Density, distribution, and orientation of water molecules inside and outside carbon nanotubes

The behavior of water molecules inside and outside 1.1, 2.8, 6.9, and 10.4 nm diameter armchair carbon nanotubes (CNTs) is predicted using molecular dynamics simulations. The effects of CNT diameter on mass density, molecular distribution, and molecular orientation are identified for both the confined and unconfined fluids. Within 1 nm of the CNT surface, unconfined water molecules assume a spatially varying density profile. The molecules distribute nonuniformly around the carbon surface and have preferred orientations. The behavior of the unconfined water molecules is invariant with CNT diameter. The behavior of the confined water, however, can be correlated to tube diameter. Inside the 10.4 nm CNT, the molecular behavior is indistinguishable from that of the unconfined fluid. Within the smaller CNTs, surface curvature effects reduce the equilibrium water density and force water molecules away from the surface. This effect changes both the molecular distribution and preferred molecular orientations.