Intermediate-range solvent templating and counterion behaviour at charged carbon nanotube surfaces

Abstract

The ordering of ions and solvent molecules around nanostructures is of profound fundamental importance, from understanding biological processes to the manipulation of nanomaterials to optimizing electrochemical devices. Classical models commonly used to describe these systems treat the solvent simplistically, an approach that endures, in part, due to the extreme difficulty of attaining experimental measurements that challenge this approximation. Here we perform total neutron scattering experiments on model systems—concentrated amide solutions of negatively charged carbon nanotubes and sodium counterions—and measure remarkably complex intermediate-range molecular solvent ordering. The charged surface orders the solvents up to ∼40 Å, even beyond its dense concentric solvation shells. Notably, the molecular orientation of solvent in direct contact with the nanotube surface itself is distinct, lying near-parallel and not interacting with desolvated sodium counterions. In contrast, beyond this layer the ordering of solvent is perpendicular to the surface. Our results underscore the critical importance of multibody interactions in solvated nanoscale systems and charged surfaces, highlighting competing ion/surface solvation effects. Neutron scattering on a model system of highly concentrated solutions of charged carbon nanotubes reveals a strong solvent ordering up to ∼40 Å around the charged nanoscale surface.