Nanoemulsions: Formation, Properties and Applications Cite This: Soft Matter, 2016, 12,2826 Ankur Gupta,A H

Soft Matter View Article Online REVIEW View Journal | View Issue Nanoemulsions: formation, properties and applications Cite this: Soft Matter, 2016, 12,2826 Ankur Gupta,a H. Burak Eral,bc T. Alan Hattona and Patrick S. Doyle*a Nanoemulsions are kinetically stable liquid-in-liquid dispersions with droplet sizes on the order of 100 nm. Their small size leads to useful properties such as high surface area per unit volume, robust stability, optically transparent appearance, and tunable rheology. Nanoemulsions are finding application in diverse areas such as drug delivery, food, cosmetics, pharmaceuticals, and material synthesis. Additionally, they serve as model systems to understand nanoscale colloidal dispersions. High and low Received 7th December 2015, energy methods are used to prepare nanoemulsions, including high pressure homogenization, Accepted 19th February 2016 ultrasonication, phase inversion temperature and emulsion inversion point, as well as recently developed DOI: 10.1039/c5sm02958a approaches such as bubble bursting method. In this review article, we summarize the major methods to prepare nanoemulsions, theories to predict droplet size, physical conditions and chemical additives Creative Commons Attribution-NonCommercial 3.0 Unported Licence. www.rsc.org/softmatter which affect droplet stability, and recent applications. 1 Introduction and water phases of the emulsion. The emulsifier also plays a role in stabilizing nanoemulsions through repulsive electro- Nanoemulsions are emulsions with droplet size on the order of static interactions and steric hindrance.1 The emulsifier used is 100 nm. A typical nanoemulsion contains oil, water and an generally a surfactant, but proteins and lipids have also been emulsifier. The addition of an emulsifier is critical for the effective in the preparation of nanoemulsions.2–12 Over the past creation of small sized droplets as it decreases the interfacial decade or more, the research focus has been on preparing nano- This article is licensed under a tension i.e., the surface energy per unit area, between the oil emulsions through various methods, broadly classified into two primary categories: high-energy and low-energy methods.13–15 High energy methods such as high pressure homogenization a Massachusetts Institute of Technology, Cambridge, MA, USA. (HPH) and ultrasonication15 consume significant energy Open Access Article. Published on 23 February 2016. Downloaded 9/23/2021 5:24:15 PM. E-mail: [email protected]; Fax: +1 617 324 0066; Tel: +1 617 253 4534 B 8 10 À1 b Delft University of Technology, The Netherlands ( 10 –10 Wkg ) to make small droplets. On the other c Utrecht University, The Netherlands hand, low energy methods exploit specific system properties Ankur Gupta is a PhD candidate H. Burak Eral is an Assistant at Massachusetts Institute of Professor in the Process & Technology in the Chemical Engi- Energy Department within the neering department working with 3ME faculty at Delft University Prof. Patrick S. Doyle and Prof. of Technology since October T. Alan Hatton. He is currently 2015. He also holds a guest working on nanoemulsions and faculty appointment in Van’t their applications. His research Hoff laboratories in University of interests are interfacial science, Utrecht. He worked at the multiphase fluid flow and Massachusetts Institute of Tech- microfluidics. nology as a postdoc fellow with Patrick S. Doyle after completing Ankur Gupta H. Burak Eral his PhD in Physics of Complex Fluids group in University of Twente in 2012. His research spans interdisciplinary fields including: soft matter, process intensification, crystallization, hydrodynamics, rheology and wetting physics. 2826 | Soft Matter, 2016, 12, 2826--2841 This journal is © The Royal Society of Chemistry 2016 View Article Online Review Soft Matter to make small droplets without consuming significant energy There is some confusion in the literature regarding a precise (B103 WkgÀ1). Phase inversion temperature (PIT)16 and definition of nanoemulsions which are often confused with emulsion inversion point (EIP)17–19 are two examples of low the thermodynamically stable microemulsions which form energy approaches for the formation of nanoemulsions. spontaneously.22,23 The major differences between classical Recently, a few novel technologies such as bubble bursting emulsions (or macroemulsions), nanoemulsions and micro- at oil/water interface20 and evaporative ripening21 have also emulsions is in droplet size range and stability characteristics, been developed for making nanoemulsions. The second section of as summarized in Fig. 1. Macroemulsions and nanoemulsions this article reviews the various methods to make nanoemulsions. are both thermodynamically unstable, i.e. given sufficient time, We also discuss the ways to control and predict droplet size based phase separation occurs. However, because of the small size of on system properties and process parameters. nanoemulsions (sometimes also referred as ‘miniemulsions’), Nanoemulsions are kinetically stable, i.e., given sufficient nanoemulsions can be kinetically stable over long time scales. time, a nanoemulsion phase separates.22,23 In Section 3, Nanoemulsion metastability has nothing to do with proximity we discuss the underlying physics behind destabilization to an equilibrium state.2 On the other hand, since microemulsions mechanisms such as flocculation, coalescence, Ostwald ripening are thermodynamically stable systems in equilibrium, they and creaming. We show that Ostwald ripening is the dominant are sensitive to changes in temperature and composition. destabilization mechanism for nanoemulsions. We also review Therefore, nanoemulsions are attractive for aforementioned the literature on the effect of parameters like nanoemulsion applications because they are relatively the least sensitive to composition and temperature on destabilization rates of nano- physical and chemical changes. Two recent studies have clarified emulsions.2,16,18,24–29 A short discussion on the trapped species the distinction between nanoemulsions and microemulsions, method for making stable nanoemulsions is also presented. and the interested readers are referred to these reports that The range of nanoemulsion applications spans diverse fields detail the differences between these two classes of liquid-in- including drug delivery,30–62 where O/W nanoemulsions have liquid dispersions.22,23 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. been used to deliver hydrophobic drugs; the food industry, where flavored nanoemulsions with improved curcumin/ b-carotene and digestibility have been prepared;5–7,9–12,63–73 and 2 Formation in cosmetic industry where nanoemulsions have been tested for skin hydration and ease of application.15,74,75 Researchers have An understanding of the physics of nanoemulsion formation also showed that many problems faced in current methods of is critical for the control of nanoemulsion droplet size. pharmaceutical crystallization processes can be avoided with Nanoemulsions are typically prepared in a two-step process nanoemulsions.76,77 Nanoemulsions have also been used as where a macroemulsion is first prepared, and is then building blocks for complex material synthesis such as com- converted to a nanoemulsion in a second step. In this section, This article is licensed under a partmentalized nanoparticles and encapsidated oil droplets.15,78–81 we describe various methods developed over the past decade The fourth section of this paper discusses important properties or so to prepare nanoemulsions, and indicate the progress on nanoemulsions and discusses a wide range of nanoemulsion made on the prediction and control of nanoemulsion applications. droplet size. Open Access Article. Published on 23 February 2016. Downloaded 9/23/2021 5:24:15 PM. T. Alan Hatton is the Ralph Patrick S. Doyle is the Robert Landau Professor of Chemical T. Haslam (1911) Professor of Engineering and Director of the Chemical Engineering at the David H. Koch School of Massachusetts Institute of Chemical Engineering Practice at Technology. He received his PhD the Massachusetts Institute of in Chemical Engineering in 1997 Technology. He received his PhD from Stanford University and in Chemical Engineering in 1981 then spent three years as a post- from University of Wisconsin. His doctoral fellow at the Institute group’s research focuses on the Curie in Paris. His awards synthesis, understanding and include the Soft Matter Lecture- application of stimuli-responsive ship, Guggenheim Fellowship, T. Alan Hatton materials, which include nano- Patrick S. Doyle and Pioneer of Miniaturization particles, nanofibers, surfactants, Lectureship. His research interests polymers and gels, with a current focus on the development of focus on soft matter, polymer electrochemical processes to facilitate chemical separations and to physics and microfluidics. mediate the transformation of captured waste to useful commodity chemicals. This journal is © The Royal Society of Chemistry 2016 Soft Matter, 2016, 12, 2826--2841 | 2827 View Article Online Soft Matter Review Fig. 1 Comparison of macroemulsions, nanoemulsions (also referred to as miniemulsions) and microemulsions with respect to size, shape, stability, Creative Commons Attribution-NonCommercial 3.0 Unported Licence. method of preparation, and polydispersity. Nanoemulsions and microemulsions have a larger surface area per unit volume than do macroemulsions because of their size. In addition, due to a strong kinetic stability, nanoemulsions are less

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