Enhanced wettability of long narrow carbon nanotubes in a double-walled hetero-structure: unraveling the effects of a boron nitride nanotube as the exterior.

Abstract

Studying the structure and dynamics of nano-confined water inside carbon nanotubes has consistently attracted the wide-spread interest of researchers. In the present work, molecular dynamics simulations indicated internal nonwetting behavior for the central region of the long and narrow single-wall carbon nanotube (5,5) (SWNT) and showed that continuous single-file water molecules are not formed through it. Unlike the SWNT, by adding boron nitride nanotubes (6,6) as an outer wall to the SWNT, a continuously long single-file water chain is formed through the double-walled carbon and boron nitride hetero-nanotube (DWHNT) and thorough internal wetting of the DWHNT is observed. The position and the number of water molecules, electrostatic potential heatmap of the nanotube's wall, free energy profile of nano-confined water, and number of hydrogen bonds between them confirmed the aforementioned results and complete internal wetting of the DWHNT. After using the boron nitride nanotube (6,6) as the outer wall, an homogeneous electrostatic potential distribution in the DWHNT and increase in the hydrophilic characteristics of the nano-channel wall are observed, bringing about gradual trapping of more water molecules through it. Finally, water molecules occupied the central region of the DWHNT and a thorough single-file water chain is formed inside the nano-channel. Water dipole orientation inside the DWHNT and their radial distribution function asserted the occurrence of the liquid-solid quasi-phase transition of single-file water molecules confined inside the long and narrow carbon nanotube (5,5) under ambient conditions.