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Wednesday, 28 December 2016

The bigger role of Nanoemulsions

In an interaction, Ankur Gupta, a PhD Candidate (2012-2017) in chemical engineering, Massachusetts Institute of Technology (MIT), Cambridge, MA with Chemical Today magazine explains the use of nanoemulsions in pharmaceuticals, food industry and how the unique properties of nanoemulsions can make smart foods.
In brief, give us an idea about your research.
I am currently working on the formation and applications of nanoemulsions. We have recently published series of articles where we proposed a scaling on controlling and predicting the droplet size of nanoemulsions. We are focusing on the application of nanoemulsions in pharmaceutics and food industry.
Techniques involved in nanoemulsions formulation methods.
Nanoemulsion synthesis can be divided into two broad categories: high-energy methods and low-energy methods. High energy methods require significant energy input ( W/kg) where droplets break and become small due to shear forces. High energy methods include high-pressure homogenizer (also known as microfluidizer) and ultrasonicator, both of which are commercially available.
Low-energy methods required lower energy input ( W/kg, achievable using a magnetic stirrer) where the low interfacial tension of the system is exploited to create small droplets. Low energy methods include emulsion inversion point (also known as phase inversion composition) and phase inversion temperature.
Materials used in formulating nanomaterials.
For nanoemulsions, typically one needs oil, water and surfactant(s). Nanoemulsions can either be oil-in-water (O/W) or water-in oil (W/O). Formulation of nanoemulsions using low energy methods requires a careful choice of surfactant since low interfacial tension is required for generating small droplets without significant energy input.
Since O/W nanoemulsions are more common in literature, I will share the common oil phases and surfactant used for O/W nanoemulsions: Oil phases:
•Alkanes such as decane, dodecane, hexadecane
• Food oils such as corn, orange, peanut
• Aromatics such as toluene, xyelene, anisole
• Complex organic oils such as mineral oil Surfactant phases:
• Ionic surfactants such as SDS, CTAB
• Non-ionic surfactants such as Tween, Span, Brij
Explain advantages and the major challenges of nanoemulsions.
The advantages of nanoemulsions are that they have -high surface area, robust stability, tunable rheology, relatively simple synthesis, easy scale-up, easy route to advanced material synthesis and diverse applications.
The challenges, however, are that nanoemulsions require advanced microscopy for visualisation, lack of techniques to create completely monodispersed nanoemulsions, and they require relatively large amount of surfactant for synthesis.
Factors affecting the stability of nanoemulsions.
Due to the small size of nanoemulsions droplets, the stability of nanoemulsions is dominated by Ostwald ripening i.e. increase in the size of bigger droplets at the expense of smaller droplets. The following factors affect the stability:
• Solubility of the oil phase
• Interfacial tension (dependent on choice of surfactant)
• Temperature
• Diffusivity of dispersed phase (affected by the choice of surfactant)
• Polydispersity of the droplet size distribution
• Other factors such as additives, cosurfactants etc.
Trapped species method is a well-known method to make nanoemulsions stable against temperature and time. In trapped species method, insoluble oil is mixed with the dispersed phase to reduce the solubility and hence quenching the Ostwald ripening rate.
How can nanoemulsion be effectively used in various sectors such as food additives, paints & surface coatings, waste-water treatment, cosmetics, antimicrobial cleaning agents?
O/W nanoemulsions have enormous potential in food additives since a high surface area of nanoemulsion allows the transport of hydrophobic nutrients through dissolution in the oil phase in an overall aqueous medium. For instance, significant research has been done on incorporating beta-carotene and curcumin as food additives in O/W nanoemulsions.
The rheology of nanoemulsions can be easily tuned depending on the droplet size. For instance, for the same droplet size, nanoemulsions can be tuned from being viscous to elastic. Thus, they are attractive for cosmetics industry as they provide a range of rheological properties that can have a better skin feel. Similarly, nanoemulsions have a potential for paint and coating industry as the rheology can be tuned without changing the composition.
Nanoemulsions can also provide an easy route to transport chemicals that are hydrophobic in nature and thus find applications in antimicrobial cleaning agents.
The effectiveness of nanoemulsions when compared to macroemulsions and microemulsions.
Macroemulsions are larger emulsions with droplet sizes on the range of 10 um – 10mm and thus are susceptible to destabilisation mechanisms such as coalescence and creaming. Further, they do not have a large surface area. On the other hand, due to their small size, nanoemulsions provide robust stability and high surface area.
Microemulsions, unlike their name suggests, are equilibrium systems with droplet size on the order of 10nm. Thus, though they have high surface area, due to their thermodynamically stable property, they are susceptible to changes in temperature, pH and composition. On the other hand, nanoemulsions are kinetically stable system and possess robust stability against temperature, dilution and pH.
The role of nanoemulsions in making of smart foods.
Nanoemulsions have enormous potential in making smart foods. Since we can make nanoemulsions with oil volume fraction as high as 40 percent, a large quantity of hydrophobic nutrients such as beta-carotene and curcumin can be supplied in an aqueous medium. Further, nanoemulsions possess robust stability and thus have a longer shelf-life. Research has shown that one can stabilise nanoemulsions with biocompatible surfactants. Further, it is easy to add colour into nanoemulsions by using oils with different colours.
Currently, nanoemulsions also play a vital role in the field of nanotechnology.
Nanoemulsions can help in nanotechnology research as they provide an excellent route for nanoparticle synthesis, and developments of advanced materials such thermo-reversible gels. Further, nanoemulsions are an excellent model system for colloidal study due to an abundant deformable surface area.
Most challenging issues in your area of research.
The most challenging issue in nanoemulsion research is the actual visualisation of nanoemulsions since they cannot be observed through usual microscopes. Thus, detailed characterization has to rely on advanced microscopy techniques such as SANS and cryo-TEM.
© Chemical Today Magazine
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