Pickering emulsion

A Ramsden emulsion, sometimes named Pickering emulsion, is an

If oil and water are mixed and small oil droplets are formed and dispersed throughout the water (oil-in-water emulsion), 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.

Particle properties such as

by the droplet and therefore will not be likely to prevent coalescence of the droplets. Particles that are partially hydrophobic are better stabilizers because they are partially wettable by both liquids and therefore bind better to the surface of the droplets. The optimal contact angle for a stable emulsion is achieved when the particle is equally wetted by the two phases (i.e. 90° contact angle). The stabilization energy is given by

${\displaystyle \Delta E\ =\pi r^{2}\gamma _{OW}(1-|\cos {\theta _{OW}}|)^{2}}$

where r is the particle radius, ${\displaystyle \gamma _{OW}}$ is the interfacial tension, and ${\displaystyle \theta _{OW}}$ is the contact angle of the particle with the interface.

When the contact angle is approximately 90°, the energy required to stabilize the system is at its minimum.^[3] Generally, the phase that preferentially wets the particle will be the continuous phase in the emulsion system. The most common type of Ramsden emulsions are oil-in-water emulsions due to the hydrophilicity of most organic particles.

One example of a Ramsden-stabilized emulsion is homogenized milk. The milk protein (casein) units are adsorbed at the surface of the milk fat globules and act as surfactants. The casein replaces the milkfat globule membrane, which is damaged during homogenization. Other examples of emulsions where Ramsden particles may be the stabilizing species are for example detergents, low-fat chocolates, mayonnaises and margarines.

Ramsden emulsions have gained increased attention and research interest during the last 20 years when the use of traditional surfactants was questioned due to environmental, health and cost issues. Synthetic nanoparticles as Ramsden emulsion stabilizers with well-defined sizes and compositions have been the primarily particles of interest until recently when also natural organic particles have gained increased attention. They are believed to have advantages such as cost-efficiency and degradability, and are issued from renewable resources.^[4] Pickering emulsions find applications for enhanced oil recovery^[5] or water remediation.^[6] Certain Pickering emulsions remain stable even under gastric conditions and show an extraordinary resistance against gastric lipolysis,^[7] facilitating their use for controlled lipid digestion and satiation^[8] or oral delivery systems.^[9]

Additionally, it has been demonstrated that the stability of the Ramsden emulsions can be improved by the use of amphiphilic "

.

It is also possible to use latex particles for Ramsden stabilization and then fuse these particles to form a permeable shell or capsule, called a colloidosome.^[11] Moreover, Ramsden emulsion droplets are also suitable templates for micro-encapsulation and the formation of closed, non-permeable capsules.^[12] 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.^[13] Pickering-stabilized microbubbles may have applications as ultrasound contrast agents.^[14][15]

See also

• Liquid marbles

References