Why Do Oil and Water Separate Again After Shaking?

Put oil and vinegar in a bottle and shake hard: the liquid turns cloudy for a while, then separates into two layers. Shaking merely breaks the oil into many suspended droplets and increases the area where oil meets water. It does not disperse individual oil molecules through the water as sugar dissolves, so the mixture is not a stable solution.

Water molecules are polar and form a constantly rearranging network of hydrogen bonds with one another. Cooking oil consists mainly of molecules with very little polarity, which cannot provide interactions strong enough to compensate for disrupting water–water contacts. To reduce the oil–water interface, small droplets collide and merge into larger ones until a continuous oil layer forms. Oil usually sits on top because common cooking oils are less dense than water; floating and immiscibility are separate questions.

Mustard, egg yolk or a little detergent can slow the separation. Their surface-active components have one region that associates with water and another that associates with oil, so they can surround droplets and lower interfacial tension, producing a more stable emulsion. This still does not mean that oil molecules have fully dissolved in water. The point is not that oil “fears” water, but that the mixed molecular arrangement is thermodynamically unfavourable.

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