Absence of Hofmeister Selectivity in Hydrophobic Ion-Exchanger Nanopores.

Dehydration governs ion selectivity in both natural ion channels and synthetic ion sieves, as ions must dehydrate to traverse subnanometer pores. Because lipophilic ions experience lower dehydration energy costs, an intrinsic lipophilic selectivity pattern known as the Hofmeister series arises. This Hofmeister selectivity is modulated in natural ion channels by electrostatic and coordinative interactions between ions and nanopore surfaces, yielding diverse selectivities. Ion selectivity has also been demonstrated in larger synthetic nanopores─up to 5 nm in diameter─functionalized with ionophores that enhance specific coordinative interactions. In these synthetic ion channels, surface hydrophobicity is critical for ion selectivity; however, its exact role remains undetermined. Here, we elucidate the contribution of hydrophobicity by comparing structurally identical hydrophilic and hydrophobic cation-exchanger gold nanopores modified with 10-mercaptodecane-1-sulfonate, a ligand lacking strong ion-coordinating functionalities. Zero-current potentiometry revealed that cation-exchanger nanopores do not discriminate among singly charged cations, except for a modest preference for H