Emulsifier could improve the emulsion various components surface tension and make it becomes stable dispersion system or emulsion.

Emulsifier is asurfactant, which compounds that typically have polar or hydrophilic part and a non-polar (lipophilic) part. Because of the feature, emulsifiers tend to have more or less solubility in water or oil. 
If the emulsifier is more soluble in water, it will generally form oil-in-water emulsions while the emulsifiers is more soluble in oil then it will form water-in-oil emulsions.

The hydrophilic-lipophilic balance (HLB) of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic. The HLB value can be used to predict the surfactant properties high value determines it’s water-soluble (lipid-insoluble) and low value turns lipid-soluble (water-insoluble).

Depends on the structure, the emulsifier is separated into anionic, cationic and non-ionic three types. Based on its source it can be divided into natural emulsifier or synthetic emulsifier. Customers could choose proper emulsifier by their request and need.

Water repellents are generally composed of hydrophobic substances but currently it mainly composed of fluorocarbons and due tothe property it have waterproof and oil proof effect.

Usually the waterproofprocess is working under water so the emulsifier needs to add into water repellent at the same time.
Fluorine free water proofing agent is composed by fat and was and it also needs the suitable emulsifier for emulsification

Over all, when the users are use water repellent they also need to consider the right emulsifier as well.