Water and Ion Transfer to Narrow Carbon Nanotubes: Roles of Exterior and Interior

Abstract: Narrow carbon nanotubes (CNTs) desalinate water, mimicking water channels of biological membranes, yet the physics behind selectivity, especially, the effect of the membrane embedding CNTs on water and ion transfer, is still unclear. Here, we report $ab$ $initio$ analysis of the energies involved in transfer of water and K$^+$ and Cl$^-$ ions from solution to empty and water-filled 0.68 nm CNTs, for different dielectric constants $\epsilon$ of the surrounding matrix. The transfer energies computed for $1 \leq \epsilon < \infty$ permit a transparent breakdown of the transfer energy to three main contributions: binding to CNT, intra-CNT hydration, and dielectric polarization of the matrix. The latter scales inversely with $\epsilon$ and is of the order $10^2$/$\epsilon$ kJ/mol for both ions, which may change ion transfer from favorable to unfavorable, depending on ion, $\epsilon$, and CNT diameter. This may have broad implications for designing and tuning selectivity of nanochannel-based devices.