# Critical Review of Techniques for Food Emulsion Characterization
## 2 Emulsion Formation and Stability  
Emulsions are colloidal dispersions created by mixing two immiscible phases, an oil phase, and an aqueous phase, in the presence of an emulsifying agent. Emulsions consist of liquid droplets (dispersed phase) dispersed in a continuous phase and can be classified into several categories based on their composition: oil-in-water (O/W), water-in-oil (W/O), and oil-in-oil (O/O). An example of the last category is an emulsion comprising a polar oil, such as propylene glycol, that is dispersed in a nonpolar oil, like paraffinic oil, or _vice versa_[25]. Emulsions can also be categorized based on droplet size, falling into macroemulsions, microemulsions, and nanoemulsions (also known as miniemulsions) [26]. Macroemulsions contain droplets larger than 0.1 \(\upmu\)m, microemulsions have droplets ranging from 10 to 100 nm, while nanoemulsion droplets range in size from 20 to 500 nm. The majority of emulsions fall into the macroemulsion category. Microemulsions are thermodynamically stable due to their low interfacial energy and can spontaneously form, however, they exhibit high sensitivity to changes in temperature and composition [27]. Macroemulsions and nanoemulsions are inherently thermodynamically unstable and exist in a state of non-equilibrium [28]. In these types of emulsions, breaking the dispersed phase into droplets enlarges the interface between the phases, resulting in a significant increase in interfacial area and energy. Hence, forming macro- and nanoemulsions does not occur spontaneously but requires an input of energy [29]. However, due to the very small size of droplets, nanoemulsions can remain kinetically stable over extended periods. It is important to note that this metastability does not result from the equilibrium state, but rather from greatly reduced rates of gravity-related sedimentation-based separation processes [30]. Unstable systems can be stabilized using surface active agents--surfactants (called also emulsifiers). Surfactants can be categorized into: (1) small molecular surfactants, such as sodium lauryl sulfate (SLS), diacetyl tartaric acid ester of mono- and diglycerides (DATEM), citric acid esters of mono and diglycerides (CITREM), Tweens 20 and 80, Spans 20, 40, 60 and 80, and Brij-97 [31]; (2) macromolecular emulsifiers, such as proteins, polysaccharides, phospholipids, and surface-active polymers; and (4) solid particles (known as Pickering particles), such as silica, clay, hydroxyapatite, starch, or chitosan particles [32]. The function of the emulsifier is to decrease the interfacial tension between immiscible phases and the resistance of the droplets to deformation, thereby reducing the shear force needed for breaking up droplets [33]. The resistance of the droplets to deformation is determined by the Laplace pressure, which refers to the difference in pressure inside and outside the droplet. Concurrently, emulsifier molecules adsorb to newly formed droplet surfaces, creating a physical barrier between them, which both stabilizes them and inhibits coalescence [29]. Emulsifiers with low molecular weight adsorb more rapidly to the droplet surface than high molecular weight surfactants and have a tendency to create a single-molecule layer at the interface of oil and water. The adsorption of a single emulsifier molecule at the interface can be described using Gibbs or Langmuir adsorption theorems [34]. In contrast, macromolecular surfactants form thicker layers with more complex morphologies, such as cross-linked multilayers or supramolecular structures [35]. The capacity of emulsifiers to stabilize the droplets arises from their amphiphilic nature [36]. Emulsifier molecules consist of a hydrophilic part (the head) and a hydrophobic segment (the tail). When introduced into an oil-in-water (O/W) emulsion, where oil droplets are dispersed in an aqueous phase, emulsifier molecules spontaneously envelop the oil droplets, with their non-polar tails reaching into the droplet [33].