Oil floating above vinegar in vinaigrette is a common sight dictated by fundamental laws of chemistry and physics. The inability of these two liquids to blend into a uniform solution stems from their inherent molecular structures. Understanding this phenomenon requires examining the forces that govern how molecules interact.
The Principle of Polarity
The reason oil and vinegar do not mix lies in molecular polarity, which describes how electrical charge is distributed across a molecule. Vinegar is mostly water, and water molecules are polar, meaning they have an uneven distribution of electric charge with slightly negative and positive ends. This polarity creates a strong attraction between water molecules and other polar substances, a trait described as hydrophilic, or “water-loving.”
Oil is composed of large lipid molecules, which are non-polar. The electric charge is balanced throughout the oil molecule, lacking the distinct positive and negative ends found in water. Non-polar molecules are attracted to other non-polar molecules but repel polar substances like vinegar, making them hydrophobic. The rule in chemistry is “like dissolves like,” meaning polar substances mix only with polar substances, forcing oil and vinegar to separate.
Layering Due to Density
While polarity explains why oil and vinegar do not blend, a difference in density dictates their physical arrangement once separated. Density measures a substance’s mass relative to its volume. The liquid with the lower density will always float on top of the liquid with the higher density.
Vinegar, which is largely water, has a density typically around 1.01 gram per milliliter (g/mL). Most common cooking oils, such as olive or vegetable oil, have a lower density of approximately 0.92 g/mL. Because oil is less dense than the water-based vinegar, the oil layer rises and rests on the surface, creating the distinct floating layer.
Creating a Temporary Mixture
Although oil and vinegar naturally resist mixing, it is possible to create a temporary blend called an emulsion. An emulsion is a mixture where tiny droplets of one liquid are suspended evenly throughout another, giving it a uniform appearance. Vigorously shaking vinaigrette breaks the oil into microscopic droplets, which are temporarily dispersed throughout the vinegar.
This temporary mixture quickly separates once shaking stops, as the oil molecules rapidly recombine. To achieve a stable emulsion, a third ingredient known as an emulsifier must be added. An emulsifier contains both a polar and a non-polar part, allowing it to bridge the oil and vinegar molecules. Common emulsifiers, such as mustard and egg yolk (which contains lecithin), surround the oil droplets, preventing them from reconnecting and stabilizing the mixture.