A depiction of the origin of selectivity via the RLF mechanism.

<p>Selectivity arises from the difference in the entropy change from unconstrained ligands () to constrained ligands () between K⁺ and Na⁺. The brown regions represent the volume in which two ligands can move about the ion. As the ligands' fluctuations become constrained, the ligands experience a greater decrease in available volume and thus entropy when coordinating a large ion than when coordinating a smaller ion.