SWIM used MEK:Heptane to purify and crystallize bufotenine.
Basically with a mix like that (of any solvents really, doesn't need to be MEK:Heptane) where one (MEK) is somewhat polar and he other solvent (hetptane) is highly non-polar, and both are miscible, you can adjust the polarity of your solvent mix by adjusting the ratio of the solvent mix. For example, 2 parts MEK and 1 part heptane would be more polar than 1 part MEK and 1 part heptane. And 1 part MEK and 2 parts heptane would be more polar than 1 part MEK and 1 part heptane.
It's just a nice way of controlling the polarity of your solvent. Some alkaloids are very polar and only soluble in MEK and not heptane. So by adding a little heptane to the MEK, eventually if you add enough, those alkaloids will crash out of the solvent mix. This is useful when trying to separate highly polar alkaloids from highly non-polar ones.
Any solvents that are miscible will work such as alcohol and heptane, acetone and xylene, etc. It's best to use solvents that are highly polar and highly non-polar, and ones that evaporate fast. So for the polar end you'd use acetone, MEK, methanol, DCM, ethyl acetate, etc., and for the non-polar end you'd use heptane usually. Naphtha, xylene, d-limonene, and most other easy to get non-polar solvents evaporate very slowly compared with heptane, so people usually use heptane or a similar fast evaporating solvent.
There are some cases where you might want one of the two solvents to evaporate very slowly. For example, you could crystallize bufotenine in xylene by first adding enough acetone to dissolve the bufotenine, and then letting it slowly evaporate. The acetone evaporates away first leaving behind the xylene. This causes bufotenine crystals to grow slowly. Bufotenine is insoluble in room temperature xylene, and so as the acetone slowly evaporates away, bufotenine crystals get larger and larger. This is a good way to grow very large bufotenine crystals.
