Rational design of ion-pair receptors for selective binding of hazardous salts

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Ion-pair receptors, i.e. molecules with disparate binding sites for a cation and an anion, have emerged as efficient materials for binding salts. The ability of binding an electroneutral ion-pair renders ion-pair receptors promising candidates for waste purification applications. Despite the advances in synthesis of such receptor molecules, a rational design of the receptors, demonstration of their function, and their application for a sustainable removal of ions has remained challenging. These challenges are related to both the absence of stringent structure-function relationships and to challenges in obtaining direct experimental evidence for the receptor-mediated ion-pair formation. Here we propose a collaborative synthetic-chemical and physico-chemical study of ion-pair formation mediated via ion-pair receptors. Using the electrical dipole moment - an intrinsic property of ion-pairs - we will provide direct evidence for receptor mediated ion-pairing. Via systematic variation of the cation, the anion, and the receptor and simultaneous spectroscopic determination of the ion-pair formation in solution, the proposed study aims at deriving general structure-function relationships. Such information will allow predicting the binding efficiency and selectivity of ion-pair receptors and thus initiate a transition from trial-and-error approaches towards a rational design of task-specific receptor molecules. The obtained relationships will be used to predict and test the optimum receptor structure for binding two prototypical toxic salts, thallium chloride and sodium arsenate. Finally, we will functionalize a polymer resin with the optimum receptor structure and test its application for selective removal of hazardous salts.