Measuring Solvent Structure Around Charged Carbon Nanotubes Using Total Neutron Scattering

The liquid-phase processing of carbon nanomaterials is a critical step towards their commercial viability, yet the fundamental physics of solvent stabilization has remained elusive. This complexity stems from the minute volumetric fraction of solvent at the nanocarbon interface, the intrinsic disorder in liquids, and the weak X-ray scattering of light elements in most solvents. We have tackled these challenges head-on using the cutting-edge total neutron scattering (TNS). TNS offers a unique light-element-specific scattering probe and the opportunity for isotopic substitution, enabling us to extract multiple constraining datasets for a single nanotube-solvent system.<br/> <br/>Leveraging TNS measurements in conjunction with established analysis techniques for pure liquids, we have, for the first time, achieved direct observation of solvent ordering around charged carbon nanotubes. This breakthrough allows us to measure solvent structures with atomistic precision, while charged nanotubes present sufficient concentrations to study their interfaces in their native homogeneous bulk state. We explore a variety of liquid-phase systems, including negatively charged nanotubes dissolved in amides (NMP and DMAc), which are ideal models for nanotube double-layer supercapacitors, and positively charged carbon nanotubes in chlorosulfonic acid. Our findings reveal that in all measured systems, the nanotubes induce strong templating effects on the solvent, forming cylindrical solvation shells and aligning the solvents along their dipoles, with ordering extending beyond these density fluctuations. For anionic nanotube systems, we can directly image the electric double layer, observing sodium counterions at the nanotube surface becoming desolvated as the solvent prefers to solvate the nanotube surface. This insight is invaluable for understanding the behavior of nanocarbons in energy storage applications.<br/> <br/>These pioneering measurements underscore the transformative potential of applying TNS to nanomaterial liquid-phase dispersions. We envision that this approach will dramatically advance our understanding of these complex high-aspect-ratio nanomaterials, paving the way for their broader application and societal benefits.