To understand the emulsifying process,we must first accept the scientific principle that oil and water do not naturally mix. Quite literally,they find each others presence repulsive.
A good illustration of the aversion is homemade oil-and-vinegar salad dressing.
When you shake or beat your salad dressing. you do more then disperse the oil into
droplets minute enough to remain temporarily suspended in the vinegar (which from now on we will call water, because that tart condiment is, in effect, mainly water ).
The second you stop to combine into units too large to be suspended in the water and thus slither their way upward, separating from the water in the process. The oil rises to the top and the water sinks because oil has a lower specific densitythan water.
If you want a stable emulsion, you need an emulsifying agent, which prevents the oil droplets from combinning into larger units.
Emulsifying agents occur naturally in many
animal substances, including egg yolks and milk.
An emulsifying agent helps to keep the oil particles from combining in three basic ways. First, the agent coats the oil, serving as a physical barrier between the droplets. Second it reduces
the water's surface tension, which, in turn, reduces the water's ability to repulse oil. Third, the agent gives the surfaces of the oil droplets identical electrical charges; since like charges repel each other, the droplets repel each other.