It takes an emulsifier (in the case of mayo, often the egg yolk) to unite the hydrophilic (oil-loving) component and the lipophilic (water-loving) component for emulsification to occur.
A stable emulsion is created when the emulsifier holds the vinegar or lemon juice and oil together and prevents them from separating. The key emulsifiers in homemade Mayonnaise are lecithin from egg yolks and a...