Probing the Structure of Salt Water under Confinement with First- Principles Molecular Dynamics and Theoretical X‑ray Absorption Spectroscopy

ABSTRACT: We investigated the structure of liquid water around cations (Na+) and anions (Cl−) confined inside of a (19,0) carbon nanotube with first-principles molecular dynamics and theoretical X-ray absorption spectroscopy (XAS). We found that the ions preferentially reside near the interface between the nanotube and the liquid. Upon confinement, the XAS signal of water molecules surrounding Na+ exhibits enhanced pre-edge and reduced post-edge features with respect to that of pure water, at variance with the solvation shell of Na+ in bulk water. Conversely, the first solvation shell of confined Cl − has a main-edge intensity comparable to that of bulk solvated Cl−, likely as a result of a high number of acceptor hydrogen bonds in the first solvation shell. Confined nonsolvating water molecules exhibit bulk-like or water-monomer-like properties, depending on whether they belong to core or interfacial layers, respectively. SECTION: Liquids; Chemical and Dynamical Processes in Solution Understanding the complex behavior of water and aqueoussolutions in confined environments,1 such as in cavities and channels, is key for a variety of fields from biology2−5 to materials science.6−8 However, many fundamental properties o