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Pickering Emulsions: The Stability and Applications of Solid-Particle Emulsions

may 22 2020

Many pharmaceuticals and food formulations contain both hydrophilic and hydrophobic components and so require emulsifiers to maintain good dispersion of these immiscible constituents for a stable product.1,2

Surfactants are common emulsifiers as they form a molecular film at the oil-water interface, with their hydrophilic end interacting with water and their hydrophobic end interacting with oil.3 This effect reduces coalescence (attraction between droplets of the same phase) and increases the stability of the emulsion.4However, surfactants are often obtained from non-renewable sources such as petrol and have been widely reported to cause health problems in some patients, including skin irritation or rupturing of red blood cells.2,4

Benefits of Pickering Emulsions

Solid particles can also be used as emulsifiers; these are known as Pickering emulsions. The solid particles can be made from non-toxic, plant-based, edible substances, such as proteins, fats, and starches, thus eliminating the environmental and health issues associated with surfactant-based emulsions.2

Another advantage of Pickering emulsions is their high stability as the adsorption of the solid particles at the oil-water interface is virtually irreversible.3 In addition, by adjusting the particle size, the droplet size in the emulsion can be controlled – another factor influencing stability. Recently, researchers have found that the mechanism of stabilization is influenced by both large (µm) and small (nm) particles; larger particles prevent droplet coalescence while smaller particles regulate the phase interface.4

Applications in Food

Vitamins are often hydrophobic and therefor prepared as emulsions, using surfactants, and are taken daily by many people to maintain a healthy lifestyle. In 2018, scientists demonstrated solid particles can be used as an alternative to surfactants as these produce highly stable vitamin emulsions.1

Using amaranth starch, Pickering emulsions of α-tocopherol, the most active form of vitamin E were prepared. The stability of the samples was characterized by light scattering technology, Turbiscan Lab Expert from Formulaction1, that enables non-destructive characterization of samples in their native form in terms of stability, dispersibility and redispersion testing.6 The authors used native and modified amaranth starch to generate emulsions with small droplet sizes. The modified starch showed high emulsifying capacity and stability of the emulsions, thanks to higher oil affinity in comparison to non-modified starch.1

Pharmaceutical and Medical Innovations 

Due to the presence of surfactants, the topical administration of antibiotics and antifungal medication can cause skin irritation in some patients, as previously mentioned. Recently, scientists have worked on creating surfactant-free solid particle emulsions based on cyclodextrins can overcome this adverse effect.2,5

The authors showed that water soluble cyclodextrins form complexes with the oil molecules. At high enough concentration of cyclodextrins, the complexes form nanoparticles able to stabilize the oil-water interphase. Different cyclodextrins (α-, β- and γ-cyclodextrins) were used in combination with liquid paraffin and isopropyl myristate, both oils widely used in topical applications. Isopropyl myristate in combination with γ-cyclodextrins showed the highest emulsifying capacity and good stability. 2,5

In addition, the Pickering emulsions they developed to deliver biocidal phytochemical oils and azole nitrates, a group of antifungals active against a wide range of fungi and Gram-positive bacteria, were highly active against E. coli and two-fold more sensitive against C. albicans and S. aureus compared to the commercial cream.5

Summary

Despite Pickering emulsions being discovered over a century ago, research into their applications is relatively new. However, with their excellent stability and non-toxic constituents, it looks like it is only a matter of time before they are widely used in healthcare applications.

References and Further Reading
1. Leal-Castañeda E. J., et al. (2018). Pickering Emulsions Stabilized with Native and Lauroylated Amaranth Starch. Food Hydrocolloids. https://doi.org/10.1016/j.foodhyd.2018.01.043.
2. Leclercq L., et al. (2016). Pickering Emulsions Based on Cyclodextrins: A Smart Solution for Antifungal Azole Derivatives Topical Delivery. European Journal of Pharmaceutical Sciences. http://dx.doi.org/10.1016/j.ejps.2015.11.017
3. Jiang H., et al. (2020). Pickering Emulsions: Versatility of Colloidal Particles and Recent Applications. Colloid and Interface Science. doi.org/10.1016/j.cocis.2020.04.010.
4. Matos M., et al. (2017). Combined Emulsifying Capacity of Polysaccharide Particles of Different Size and Shape. Carbohydrate Polymers. http://dx.doi.org/doi:10.1016/j.carbpol.2017.04.006.
5. Leclercq L., et al. (2020). Phytochemicals and Cyclodextrins Based-Pickering Emulsions: Natural Potentiators of Antibacterial, Antifungal and Antibiofilm Activity. Langmuir. DOI: 10.1021/acs.langmuir.0c00314.
6. Formulaction.com. Turbiscan Lab. (2020). https://www.formulaction.com/en/products-and-technologies/product-range/turbiscan-lab  

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