Historical PerspectiveA critical review of synthesis procedures, applications and future potential of nanoemulsions
Graphical abstract
Introduction
Emulsion is a mixture of two or more liquids that are immiscible, where the dispersed phase (droplets) is dispersed throughout the continuous phase, having interfacial boundary and are colloidal systems [1]. Nanoemulsions are transparent/ translucent oil in water emulsions with droplet diameter of 20 to 200 nm [2]. They are heterogeneous dispersions of two immiscible phases of oil and water, thermodynamically stable and can be stabilized by interfacial layer of surfactants and cosurfactants. The main reason behind the stability of the droplet is the Brownian motion by the droplets of oil/water [3]. Nanoemulsion is a translucent system compared to ordinary emulsion or sometimes microemulsion [4]. The isotropic nanoemulsion system is a very fragile one. Slightest change in conditions can cause destabilization, the reasons are nature of oil phase and addition of polymers to thicken the nanoemulsion gel. Large surface area for exchange accounts for lack of stability thermodynamically [5]. Disadvantages include high manufacturing cost, Ostwald ripening and availability of the surfactants and co-surfactants. Nanoemulsions are unstable due to Ostwald's ripening a process where growth of a lager droplet occurs when smaller droplets disintegrate and deposit on the larger ones [4].
This problem was solved by introducing self nano emulsifying drug delivery systems composed of surfactants and co-surfactants [6]. They form nanoemulsions on mild agitation followed by injections into the aqueous media.
Section snippets
Single emulsions
These are water in oil or oil in water emulsions. These are prepared by a single step process. Here single surfactant is used along with the immiscible liquid which exists as buffer as a surface stabilizing layer in between two layers. In water in oil emulsion water droplets are dispersed in oil, whereas in the other one, oil droplets are dispersed in water solution [7].
Double emulsions
These are water/oil/water or oil/water/oil emulsions. They are prepared as a two-step process and two different surfactants
Methods of preparation
The main components involved in the preparation of nanoemulsions are
- a.
Oil (for solubilization of the lipophilic molecules/ drug)
- b.
Surfactant
- c.
Water
- d.
Co surfactant (enhance the action of surfactant)
Many methods have been implemented to prepare nanosized droplets of 100–600 nm which is the suitable range for majority of applications. This requires a lot of energy. The procedure should not only produce nano-size droplets but it should also maintain the stability of the droplets. The widely used methods
Applications
Nanoemulsions have proved to be a better alternative due to there various properties which resulted in their increases usage in different applications. Table 1 represents the nanoemulsion application based on its properties. Fig. 6 represents various aspects of synthesis and possible applications of nanoemulsions. Various patents on different aspects of nanoemulsion including applications, is presented in Table 2 [33].
Future potential and challenges
A number of advantages have been attributed to nanoemulsions including efficient and regulated rate of drug release, uncompromised and prolonged efficacy, tunable drug uptake, lower side (adverse) effects and protection of drugs from enzymatic or oxidative degradation. Inspite of such promising potential, they do suffer from some major limitations. Formulation of nanoemulsion is a costly process since it involves expensive instrumentation and techniques for droplet size reduction. The
Conclusions
As of today, nanoemulsion systems have distinct and more advantages over the disadvantages. Problems in storage due Ostwald ripening can also be addressed by optimizing the synthesis media and parameters to a great extent. Though fragile systems, there have been a wide range of applications ranging from disinfectants to drug delivery systems to providing food-grade system as well as cosmetics and, have proved to be more efficient than the traditional systems due to its entropy and increased
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
The authors express gratitude to the Vellore Institute of Technology for all the support, assistance, and constant encouragement to pursue this work.
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