A vinaigrette separates because oil and vinegar are chemically dissimilar liquids and a basic dressing is only a temporary emulsion. Shaking breaks one phase into tiny droplets dispersed through the other, but without enough stabilizing material those droplets gradually collide, merge and form separate layers again. Separation is therefore normal, not evidence that the dressing has failed. The practical question is how long you want the mixture to stay combined and what texture you want on the salad.
Oil and water-based vinegar do not naturally stay mixed
Vinegar is mostly water with dissolved acetic acid and flavor compounds, while oil is dominated by nonpolar fat molecules. Because the two phases interact poorly, mixing them requires mechanical energy. Whisking, shaking or blending divides one liquid into many small droplets suspended in the other. The smaller the droplets, the slower they can rise, sink and join together again, so a blender can make a vinaigrette that looks creamy for longer than a gentle stir. But droplet size alone does not make the system permanently stable.
An emulsifier slows the droplets from finding one another
Ingredients such as mustard, egg yolk and some gums contain molecules or particles that can sit at the boundary between oil and water. They help coat droplets and make coalescence more difficult. Mustard also adds fine solids and viscosity, both of which slow movement through the dressing. Garlic paste, miso and certain purees can have a similar practical effect even though their chemistry differs. A classic oil-and-vinegar dressing without a strong emulsifier is expected to separate; just shake it again before serving.
The way you add the oil changes the initial emulsion
For a finer, longer-lasting emulsion, whisk the vinegar, salt, mustard and other water-based ingredients first, then add oil gradually while whisking continuously. A thin stream gives the whisk time to break each addition into small droplets. Pouring all the oil in at once creates larger pools that are harder to disperse. A jar method works too: leave enough empty space so vigorous shaking can move the liquids. An immersion blender can create an especially fine dispersion when the batch is large enough to cover the blade.
Temperature and ingredient ratios affect texture, not just stability
Cold oil becomes more viscous, which can slow separation but also make a dressing feel heavy. Olive oil may partially solidify in the refrigerator, so a chilled vinaigrette can look cloudy or lumpy without being spoiled. Let it warm briefly and shake again. Too much watery ingredient makes the dressing thin, while very high oil content can be difficult to emulsify without more mustard or another stabilizer. Salt should dissolve in the vinegar phase before the oil goes in because it dissolves poorly in pure fat.
Storage changes the appearance without changing the basic principle. In the refrigerator, a vinaigrette can separate more slowly because viscosity rises, yet olive oil may also turn cloudy or partially solid. That is a physical temperature effect. Bring the bottle toward room temperature, then shake until the phases redisperse. A dressing containing fresh garlic, herbs, dairy or egg may have a shorter safe refrigerator life than a simple oil-and-vinegar mixture, so emulsion stability and food safety should be treated as separate questions.
For service, dress delicate leaves at the last moment. Even a perfectly stable emulsion eventually softens lettuce because salt and acid draw moisture from plant tissue. A vinaigrette that stays creamy for hours is useful for storage, but it does not make a dressed salad equally stable.
The core idea is straightforward: vinaigrette separates because oil and vinegar prefer separate phases; whisking creates a temporary dispersion and emulsifiers make that dispersion last longer. If a simple homemade dressing separates in the bottle, nothing is wrong—shake it, taste it and dress the salad immediately.
Sources
- UCLA Science and Food — Vinaigrette — Explains polarity, phase separation and the role of emulsifiers in vinaigrette.
- Professional Cooking — Temporary Emulsions — Culinary reference on temporary oil-and-vinegar emulsions and droplet size.