# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Determination of Zeta Potential  
The droplets of most emulsions are electrically charged due to the adsorption of charged or ionizable emulsifier molecules to their surface [28]. The charge of a droplet is determined by the type and concentration of adsorbed molecules, as well as the pH and composition of the continuous phase, particularly the presence of other charged species, such as ions or macromolecules. The most commonly analyzed parameter describing droplet surface charge is the \(\zeta\) potential (zeta potential), defined as the potential difference between the charged droplet surface and the continuous phase. Zeta potential is a quantitative measure that allows the prediction and tracking of emulsion stability [139]. An increase in the zeta potential increases emulsion stability due to higher repulsive electrostatic forces between the droplets, preventing droplet coalescence and flocculation, which ultimately enhances the stability of the emulsion [120]. In general, a high negative or positive zeta potential (\(\pm\)30 mV) indicates a stable emulsion [140], whereas zeta potential values approaching 0 indicate non-stable systems [141]. Emulsion zeta potential can be measured with micro-electrophoretic analyzers, such as Zeta PALS [142] or Malvern Zetasizer [143]. These analyzers estimate zeta potential based on the electrophoretic mobility of emulsion droplets under an electric field. The direction of droplet movement is used to determine the sign of the electrical charge, while the velocity of the movement gives information on the magnitude of the electrical charge at the droplet surface. Emulsion droplet or particle surface charge influences ion distribution, creating an electrical double layer around the particle. Charged solid surfaces attract counter-ions from the solution, forming a surface potential that decreases with increasing distance from the particle surface. In liquid emulsions, the droplets are surrounded by two layers: one with strongly bound ions (inner layer) and another with loosely associated ions (outer diffuse layer), forming the electric double layer [16]. As the emulsion droplet moves, ions within the boundary move with it, while those beyond the boundary remain in the bulk dispersion. The interface between stationary and diffuse layers of counter-ions is called the \"shear plane\", and its potential is the zeta potential (Figure 12). Figure 12: Schematic diagram of an electrical double layer formed around an emulsion droplet. The emulsion droplet (represented by the orange circle) is surrounded by two layers of ions. Strongly bound ions form the inner layer (indicated with a blue ring), while loosely bound ions—the outer diffuse layer (indicated with a red ring). The correlation between the zeta potential and the distance from the droplet surface is schematically depicted in the graph. As mentioned above, the zeta potential value depends on the type and concentration of charges on the droplet surface and ions present in the continuous phase, as well as the pH of the system [144]. The interaction between droplets and surrounding ions plays a crucial role in determining the stability and behavior of emulsions over time [88]. A higher zeta potential value signifies stronger repulsion between droplets, which ultimately enhances the stability of the emulsion [145]. These repulsive forces prevent the coalescence and flocculation of droplets and thereby ensure their uniform dispersion within the emulsion [146]. When the zeta potential of the emulsion approaches zero, it becomes extremely unstable [147]. Stabilization of such systems can be achieved by the addition of charged molecules. For example, it has been demonstrated that unstable emulsions of rice bran protein-stabilized emulsions could be effectively stabilized by incorporating (+)-catechin in the formulation [148].