Molecular simulation of solute hydration structure in nanoscale confinement

A series of molecular dynamics simulations were performed to investigate the effect of carbon nanotube diameter on the hydration structure of five different solutes (K⁺, Mg²⁺, Cl^-, K^- and K⁰) inside carbon nanotubes (CNTs). Simulation results reveal different hydration processes for monocharge, bicharge, and neutral solutes in CNTs. Coordination numbers of monocharge solutes decrease significantly only inside narrow CNTs with diameters less than 0.73 nm. The coordination number of neutral solute is, however, sensitive to the CNT diameter and decrease monotonically as CNT diameters decrease in all the CNTs used in this work. Only the positive monocharge solute has the order of its coordination shell structure vary considerably with a change in CNT diameter. The shells of the other solutes appear to be bulk like in all the CNTs used in this work. The shell order of K⁺ decreases as CNT diameter decreases for diameters larger than 1.0 nm, and increases as CNT diameter decreases for diameters less than 1.0 nm. Inside the two narrow CNTs with diameters of 0.6 and 0.73 nm, the shell order of K⁺ is even higher than that found in bulk solution. The hydration of bicharge solute is found to be identical to that in bulk solution in all the CNTs used in this work, even in the narrow CNT with a diameter of 0.6 nm.