Molecular dynamics study on diameter effect in structure of ethanol molecules confined in single-walled carbon nanotubes

Equilibrium molecular dynamics simulations have been performed to investigate the structural characteristics of ethanol molecules confined in single-walled, pristine armchair carbon nanotubes with a length of 2.5 nm and diameters ranging from 0.68 to 1.35 nm in an open ethanol reservoir at 298.0 K and 100.0 kPa by all-atom and united-atom models. Both models present similar results. Structural properties of confined ethanol molecules are analyzed in terms of the average number of hydrogen bonds, radial density distributions of methyl and hydroxyl groups, orientation distributions of the methyl-methylene bond, oxygen-hydrogen bond and dipole moment, and molecular conformations as a function of the diameter of carbon nanotubes. The results indicate that the behavior of the nonpolar part of confined ethanol molecules changes monotonically with the diameter, whereas that of the polar part changes non-monotonically. The different dependence on diameter indicates that the wall-fluid interactions determine the behavior of nonpolar groups, whereas that of polar groups is determined by the fluid-fluid interactions. Only in the nanotube with a diameter of 1.081 nm did the confined ethanol molecules have a highly preferred dipole orientation. The conformational equilibrium also varies considerably with the diameter non-monotonically. The largest proportion of gauche ethanol corresponds to the most preferred dipole orientation.