ is the interfacial tension, and $heta\_\{OW\}$ is the contact angle. When the contact angle is approximately 90°, the energy required to stabilize the system is at its minimum. [Velikov,K.P., Velev, O.D. "Stabilization of thin films, foams, emulsions and bifluid gels with surface-active solid particles" In "Colloid Stability and Application in Pharmacy", T. F. Tadros, Ed., Wiley-VCH Publ. Weinheim, 2007, pp. 277-306]

Generally the phase that preferentially wets the particle will be the continuous phase in the emulsion system.

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