Nano-emulsions

Abstract

The formation, properties and applications of nano-emulsions (also referred to as miniemulsions, ultrafine emulsions, submicron emulsions) are reviewed and summarized. Nano-emulsion droplet sizes fall typically in the range of 20–200 nm and show narrow size distributions. Although most of the publications on either oil-in-water (O/W) or water-in-oil (W/O) nano-emulsions report their formation by dispersion or high-energy emulsification methods, an increased interest is observed in the study of nano-emulsion formation by condensation or low-energy emulsification methods (based on the phase transitions that take place during the emulsification process). Phase behaviour studies have shown that the size of the droplets is governed by the surfactant phase structure (bicontinuous microemulsion or lamellar) at the inversion point induced by either temperature or composition. Studies on nano-emulsion formation by the phase inversion temperature (PIT) method have shown a relation between minimum droplet size and complete solubilization of the oil in a microemulsion bicontinuous phase independently of whether the initial phase equilibria is single or multiphase. Due to their small droplet size nano-emulsions possess stability against sedimentation or creaming with Ostwald ripening forming the main mechanism of nano-emulsion breakdown. The main application of nano-emulsions is the preparation of nanoparticles using a polymerizable monomer as the disperse phase (the so-called miniemulsion polymerization method) where nano-emulsion droplets act as nanoreactors. Novel complex polymeric materials as well as hybrid organic/inorganic materials, such as magnetic polymeric nanospheres, are among the new applications developed. Another interesting application which is experiencing an active development is the use of nano-emulsions as formulations, namely, for controlled drug delivery and targeting.

Introduction

Emulsions with droplet size in the nanometric scale (typically in the range 20–200 nm) are often referred to in the literature as miniemulsions [1], nano-emulsions [2], [3], ultrafine emulsions [2], submicron emulsions [4], etc. The term nano-emulsion [5•], [6•] is preferred because in addition to give an idea of the nanoscale size range of the droplets it is concise and it avoids misinterpretation with the term microemulsion (which are thermodynamically stable systems). Due to their characteristic size, nano-emulsions appear transparent or translucent to the naked eye (Fig. 1) and possess stability against sedimentation or creaming. These properties make nano-emulsions of interest for fundamental studies and for practical applications (e.g. chemical, pharmaceutical, cosmetic, etc. fields). Oil-in-water (O/W) type nano-emulsions have been investigated since long ago, and have been reviewed thoroughly [1], [4], [5•], [6•], [7], [8••], [9•], specially as nanoreactors for polymerization [7], [8••], [9•]. In contrast, water-in-oil (W/O) nano-emulsions have been described for the first time recently [10••], [11••]. Both types of nano-emulsions are experiencing a very active development as reflected by the numerous publications and patents. In this review, the attention is mainly focused to nano-emulsion formation, with special emphasis on low-energy emulsification methods. Some recent contributions on nano-emulsion properties and applications are also discussed.