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The other day I made a salad dressing based on oil and vinegar. To my understanding, there is a positive energy associated with the surface between the oil and vinegar. The most stable state is also the state of minimum energy, hence the mixture tends to separate into two layers, minimizing the total surface energy of the mixture.

Now, adding mustard had the effect of stabilizing the emulsion. The salad dressing didn't separate nearly so much or so quickly. This makes me wonder:

In general, what causes an emulsion to be stable?

For stable emulsions, why don't the two liquids completely mix together?

Are there emulsions that are unstable energetically, but tend to separate so slowly that they appear stable? If so, what causes them, and how do these differ from stable emulsions?

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  • $\begingroup$ In a stable emulsion the total energy, i.e. mixing energy plus surface energy would have to have minimum at a finite droplet size. In practice this may not be true for most emulsions and they may only be meta-stabile and exist for a more or less useful time before they separate. The food and paint industries had to spend considerable effort to find emulsifiers that guarantee long term stability. While I do not know the exact physics behind these I would assume that the emulsifier concentration has to be just right to have the emulsifier cover the required surface area of the droplets. $\endgroup$
    – CuriousOne
    Commented Dec 26, 2015 at 21:45
  • $\begingroup$ You also want to look at the Zeta potentia, e.g. here: www3.nd.edu/~rroeder/ame60647/slides/zeta.pdf $\endgroup$
    – Gert
    Commented Dec 27, 2015 at 0:26

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With the exception of some microemulsions an emulsion is always thermodynamically unstable. That is because the interfacial tension is always greater than zero so it always costs energy to increase the interfacial area.

Emulsions exist only because they are kinetically stable. If you calculate the energy as two emulsion droplets approach and merge it will look something like this:

Kinetic stability

At intermediate distances between the drops the energy goes up, and this creates a repulsive force that keeps the droplets apart.

The barrier can have many causes. For example emulsions in water are often charged - typically the droplets carry a negative charge. The droplets then repel each other due to this electrostatic repulsion. Another common phenomenon is steric stabilisation where the droplets have some large floppy polymer adsorbed at the surface. Your system is one of the rarer cases because it is a Pickering emulsion. The mustard particles adsorb at the oil water interface and they create the barrier that stops the droplets from merging.

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