I was asked about why the protactinium produced by the decay of uranyl nitrate ends up in the organic layer. Protactinium is not soluble in water - well, at least the isotope produced in this decay chain is not. So in a way, it must precipitate out on an atomic scale. I wondered if these atoms just got picked up by the organic layer on shaking because it "sticks" to the solvent in some kind of surface tension effect. I came up with an analogue experiment. I put sand into water - it doesn't dissolve. I added a layer of oil.
Then I gave it a big shake...
