# Critical Review of Techniques for Food Emulsion Characterization
## 2 Emulsion Formation and Stability
Surfactant mixtures, including combinations of ionic/nonionic or nonionic/nonionic emulsifiers, can also be employed to enhance emulsification and improve the stability of the formulation. When selecting the most suitable emulsifier, a key factor to consider is its hydrophilic-lipophilic balance (HLB), which describes the emulsifier affinity for oil and water phases. It is utilized to predict the emulsifier capability to form a specific type of emulsion. HLB is linked to the emulsifier solubility in water and oil, as well as the size and strength  
Figure 1: Emulsion destabilization mechanisms. Orange and blue areas represent the oil and water phases, respectively. Reprinted from [43], published under CC license CC BY-NC 3.0. Copyright © 2023 Royal Society of Chemistry. of its hydrophilic and lipophilic moieties [48]. HBL is calculated based on the weight percentage of the hydrophilic groups compared to the hydrophobic groups in a molecule and spans from 0 to 20, with an HLB of 10 indicating an equal attraction to both phases. Emulsifiers with a higher HLB value are more inclined towards being hydrophilic and typically form oil-in-water emulsions, while those with lower values tend to be more hydrophobic and favor the formation of water-in-oil emulsions. To achieve maximum stability of emulsions, it is essential to align the HLB value with that of the non-polar oil phase being utilized [49]. For instance, emulsifiers with an HLB ranging from 7 to 8 are generally required for emulsifying vegetable oils, whereas surfactants with a higher HLB of around 14 are better suited for creating castor oil emulsions. Nevertheless, the hydrophilic-lipophilic balance concept is not without its constraints. Notably, it may fail to accurately predict the resultant emulsion type formed by specific emulsifiers, owing to variables, such as temperature or emulsifier concentration, during the emulsification process [50]. These factors can lead to the same emulsifier giving rise to both water-in-oil (W/O) and oil-in-water (O/W) emulsions. Furthermore, O/W emulsions can be generated using emulsifiers exhibiting HLB values across the entire spectrum, and theoretical HLB calculations prove inadequate for blends comprising diverse emulsifying agents. In response to these challenges, the hydrophilic-lipophilic difference (HLD) concept has been introduced. HLD offers a more precise characterization of emulsifiers [51] as it takes into account various parameters shaping the emulsion, encompassing surfactant concentration and type, hydrophobicity of the oil phase, salinity, temperature, and the characteristic curvature (\(C_{c}\)) of the emulsifier [52]. The \(C_{c}\) value indicates whether the emulsifier tends to form a curvature around an oil droplet in water (negative \(C_{c}\)) or a water droplet in oil (positive \(C_{c}\)). Typically, hydrophilic emulsifiers exhibit negative \(C_{c}\) values; for example, sodium lauryl sulfate (SLS) possesses a \(C_{c}\) value of \(-2.3\). Conversely, hydrophobic emulsifiers demonstrate positive \(C_{c}\) values; dioctyl sodium sulfosuccinate, for instance, has a \(C_{c}\) value of 2.6. As previously mentioned, emulsions can also be stabilized with solid particles, known as Pickering particles. Pickering particles accumulate at the water-oil interface and create a steric barrier preventing droplet coalescence. These particles include inorganic particles (e.g., silica, clay, hydroxyapatite particles), organic particles (e.g., starch, ovotranserrin, chitosan particles) [53], and Janus particles (particles whose surfaces have two or more distinct physical properties, e.g., hydrophilic and hydrophobic). They provide stability to emulsions by being partially wetted by both the oil and water phases [54]. The formation of oil-in-water (O/W) Pickering emulsion or water-in-oil (W/O) Pickering emulsion depends on the wettability of solid particles at the interface between oil and water.