Generalized Framework for Modeling Water Confined Inside Metallic and Semiconducting Carbon Nanotubes

Carbon nanotubes (CNTs) with diameters in the 0.8 nm – 2 nm range are emerging as important material platforms to investigate confinement-induced effects on water and electrolyte solutions, which has wide-ranging applications in electrochemistry, energy, and membrane science. Although previous molecular dynamics simulation studies have attempted to elucidate the nature of water-CNT dispersion interactions modeled using pair-wise additive potentials, including the Lennard-Jones potential, it remains unclear how the anisotropic polarizability tensors of CNTs, i.e., the different polarizability components along the radial and axial directions, govern the properties of polar fluids such as water, under nanoscale confinement. Importantly, while both metallic and semiconducting CNTs have a radial polarizability which scales quadratically with the CNT radius, these CNTs differ significantly in their axial polarizability which depends on the band gap. Here, we present a generalized theoretical framework for modeling water confined inside metallic and semiconducting CNTs, and investigate the thermodynamic (e.g., density, hydrogen-bonding network) and transport (e.g., friction coefficient) properties of confined water molecules as a function of the CNT radial and axial polarizability. Specifically, we show how the water-induced electronic polarization of the CNT results in the generation of an electrostatic pressure which then acts back on the confined water molecules, including influencing the water density and structuring. Overall, our study highlights the central role of electronic polarization effects in nanofluidics involving polar fluids confined inside strongly polarizable nanomaterials.