Concentric Structure of Coexistent KCl Core Solution and Water Adlayer Inside a Highly Hydrophobic Single-Wall Carbon Nanotube

The concept of hydrated ions, which is described by the hydration number and the size of the hydrated ion determined with X-ray diffraction measurements, has contributed to understanding ionic solutions. However, we do not understand the effect of ions on the hydrogen-bonded quasi-tetrahedral (Td) clusters of H2O molecules and the beyond-hydrated ion structures, regardless of their crucial importance. The present study applied the intensive confinement effect of a highly hydrophobic single-wall carbon nanotube (SWCNT) to elucidate the correlation of ions with quasi-Td clusters and beyond-hydrated ion structures using KCl solution at 298 K. In situ X-ray diffraction revealed that the water structure confined inside the SWCNT was insensitive to the presence of ions confined in the SWCNT. The GCMC simulation studies showed the stably separated structure of the KCl solution in the central core and the adsorbed water layer akin to the graphene wall of the SWCNT. Newly clarified coexistence of the KCl solution and the water adlayer inside the SWCNT should give new insights into understanding localized heterogeneities in bulk ionic solutions.