Emulsifiers are substances that enable the formation of stable emulsions from mixtures of two or more immiscible components. Their working principle is that during emulsification, the dispersed phase is dispersed in the continuous phase as microdroplets (micrometer-scale). The emulsifier reduces the interfacial tension between the components in the mixture and forms a relatively strong film on the surface of the microdroplets, or an electric double layer due to the charge provided by the emulsifier, preventing the microdroplets from agglomerating and maintaining a homogeneous emulsion. From a phase perspective, an emulsion is still a heterogeneous system. The dispersed phase in an emulsion can be an aqueous phase or an oil phase, but is mostly an oil phase; the continuous phase can also be an oil phase or an aqueous phase, but is mostly an aqueous phase.
Emulsifiers are surfactants, containing both hydrophilic and lipophilic groups in their molecules. To express the hydrophilicity or lipophilicity of an emulsifier, the "hydrophilic-lipophilic balance value (HLB value)" is commonly used. The lower the HLB value, the stronger the lipophilicity; conversely, the higher the HLB value, the stronger the hydrophilicity. Different emulsifiers have different HLB values, and in order to obtain a stable emulsion, it is necessary to select a suitable emulsifier.
