Add oil to water and the two liquids will never mix. At least not until an emulsifier is added. Emulsifiers are molecules with one water-loving (hydrophilic) and one oil-loving (hydrophobic) end. They make it possible for water and oil to become finely dispersed in each other, creating a stable, homogeneous, smooth emulsion.
Source: www.eufic.org/article/en/food-safety-quality/food-additives/artid/emulsifiers-food/