The Essential Properties of Nanoemulsions: Basics of Nanoemulsion

Introduction

Nanoemulsions (NE) are heterogeneous submicron-sized mixtures of lipid and aqueous phases stabilised by emulsifying agents which are also recognised as ultrafine emulsions (Khaleel et al., 2020; Sharma et al., 2010; Gurpret et al., 2018; Patel et al., 2012; Gupta et al., 2016; Eral et al., 2014; Helgeson et al., 2012; Wu et al., 2013; An et al., 2013; Chen et al., 2011; Khaled et al., 2016; Trujilloet al., 2016; Pathaket al., 2017; Zhao et al., 2021; Aswathanarayan et al., 2019; Zhang et al., 2018; Hamed et al., 2015; Ganta et al., 2014; Bainun et al., 2015; Jaiswal et al., 2015; Pandey et al., 2018; Zhang et al., 2013; Zhang et al., 2011; Aboofazeli et al., 2010; Solans et al., 2005; Burapapadh et al., 2010). Nanoemulsions are formulated when two immiscible liquids have been emulsified with emulsifiers, leading to the formation of colloidal dispersive systems which are thermodynamically stable (Helgesonet al., 2016; Jaiswalet al., 2015; Sarker et al., 2005) and are fine oil-in-water dispersions with droplets ranging in diameter from 100 to 600 nm (Sharma et al., 2010; Bouchemalet al., 2004; Pagar et al., 2019; Sutradharet al., 2013). Nanoemulsions are dispersed particle collections used in pharmaceuticals, biomedical aids, and vehicles with promising contribution in cosmetics, diagnostics, drug therapies, and biotechnologies (Gurpret et al., 2018; Pagar et al., 2019; Sutradharet al., 2013; Sivakumar et al., 2014; Sharmaet al., 2012; Sonnevilleet al., 2018; Khannaet al., 2018) and are furthermore renowned as multiphase colloidal dispersion, which is perceived by its stability and clarity (Shethet al., 2020 ; Calderóet al., 2016; Demisliet al., 2020; Renet al., 2018; Pavoni et al., 2020; Rehman et al., 2017; Namet al., 2012; Streck et al., 2016; Lococoet al., 2012; Cheng et al., 2020). Gupta et al. (2016) revealed that nanoemulsions have very high kinetic stability (Sarheedet al., 2020; Liuet al., 2019; Zhang et al., 2014; Sadeghpouret al., 2015; Barreset al., 2017; Hashtjin et al., 2015; Yu et al., 2012). As a consequence, nanoemulsions can be kinetically stable for longer periods of time. The interfacial tension between oil and water appears to be exceptionally low in nanoemulsions, and they appear to be transparent due to droplet sizes that are less than 25% of the wavelength of visible light (Gupta et al., 2016; Gabaet al., 2019; Namrathaet al., 2021). It forms quickly and, in some cases, spontaneously, with little or no high-energy input. The research work focused on the formulation of nanoemulsions utilizing various approaches, which are broadly categorised into two essential classes: high-energy techniques and low-energy techniques (Gupta et al., 2016; Kumaret al., 2019; Kottaet al., 2013; McClementset al., 2013]. Low energy approaches to nanoemulsion development, on the other hand, entail Phase Inversion Temperature (PIT) and Emulsion Inversion Point (EIP) (Liuet al., 2019; Kumaret al., 2019; Salemet al., 2019; Choradiyaet al., 2021; Mishraet al., 2018). High-energy approaches like High-Pressure Homogenization (HPH) and ultrasonication requisites ginormous energy to yield small droplets (Khaleel et al., 2020; Calligariset al., 2016). In addition to the surfactant, the oil phase, and the water phase, a co-surfactant or co-solvent is prevalently used (Kumaret al., 2019). Emulsion droplet coalescence or coagulation is effectively repressed due to the smaller droplet size of emulsions. Additionally, it also facilitates the suppression of emulsion precipitation and also helps to deliver the active agents (Patel et al., 2012). Eral et al. (2014), Gupta et al. (2016) and Li et al. (2021) had similar thoughts on illustrating that many of the problems encountered in prevailing pharmaceutical crystallisation practises could be eliminated by using nanoemulsions. Droplet size range and stability are the predominant significant variations among conventional emulsions (or macroemulsions), nanoemulsions, and microemulsions (Gupta et al., 2016; Mcclementset al., 2012). A research conducted by Helgeson et al., (2012) reported the utilization of nanoemulsions to be building blocks for the conglomeratisation of complex substances, including compartmentalised nanoparticles as well as oil droplets capsidated in a protein shell which was in accordance with (Wu et al., 2013) and (An et al., 2013). Macroemulsions, as contrasted to nanoemulsions, are thermodynamically stable, equilibrium systems that are temperature and composition sensitive (Guptaet al., 2016). Since nanoemulsions are considered to be less vulnerable to physicochemical changes than conventional emulsions, they are a desirable tool for the aforementioned applications (Gupta et al., 2016; Azeemet al., 2009). Owing to the convergence of polymer and perhaps even electrostatic layers encircling the small nanoemulsion droplets, gel-like attributes and possibly high viscosity can indeed be incorporated at abated droplet concentrations (Chen et al., 2011; Alliodet al., 2019; Fathordoobadyet al., 2021). Nanoemulsions could be engineered to improve the biological accessibility of bioactive molecules that are either entombed within them or maybe even consumed alongside them (Khaled et al., 2016; Trujilloet al., 2016). This chapter presents an analysis of the chemical and physical properties of nanoemulsions, with an emphasis on rheological attributes and its significance. Furthermore, the correlation among both the structure and function of nanoemulsions is succinctly summarized.