text: The majority of synthetic ion channels follow an Eisenman I sequence (Cs > Rb > K > Na >> Li) in their selectivity for alkali metal cations, suggesting that the origin of the selectivity is governed by the difference in energy required to remove water from a fully hydrated cation.  A few synthetic channels show other patterns of ion selectivity, and only a single instance in which a synthetic channel following the opposite selectivity sequence (Eisenman XI; Cs    ) had been reported.