Why don't Hydrocarbons and Water Molecules mix, and why might an emulsifier fix this?

Hydrocarbons are long chained, non-polar molecules of Carbon and Hydrogen. As the chain length increases the boiling point of oil increases and the intermolecular forces get stronger. At smaller chain lengths molecules are volatile and become a gas easily as these forces are weak. The 'Van der Waals' forces which hold these molecules are strong when molcules are long.

Water is a small molecule, however due to the electrostatic pull of the oxygen molecule on the hydrogen's electrons, it is also very polar. Electrostatic forces between positively charged hydrogen atoms and negatively charged oxygen atoms are much stronger than the van der Waals forces from the oil; therefore water molecules attract each other very strongly, and other polar molecules. As they form only very weak interactions with hydrocarbons, they separate.

Emulsifiers contain a polar and non-polar part of the molecule; therefore one part of the molecule can attract water and one part can attract the hydrocarbon, helping the two types of molecule mix.