The Nanopore Inner-Sphere Enhancement (NISE) Effect: Overview of Theory, Data, and Current Evidence.

The adsorption of cations on solid surfaces is generally strong for divalent cations and weak for monovalent cations. The divalent cations often will adsorb via inner-sphere mechanisms, while the monovalent cations will typically adsorb via outer-sphere mechanisms. In constrained environments, where the dimensions are circa the diameter of the hydrated atoms, the adsorption behavior may change significantly. Specifically, the retention of monovalent cations can exceed that of divalent cations in these nanoporous environments. This is very counterintuitive and cannot be explained by traditional adsorption theories. The Nanopore Inner-Sphere Enhancement (NISE) theory does offer a viable explanation for these observations. The processes involved in these environments will be presented along with adsorption data and spectroscopic evidence that supports the NISE theory.