The stability of oil-in-water emulsions is significantly affected by the pH of the system. The pH influences the formation, stability, and properties of emulsions by affecting the behavior of emulsifying agents, which are substances that help to stabilize the emulsion by reducing the interfacial tension between the oil and water phases. Emulsifying agents can be classified into three main categories: surfactants, proteins, and polysaccharides. The role of pH in the formation and stability of emulsions using these different types of emulsifying agents is discussed below.1. Surfactants: Surfactants are amphiphilic molecules that consist of a hydrophilic water-loving head and a hydrophobic water-hating tail. They can be classified into four types based on the charge of their hydrophilic head: anionic, cationic, nonionic, and zwitterionic. The pH of the system can affect the charge of the surfactant head, which in turn influences the stability of the emulsion.- Anionic surfactants, such as sodium dodecyl sulfate SDS , have a negatively charged head. At low pH values, the negative charge on the head is neutralized, leading to a decrease in the repulsive forces between the surfactant molecules and a reduction in the emulsion stability.- Cationic surfactants, such as cetyltrimethylammonium bromide CTAB , have a positively charged head. At high pH values, the positive charge on the head is neutralized, resulting in a decrease in the emulsion stability.- Nonionic surfactants, such as polyoxyethylene sorbitan monooleate Tween 80 , do not have a charged head and are less sensitive to pH changes. However, extreme pH values can still affect their stability due to changes in the solubility of the surfactant.- Zwitterionic surfactants, such as lecithin, have both positive and negative charges on their head. Their stability is affected by pH changes, as the balance between the positive and negative charges can be disrupted, leading to changes in the emulsion stability.2. Proteins: Proteins are natural emulsifying agents that can stabilize emulsions through their amphiphilic nature and ability to form a protective layer around the oil droplets. The pH of the system can affect the protein's charge, solubility, and conformation, which in turn influence the emulsion stability.- At the isoelectric point pI of a protein, its net charge is zero, and the protein is least soluble. This can lead to protein aggregation and a decrease in the emulsion stability.- Away from the pI, the protein carries a net charge, which increases its solubility and the repulsive forces between the oil droplets, leading to a more stable emulsion.- Extreme pH values can cause protein denaturation, which may either improve or reduce the emulsion stability, depending on the specific protein and its conformational changes.3. Polysaccharides: Polysaccharides, such as gum arabic and xanthan gum, are natural emulsifying agents that stabilize emulsions through their ability to form a thick, viscous layer around the oil droplets. The pH of the system can affect the polysaccharide's charge, solubility, and conformation, which in turn influence the emulsion stability.- Polysaccharides with ionizable groups, such as carboxyl or amino groups, can be affected by pH changes. At low pH values, the ionizable groups are protonated, leading to a decrease in the repulsive forces between the oil droplets and a reduction in the emulsion stability.- Non-ionizable polysaccharides are less sensitive to pH changes, but extreme pH values can still affect their solubility and conformation, leading to changes in the emulsion stability.In conclusion, the pH of the system plays a crucial role in the formation and stability of oil-in-water emulsions by affecting the behavior of different types of emulsifying agents. The optimal pH for emulsion stability depends on the specific emulsifying agent used and its sensitivity to pH changes. To achieve the desired emulsion stability, it is essential to carefully control the pH of the system and select the appropriate emulsifying agent based on its properties and sensitivity to pH changes.