What can I do if I don't get a phase separation between my organic and water phases?

Question

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?

Answer 1

I know you can add a bunch of salt like NaCl to your aqueous phase to make it more polar. Some solvents are pretty tough to work with, like THF is surprisingly H2O soluble. There are probably a couple other things you can do also, so others could chime in.