# Critical Review of Techniques for Food Emulsion Characterization
## 2 Emulsion Formation and Stability
If one liquid has better wetting properties with solid particles compared to the other, it will become the continuous phase, while the other becomes dispersed. O/W emulsions are created when the phase contact angle \(\theta\) is less than 90\({}^{\circ}\) (e.g., emulsions stabilized with silica or clay particles), whereas W/O emulsions form when \(\theta>90^{\circ}\) (e.g., emulsions stabilized with carbon black). However, for a particle to effectively act as a stabilizer in either case, \(\theta\) needs to be relatively close to 90\({}^{\circ}\). It is worth noting that Pickering particles attach to the interface irreversibly and thus provide more efficient emulsion stabilization than conventional surfactants that adsorb to the water-oil interface [55]. The stability of emulsions is also significantly affected by emulsifier concentration [56]. While higher emulsifier concentrations generally result in more stable emulsions [57], there are instances where this may not be the case due to different inter-droplet interactions in various emulsion systems. Additionally, there is an emulsifier concentration limit, beyond which the stability of the emulsion may decrease. This is often referred to as \"over-emulsification\". Over-emulsification is a phenomenon that occurs when an excessive amount of emulsifier is added to an emulsion, leading to its destabilization. The mechanism of over-emulsification involves an imbalance in the concentration of emulsifier at the interface [58]. When the emulsifier concentration surpasses a certain threshold, there is an excess of emulsifier molecules at the interface, resulting in repulsion between these molecules. This repulsion disrupts the formation of a stable interfacial film, leading to a reduction in the emulsifier's ability to lower interfacial tension. The rate of emulsion destabilization is also influenced by factors such as pH, ionic strength, temperature, and the viscosity of the continuous phase [43; 59]. These variables significantly impact electrostatic interactions and the repulsive barrier between droplets [59]. They have a particularly profound effect on emulsions that are stabilized with ionic emulsifiers. In the context of emulsion stability, pH and ionic strength play a critical role in influencing droplet surface charge density, molecular flexibility, and electrostatic and steric interactions [58]. Emulsion droplets acquire their electrical charge primarily through the adsorption of ionized species, such as ionic emulsifiers, polyelectrolytes, or ions, from the surrounding solution onto their surface. The sign and magnitude of droplet surface charge are influenced by the kind and concentration of molecules adsorbed to the surface. The ionic strength of the solution influences the extent and strength of intra- and intermolecular electrostatic interactions, impacting the interfacial layer structure and thickness, and repulsion between the droplets [2]. An increase in the ionic strength of the continuous phase reduces the electrostatic repulsion between droplets due to the accumulation of counter-ions around their surfaces, consequently increasing the likelihood of the droplets flocculating or coalescing. The magnitude of this phenomenon is influenced by the concentration and valency of the counter-ions present in the solution. Multivalent counter-ions, such as Ca\({}^{2+}\), Fe\({}^{2+}\), and Fe\({}^{3+}\), exert a more significant effect on the electrostatic forces between droplets than monovalent ions, like Na\({}^{+}\), Cl\({}^{-}\), and K\({}^{+}\). To modify droplet surface charge, polyelectrolytes containing weakly acidic or basic ionizable groups can also be added to the emulsions. Negatively charged groups, such as sulfate, phosphate, or carbonate groups with pKa values around 1-2 and 4-5, respectively, can be provided by incorporating anionic polyelectrolytes (e.g., pectin, carrageenan, alginate) in the formulations, whereas positively charged groups, such as amino or imino groups (with pKa values around 7-11), by adding cationic polyelectrolytes.