Carbon Polarization and Carbon Nanotube Stacking Impacts the Behavior of Water in Nanoconfinement.

The structural and dynamical properties of water confined in subnanometer, single-walled carbon nanotubes (CNTs) are investigated using molecular dynamics simulations. With radii ranging from 4 to 8 Å, we assess confinement-size effects using fully flexible CNTs with explicit carbon polarizability and the MB-pol water model. Density distributions reveal, that single-file water chains form in the narrow (10,0) CNT, pentagonal ring structures form in the intermediate (15,0) CNT, and a two-layer arrangement forms in the wider (20,0) CNT. Angular distributions show that the influence of carbon polarization on water orientation grows with CNT diameter, accompanied by a pronounced tilt of water molecules toward the CNT wall. Comparisons with the TIP4P/2005f model indicate that while this classical model captures structural trends, MB-pol affords a deeper description of interfacial water behavior by enabling direct simulations of infrared spectra from dipole trajectories and consistently accounting for polarization effects. Finally, simulations of vertically aligned CNT arrays demonstrate that intertube interactions significantly impact water dynamics: in stacked (20,0) configurations, intermittent hydrogen-bond lifetimes decrease by 20-25% and layer residence times drop by up to 20% relative to isolated CNTs. These findings illustrate the complex relationship between water structure and dynamics in relation to the size of the confinement and show that carbon polarizability and explicitly considering multiple CNTs are important factors to consider in modeling confined water.