Oil and water, as water is the main component of vinegar, are molecular opposites when it comes to polarity. They repel each other when combined due to their molecular structure, not due to their consistency. It is impossible to combine the two without the help of a third-party emulsifier.
Molecules
Vinegar is made up of about 95 percent water and 5 percent fermented ethanol, known as acetic acid. Oil is a liquid fat containing molecules known as lipids. Lipids and water molecules repel one another.
Charge
Water molecules contain both a positive and negative charge and are known as polar molecules. Lipids are non-polar as they contain only one charge.
Polarity
Polar and non-polar molecules do not bind together. It takes more energy for a molecule to bond with a molecule of an opposite polarity; therefore, water bonds with other water molecules before bonding with a lipid.
Oil and Water
Oil and water appear to slip past each other when combined due to their molecular polarity. Layers are formed due to the greater density of water; oil is less dense and, therefore, floats to the top.
Emulsifiers
The only way to combine water with oil is to add an emulsifier. Emulsifiers are compounds that create a "cap" around the oil (non-polar molecule) that allows it to bind with the water. For culinary purposes, egg is one such emulsifier.
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