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Maybe this question is better classified as "physics" than chemistry, but I'm trying it here. Suppose I'm concerned about a substance S that is highly soluble in water and less soluble in oil. I have a batch of oil that is nonetheless contaminated with traces of S, and I want to remove as much S from it as feasible.

I suspect that at an interface between this oil and water, S would diffuse from oil into water, since it's more soluble in water than in oil. I also suspect that diffusion of S across the oil-water interface would be slow, and I'd like to speed up the process if possible. In principle, I can think of three ways of doing that:

  • Heating a vessel of oil and water to speed up diffusion
  • Increasing the surface area of the oil-water interface by spreading the oil thinly over a large area of water
  • Making an emulsion, either by mechanical agitation or with the addition of some emulsifying agent, and then breaking the emulsion later, hoping that S breaks out preferentially into the water fraction.

The last option seems most promising to me, but I don't know if it really works the way I suspect it does. Any pointers on how I might "wash" this oil? I am relatively unconcerned about traces of water remaining in the oil.

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    $\begingroup$ Option 3 is routinely done by agitating during liquid-liquid extration. Using emulsifiers would be wrong move, as you can get into troubles to separate oil and liquid later and oil would get contaminated by the emusifier. Getting fine emulsion is not really necessery. Extraction may or may not to be done repeatedly. $\endgroup$
    – Poutnik
    Aug 10 '21 at 9:13
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    $\begingroup$ This is performed on a daily basis in organic synthesis laboratories - what you're describing is liquid-liquid extraction, which is commonly performed inside a separatory funnel. See, for example, this video as one out of many available on YouTube. $\endgroup$ Aug 10 '21 at 9:13
  • $\begingroup$ Goes to show that I don't know jack about the practice of chemistry -- thanks both for pointing me in the direction of liquid-liquid extraction! I'm sure I'll be able to find more info now that I know the right term. $\endgroup$
    – James
    Aug 10 '21 at 10:50
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    $\begingroup$ The comments above are somewhat correct but we don't treat oils like that. Rather your oil should be in solution. After extraction and separation the organic phase is dried and the solvent removed. Otherwise you run in the problem of your point 3, and assuming you can break the emulsion or wait enough, you might end up with less S but surely more water instead. It will very much depend on what the oil is, what would be its use, and its amount. $\endgroup$
    – Alchimista
    Aug 10 '21 at 15:18

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