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

TL;DR

This study uses ab initio calculations to analyze how water and ions transfer into narrow carbon nanotubes, revealing how the surrounding dielectric environment influences selectivity and transfer energies, with implications for nanochannel device design.

Contribution

The paper provides a detailed ab initio analysis of water and ion transfer energies in CNTs, highlighting the role of dielectric polarization in transfer selectivity, which was previously unclear.

Findings

Dielectric polarization significantly affects ion transfer energies.

Transfer energies depend on CNT and ion properties, influenced by dielectric constant.

Insights can guide the design of nanochannels with tailored selectivity.

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.