A bitumen emulsifier is a chemical additive used to stabilize the mixture of bitumen and water in an emulsion. It works by reducing the surface tension between the bitumen droplets and the water, allowing the bitumen to disperse evenly throughout the aqueous phase. Emulsifiers are typically surfactants that can be anionic, cationic, or non-ionic, affecting the charge of the bitumen droplets and influencing how the emulsion interacts with aggregates. The choice of emulsifier impacts the setting time, adhesion properties, and overall stability of the bitumen emulsion.