Solvent extraction in a separation funnel is a very common method in preparative organic chemistry. But sometimes you don't get a nice phase separation between the organic and the water phase.

What are the possible causes that can prevent the formation of two distinct phases, and how can you force a phase separation when you encounter such situations?

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    $\begingroup$ sometimes I try just waiting, acidifying the solution, heat, an ultrasonic bath, centrifuge, filtering through sodium sulfate... even gasp adding salt. $\endgroup$ – Chris Apr 26 '12 at 8:35
  • $\begingroup$ For "possible causes", I should think that having a surfactant present would up the chances of forming a stable emulsion, preventing separation. I don't imagine that anyone would be knowingly trying to purify a surfactant by solvent-solvent extraction, but it's something worth bearing in mind if some surface-active-agent were produced as a by-product / was an unreacted reagent / was present from an earlier reaction step and had not been fully removed. $\endgroup$ – owjburnham Jan 25 '17 at 12:06

One such cause, where separation does not take place is the nature of the organic solvent. Its better to check the miscibility of the solvent with water. There are several websites, where one can get this info. For example 1 and 2.

Even with hydrophobic solvent, if the problem persists, one can add NaCl and then check the separation.

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    $\begingroup$ Adding salt to the mixture augments the ionic strength of water, therefore increasing the "difference" between water and your other solvent. I've found that this usually works. $\endgroup$ – CHM Apr 26 '12 at 21:15
  • $\begingroup$ Yes, assuming your solvent isn't miscible in water, adding brine (concentrated solution of NaCl in DI water) usually breaks the emulsion. $\endgroup$ – scientifics Jun 21 '12 at 20:34

Well, shaking a sep-funnel is a great way of creating a horrible intractable emulsion with no clear phase boundary, so don't do that (also, the stopper might pop off as soon as you set it down).

As for breaking an emulsion, if you have the time, just leave the sep-funnel somewhere and wait a few days. I seem to recall the first and last time I shook a sep-funnel the lab tech added some salt to the water/diethyl ether emulsion however I'm not clear on this.

Finally, consider sacrificing part of your product by drawing the funnel off to the very base of where the emulsion begins, draining the emulsion layer entirely and then taking the remainder. If you have blobs of one phase in the other, you may be able to remove them with a pipette.

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    $\begingroup$ I'm not sure why you made the comment about creating the emulsion/popping the stopper off by shaking the sep funnel. In order to get a good extraction, you need to shake the funnel. To prevent the stopper from popping off, you need to use proper technique and vent the funnel from the stopcock periodically as you shake it. $\endgroup$ – scientifics Jun 21 '12 at 20:35
  • $\begingroup$ @scientifics - regarding venting the sep funnel, you are of course correct, and any incidents involving stoppers can be attributed to inadequate venting. However, as far as emulsifying phases are concerned, IMHO protracted swirling is preferable to shaking. $\endgroup$ – Richard Terrett Jun 22 '12 at 3:38

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