Fan Li

Ion and water transport through subnanometer carbon nanotubes (CNTs) exhibits numerous anomalies absent in bulk solutions, yet the interplay among the confined water structure, external electric fields, and ion transport remains poorly understood. Here, we use systematic nonequilibrium molecular dynamics simulations to study single-file water confined in a CNT of radius 0.37 nm, both in the absence and presence of a single Na+ ion, under applied electric fields ranging from 0 to 0.2 V Å–1. In the Na+–water–CNT system, the electroosmotic flow (EOF) velocity increases nonlinearly with the field. Such nonlinear enhancement is attributed to water ordering that is exclusive to the ion-containing system. This enhanced ordering is evidenced by a larger average number of hydrogen bonds per water molecule, longer hydrogen-bond lifetimes, a reduced interoxygen distance with a higher first peak in the oxygen–oxygen radial distribution function, and a decrease in free energy. Further relative permittivity calculations reveal that the effective permittivity decreases with increasing electric field, leading to a monotonic rise in the Born energy difference between the Na+–water–CNT and water–CNT systems. This increase in Born energy likely drives the observed water ordering in Na+–water–CNT. Our findings uncover a previously unrecognized coupling mechanism among ions, electric fields, and water structure under extreme confinement.

Water confined by hydrophilic materials shows unique transport properties compared to bulk water thereby offering new opportunities for development of nano-fluidic devices. Recent experimental and numerical studies showed that nano-confined water undergoes liquid-to-solid phase-like transitions depending on the degree of confinement. In the case of water confined by graphene layers, the Van der Waals forces are known to deform the graphene layers, whose bending leads to further non-uniform confinement effects. Despite the extensive studies of nano-confined water at equilibrium conditions, the interplay between the confinement and rheological water properties, such as viscosity, slip length and normal stress differences under shear flow conditions, is poorly understood. The current investigation uses a validated all-atom non-equilibrium molecular dynamics model to simultaneously analyse continuum transport and atomistic structure properties of water in a slit between two moving graphene walls under Couette flow conditions. A range of different slit widths and velocity strain rates are considered. It is shown that under the sub-nanometer confinement, water loses its rotational symmetry of a Newtonian fluid. In such conditions, water transforms into ice, where the atomistic structure is completely insensitive to the applied shear force and which behaves like a frozen slab sliding between the graphene walls. This leads to the shear viscosity increase, although not as dramatic as the normal force increase that contributes to the increased friction force reported in previous experimental studies. On the other end of the spectra, for flows at large velocity strain rates in moderate to large slits between the graphene walls, water is in the liquid state and reveals a shear thinning behavior. In this case, water exhibits a constant slip length on the wall, which is typical of liquids in the vicinity of hydrophobic surfaces.