Miniemulsion

A miniemulsion (also known as nanoemulsion) is a special case of emulsion. A miniemulsion is obtained by shearing a mixture comprising two immiscible liquid phases (for example, oil and water), one or more surfactants and, possibly, one or more co-surfactants (typical examples are hexadecane or cetyl alcohol).

IUPAC definition
Mini-emulsion: emulsion in which the particles of the dispersed phase have diameters in the range from approximately 50 nm to 1 μm.

Note 1: Mini-emulsions are usually stabilized against diffusion degradation (Ostwald ripening (ref.[1] )) by a compound insoluble in the continuous phase.

Note 2: The dispersed phase contains mixed stabilizers, e.g., an ionic surfactant, such as sodium dodecyl sulfate (n-dodecyl sulfate sodium) and a short aliphatic chain alcohol ("co-surfactant") for colloidal stability, or a water-insoluble compound, such as a hydrocarbon ("co-stabilizer" frequently and improperly called a "co-surfactant") limiting diffusion degradation. Mini-emulsions are usually stable for at least several days.[2]

Mini-emulsion polymerization: Polymerization of a mini-emulsion of monomer in which all of the polymerization occurs within preexisting monomer particles without the formation of new particles.[3]

There are two general types of methods for preparing miniemulsions: high-energy methods and low-energy methods. For the high-energy methods, the shearing proceeds usually via exposure to high power ultrasound[4][5][6] of the mixture or with a high-pressure homogenizer, which are high-shearing processes. For the low-energy methods, the water-in-oil emulsion is usually prepared and then transformed into an oil-in-water miniemulsion by changing either composition or temperature. The water-in-oil emulsion is diluted dropwise with water to an inversion point or gradually cooled to a phase inversion temperature. The emulsion inversion point and phase inversion temperature cause a significant decrease in the interfacial tension between two liquids, thereby generating very tiny oil droplets dispersed in the water.[7]

Miniemulsions are kinetically stable but thermodynamically unstable. Oil and water are incompatible in nature, and the interface between them is not favored. Therefore, given a sufficient amount of time, the oil and water in miniemulsions separate again. Various mechanisms such as gravitational separation, flocculation, coalescence, and Ostwald ripening result in instability.[8] In an ideal miniemulsion system, coalescence and Ostwald ripening are suppressed thanks to the presence of the surfactant and co-surfactant.[4] With the addition of surfactants, stable droplets are then obtained, which have typically a size between 50 and 500 nm.

Miniemulsions have wide application in the synthesis of nanomaterials and in the pharmaceutical and food industries. For example, miniemulsion-based processes are, therefore, particularly adapted for the generation of nanomaterials. There is a fundamental difference between traditional emulsion polymerisation and a miniemulsion polymerisation. Particle formation in the former is a mixture of micellar and homogeneous nucleation, particles formed via miniemulsion however are mainly formed by droplet nucleation. In the pharmaceutical industry, oil droplets act as tiny containers that carry water-insoluble drugs, and the water provides a mild environment that is compatible with the human body. Moreover, miniemulsions that carry drugs allow the drugs to crystallize in a controlled size with a good dissolution rate. Finally, in the food industry, miniemulsions can not only be loaded with water-insoluble nutrients, such as beta-carotene and curcumin, but also improve the nutrients' digestibility.[7]

References
  1. Richard G. Jones; Edward S. Wilks; W. Val Metanomski; Jaroslav Kahovec; Michael Hess; Robert Stepto; Tatsuki Kitayama, eds. (2009). Compendium of Polymer Terminology and Nomenclature (IUPAC Recommendations 2008) ("The Purple Book"). RSC Publishing. ISBN 978-1-84755-942-5.
  2. Slomkowski, Stanislaw; Alemán, José V.; Gilbert, Robert G.; Hess, Michael; Horie, Kazuyuki; Jones, Richard G.; Kubisa, Przemyslaw; Meisel, Ingrid; Mormann, Werner; Penczek, Stanisław; Stepto, Robert F. T. (2011). "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry. 83 (12): 2229–2259. doi:10.1351/PAC-REC-10-06-03.
  3. Slomkowski, Stanislaw; Alemán, José V.; Gilbert, Robert G.; Hess, Michael; Horie, Kazuyuki; Jones, Richard G.; Kubisa, Przemyslaw; Meisel, Ingrid; Mormann, Werner; Penczek, Stanisław; Stepto, Robert F. T. (2011). "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry. 83 (12): 2229–2259. doi:10.1351/PAC-REC-10-06-03.
  4. Mason TG, Wilking JN, Meleson K, Chang CB, Graves SM, "Nanoemulsions: formation, structure, and physical properties", Journal of Physics: Condensed Matter, 2006, 18(41): R635-R666
  5. Peshkovsky A, Peshkovsky S, "Acoustic Cavitation Theory and Equipment Design Principles for Industrial Applications of High-Intensity Ultrasound", Physics Research and Technology, Nova Science Pub. Inc., October 31, 2010, ISBN 1-61761-093-3
  6. "Translucent Oil-in-Water Nanoemulsions", Industrial Sonomechanics, LLC, 2011
    "Nanoemulsions Used for Parenteral Nutrition", Industrial Sonomechanics, LLC, 2011
    "Drug-Carrier Liposomes and Nanoemulsions", Industrial Sonomechanics, LLC, 2011
  7. Gupta, Ankur; Eral, H. Burak; Hatton, T. Alan; Doyle, Patrick S. (2016). "Nanoemulsions: formation, properties and applications". Soft Matter. 12 (11): http://pubs.rsc.org/-/content/articlehtml/2016/sm/c5sm02958a. doi:10.1039/C5SM02958A. hdl:1721.1/107439. PMID 26924445.
  8. Jafari, Seid Mahdi; McClements, D. Julian (2018). Nanoemulsions: Formulation, Applications, and Characterization 1st Edition. Academic Press. p. 10. ISBN 978-0128118382.
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