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Pickering emulsion

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A Ramsden emulsion, sometimes named Pickering emulsion, is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the water and oil phases. Typically, the emulsions are either water-in-oil or oil-in-water emulsions, but other more complex systems such as water-in-water, oil-in-oil, water-in-oil-in-water, and oil-in-water-in-oil also do exist. Pickering emulsions were named after S.U. Pickering, who described the phenomenon in 1907, although the effect was first recognized by Walter Ramsden in 1903.[1][2]

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 hydrophobicity, shape, and size, as well as the electrolyte concentration of the continuous phase and the volume ratio of the two phases 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 of the particle. 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 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

where r is the particle radius, is the interfacial tension, and 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 "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.[10] This is evident when observing emulsion stabilization using polyelectrolytes.

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

References

  1. ^ Pickering, Spencer Umfreville (1907). "Emulsions". Journal of the Chemical Society, Transactions. 91: 2001–2021. doi:10.1039/CT9079102001.
  2. ^ Ramsden, W (1903). "Separation of Solids in the Surface-layers of Solutions and 'Suspensions'". Proceedings of the Royal Society of London. 72 (477–486): 156–164. doi:10.1098/rspl.1903.0034.
  3. ^ Velikov, Krassimir P.; Velev, Orlin D. (2014). Colloid Stability. pp. 277–306. doi:10.1002/9783527631193.ch35. ISBN 9783527631193.
  4. ^ Dupont, Hanaé; Maingret, Valentin; Schmitt, Véronique; Héroguez, Valérie (2021-06-08). "New Insights into the Formulation and Polymerization of Pickering Emulsions Stabilized by Natural Organic Particles". Macromolecules. 54 (11): 4945–4970. Bibcode:2021MaMol..54.4945D. doi:10.1021/acs.macromol.1c00225. ISSN 0024-9297. S2CID 233595006.
  5. ^ Sharma, T.; Velmurugan, N.; Patel, P.; Chon, B. H.; Sangwai, J. S. (17 September 2015). "Use of Oil-in-water Pickering Emulsion Stabilized by Nanoparticles in Combination With Polymer Flood for Enhanced Oil Recovery". Petroleum Science and Technology. 33 (17–18): 1595–1604. doi:10.1080/10916466.2015.1079534. S2CID 99044892.
  6. ^ Heise, Katja; Jonkergouw, Christopher; Anaya‐Plaza, Eduardo; Guccini, Valentina; Pääkkönen, Timo; Linder, Markus B.; Kontturi, Eero; Kostiainen, Mauri A. (September 2022). "Electrolyte‐Controlled Permeability in Nanocellulose‐Stabilized Emulsions". Advanced Materials Interfaces. 9 (26): 2200943. doi:10.1002/admi.202200943.
  7. ^ Scheuble, Nathalie; Schaffner, Joschka; Schumacher, Manuel; Windhab, Erich J.; Liu, Dian; Parker, Helen; Steingoetter, Andreas; Fischer, Peter (30 May 2018). "Tailoring Emulsions for Controlled Lipid Release: Establishing in vitro–in Vivo Correlation for Digestion of Lipids". ACS Applied Materials & Interfaces. 10 (21): 17571–17581. doi:10.1021/acsami.8b02637. PMID 29708724.
  8. ^ Bertsch, Pascal; Steingoetter, Andreas; Arnold, Myrtha; Scheuble, Nathalie; Bergfreund, Jotam; Fedele, Shahana; Liu, Dian; Parker, Helen L.; Langhans, Wolfgang; Rehfeld, Jens F.; Fischer, Peter (2022). "Lipid emulsion interfacial design modulates human in vivo digestion and satiation hormone response". Food & Function. 13 (17): 9010–9020. doi:10.1039/D2FO01247B. PMC 9426722. PMID 35942900.
  9. ^ Mwangi, William Wachira; Lim, Hui Peng; Low, Liang Ee; Tey, Beng Ti; Chan, Eng Seng (June 2020). "Food-grade Pickering emulsions for encapsulation and delivery of bioactives". Trends in Food Science & Technology. 100: 320–332. doi:10.1016/j.tifs.2020.04.020. S2CID 218967470.
  10. ^ Binks, B. P.; Fletcher, P. D. I. (2001). "Particles Adsorbed at the Oil−Water Interface: A Theoretical Comparison between Spheres of Uniform Wettability and "Janus" Particles". Langmuir. 17 (16): 4708–4710. doi:10.1021/la0103315. ISSN 0743-7463.
  11. ^ Dinsmore, A. D. (2002). "Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles". Science. 298 (5595): 1006–1009. Bibcode:2002Sci...298.1006D. CiteSeerX 10.1.1.476.7703. doi:10.1126/science.1074868. ISSN 0036-8075. PMID 12411700. S2CID 2333453.
  12. ^ Joris Salari (12 May 2011). "Pickering emulsions, colloidosomes &micro-encapsulation". Slideshare.
  13. ^ Poortinga, Albert T. (2008). "Microcapsules from Self-Assembled Colloidal Particles Using Aqueous Phase-Separated Polymer Solutions". Langmuir. 24 (5): 1644–1647. doi:10.1021/la703441e. ISSN 0743-7463. PMID 18220438.
  14. ^ Anderton N, Carlson CS, Matsumoto R, Shimizu RI, Poortinga AT, Kudo N, Postema M (2022). "On the rigidity of four hundred Pickering-stabilised microbubbles". Japanese Journal of Applied Physics. 61 (SG): SG8001. Bibcode:2022JaJAP..61G8001A. doi:10.35848/1347-4065/ac4adc. S2CID 245915590.
  15. ^ Anderton N, Carlson CS, Matsumoto R, Shimizu RI, Poortinga AT, Kudo N, Postema M (2022). "First-cycle oscillation excursions of Pickering-stabilised microbubbles subjected to a high-amplitude ultrasound pulse". Current Directions in Biomedical Engineering. 8 (2): 30–32. doi:10.1515/cdbme-2022-1009. S2CID 251981644.
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