Nanoemulsion formation by low-energy methods: a review

Abstract

The interest in nanoemulsions has experienced a continuous increase in the last years as evidenced by the numerous publications and comprehensive reviews on the subject. This enormous interest is triggered by the wide range of applications, namely in the pharmaceutical, cosmetic, food, chemical industries. Nanoemulsions (submicrometer-size droplets) have advantages over conventional emulsions (micrometer-size droplets) due to their small droplet size; it stipulates their stability against sedimentation or creaming and a transparent or translucent optical aspect (similar to that of microemul-sions). Nanoemulsions are commonly prepared by high-energy methods using mechanical devices, which can produce intense disruptive forces, for example, high pressure homogenizers and ultra-sound generators. Nanoemulsion formation by these methods is quite straightforward as the higher the energy input is, the smaller is the droplet size. However, the level of energy required to obtain nanometer-scaled droplets is very high, and therefore, cost-inefficient, especially considering that only a small amount of the energy produced is used for emulsification. In contrast, low-energy emul-sification methods using the internal chemical energy of the system are often more energy efficient as only simple stirring is needed, and generally allow producing a smaller droplet size than high-energy methods. It has been also claimed that high-energy methods allow preparing nanoemulsions at higher oil-to-surfactant ratios than low-energy methods. The results obtained confirm that both PIT and PIC have the same mechanisms. However, there are still issues to be solved. One of them concerns the possibility to obtain nanoemulsions with the minimum droplet size and low polydispersity by the PIC method. It is likely that the kinetics of the emulsification process plays an important role in this emul-sification method, which has not been taken sufficiently into account. Therefore, more research effort needs to be done on this subject. A more comprehensive knowledge on the mechanisms involved in nanoemulsion formation by low-energy methods will allow their optimization and consequently will extend the fields of their application. The interest in nanoemulsions has experienced a continuous increase in the last years as evidenced by the numerous publications and comprehensive reviews [12, 14, 19, 22, 31] on the subject. This enormous interest is triggered by the wide range of applications, namely in the pharmaceutical [2, 3, 6, 9, 10, 13, 19, 22, 28, 36, 37], cosmetic [1, 7, 34, 40], food [15, 26, 27, 29], chemical [5, 17, 23, 25], etc., industries. Nanoemulsions (sub-micrometer-size droplets) have advantages over conventional emulsions (micrometer-size droplets) due to their small droplet size; it stipulates their stability against sedimentation or creaming and a transparent or translucent optical aspect (similar to that of microemulsions). However, nanoemulsions, in contrast to microemul-sions, which are thermodynamically stable, are non-equilibrium systems, which may undergo flocculation, coalescence and/or Ostwald ripening. Nevertheless, with an appropriate selection of the system components, composition and preparation method, nanoemulsions with a high kinetic stability can be obtained. It is generally accepted [21, 30, 33] that the nanoemulsion main breakdown process is Ostwald ripening (diffusion of molecules of the disperse phase from small to big droplets). However , recent reports have shown flocculation to be a possible breakdown mechanism for nanoemulsions formulated with mixed nonionic-ionic surfactants [38, 39]. Nanoemulsions are commonly prepared by high-energy methods using mechanical devices, which can produce intense disruptive forces, for example, high-shear stirrers, high pressure homogenizers and ultra-sound generators. Nanoemulsion formation by these methods is quite straightforward as the higher the energy input is, the smaller is the droplet size. However, the level of energy required to obtain nanometer-scaled drop-lets is very high, and therefore, cost-inefficient, especially considering that only a small amount (about 0.1%) of the energy produced is used for emulsification [32]. In contrast, low-energy emulsification methods using the internal chemical energy of the system are often more energy efficient as only simple stirring is needed, and generally allow producing a smaller droplet size than high-energy methods [36]. Nevertheless, depending on the system and composition variables, similar droplet sizes can be obtained by both types of methods [41]. It has been also claimed that high-energy methods allow preparing nanoemulsions at higher oil-to-surfactant ratios than low-energy methods [41]. However, nanoe-mulsions with high oil-to-surfactant ratios prepared by low-energy methods have also been reported [9]. Low-energy emulsification methods Low-energy approaches rely on the spontaneous formation of tiny oil droplets within oil-water-emulsifier