Salt Solutions in Carbon Nanotubes: The Role of Cation−π Interactions

Understanding the structure of aqueous
electrolytes at interfaces
is essential for predicting and optimizing device performance for
a wide variety of emerging energy and environmental technologies.
In this work, we investigate the structure of two common salt solutions,
NaCl and KCl, at a hydrophobic interface within narrow carbon nanotubes
(CNTs). Using a combination of first-principles and classical molecular
dynamics simulations in conjunction with molecular orbital analysis,
we find that the solvation structure of the cations in the CNTs can
deviate substantially from the conventional weakly interacting hydrophobic
picture. Instead, interactions between solvated ions and π orbitals
of the CNTs are found to play a critically important role. Specifically,
the ion solvation structure is ultimately determined by a complex
interplay between cation−π interactions and the intrinsic
flexibility of the solvation shell. In the case of K+,
these effects result in an unusually strong propensity to partially
desolvate and reside closer to the carbon wall than both Na+ and Cl–, in sharp contrast with the known ion
ordering at the water–vapor interface.