Enhanced cation recognition by a macrocyclic ionophore at the air-solution interface probed by mass spectrometry.

Interfacial environments have the potential to drive unexpected events of supramolecular recognition, leading to advances in the development of novel functional materials and molecular sensing techniques. We present experimental evidence for a noticeable enhancement of the cation binding specificity of a prototype calixarene macrocycle (cesium ionophore II) at the air-solution interface, in comparison to bulk solution and to isolated solvent-less conditions. A rationalization of this intriguing finding is outlined, with the support of quantum calculations, in terms of the 'half-solvation' conditions provided by the interface and of conformational effects posed by the backbone structure and the side chains of the macrocyclic ionophore. The investigation involves the introduction of a mass spectrometry method to determine the relative abundances of interfacial complexes that should be of general application in the field and guide future advances in analytical techniques based on molecular recognition.