not sure how to answer clearly but here goes the concept of there being a need for a minimum difference in boiling points is erroneous and probably comes from rules of thumb for separating two pure phases in a single distillation using a simple apparatus that is if you can boil off of phase a without boiling off condensing phase b think back to wwii in norway when heavy water was separated from normal water by multiple distillation the boiling point difference is only degrees yet heavy water was successfully produced