Long-range solvent templating and counterion behavior at a charged, nanoscale surfaces

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Abstract
The ordering of solvent molecules around nanostructures is of broad scientific importance from understanding biological processes to the manipulation of nanomaterials in liquids, to optimiz-ing the performance of electrochemical devices. However, experimental measurements of this solvent ordering are scarce. Herein, the structures of concentrated solutions of anionic carbon nanotubes in two amidic solvents are measured via total neutron scattering leading to a previ-ously unattainable picture of nanoparticle solvation, revealing a far richer structure than hitherto assumed. The solvent molecules form densely-packed, concentric shells around the charged carbon nanotubes, ordered according to two distinct orientations due to competing effects. Firstly, a surface monolayer that maximizes attractive interactions between anionic surface and solvent molecular dipole moment. The second orientation extends ~40 Å into the bulk solvent, with molecules preferentially oriented perpendicularly to the first layer due to dipole interac-tions with the charged nanoparticle. This complex, long-ranged order drastically differs from the simplistic treatment of the solvent in classical models commonly employed to understand these systems. Moreover, our analysis indicates that a fraction of counterions condense on the sur-face as a Stern layer which are desolvated due to the solvent preferentially interacting with the charged surface. Ions at further distances are fully solvated, and their distribution is strongly in-fluenced by the dense solvation shells around the nanotube. Our results thus underscore the critical importance of multi-body interactions in solvated nanoscale systems, highlighting new competing ion/surface solvation effects crucial for understanding electrolyte-surface interfaces in supercapacitors, batteries, and electrolytically-gated devices.
Funding Information
  • Engineering and Physical Sciences Research Council (EP/R513143/1, EP/L015277/1)
  • Graphene Flagship (785219)

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