Charged surfaces and solvents interact in many different systems, from paints and dyes to Net Zero technologies such as batteries and supercapacitors, and even in the clay underneath the city of London. Most scientists ignore the solvent in these systems but, in this study, a team from UCL and their collaborators from Imperial College London, ISIS and the UK Catalysis Hub used the NIMROD beamline to measure the solvent structure around a charged carbon nanotube, a model system chosen to interrogate the solvent ordering at charged surfaces in unprecedented detail.

“There is a common simplification that assumes solvents are made of 'infinitely small molecules', and it works well when you are looking at large systems. However, for small systems where you have to look closer, eventually you can no longer ignore the fact that solvents are made of real molecules which take up real space." says Adam Clancy from UCL, one of the paper authors. “In this study we looked into a regime that sits in the middle of the interactions between individual molecules, and giant systems where you can get away with simplifications that let you ignore the solvent. Unfortunately, this medium length scale is hard to look at."  

This challenge is what drew the researchers to ISIS. The NIMROD beamline is optimised for this type of study. “It's the only place in the world to give us a window into this behaviour," says ISIS scientist, Tom Headen. “This is because we can look at the very small and the slightly small at the same time in detail."

Using NIMROD, it's possible to look at where nearest neighbour atoms are, on the sub-nanometre scale, up to many nanometres, which correlates to the length scale of the nanotubes themselves and the arrangement of the solvent molecules around them. Because of the unique interaction between neutrons and hydrogen, it also means the team could see where the hydrogen atoms are in the system.

“NIMROD allowed us to see complex liquid structures that no one else could see before," explains Camilla Di Mino from UCL, lead author of the paper. “We found that the solvent behaviour was actually complex, intricate and unusual."

In their study, published in Nature Nanotechnology, the team created a model system using carbon nanotubes. They have a well-defined surface, and it's therefore possible to build a computer model replica of this system. Other systems would be harder to replicate and measure in this detail. The carbon nanotubes can also dissolve to relatively high concentrations in the liquid solvents, which is also needed for the experiment.

The team can then take the learnings from this model system to apply to real-world environments with wide reaching impact.

They found that the 3-dimensional shape of the solvent molecule is very important. These molecules compete with the ions in solution in how they order around the surface of the nanotubes. This influence reaches far out into the bulk solvent, as if the surface is acting as a template for the solvent.

This shows that solvent ordering is far more complex than previously thought. “The solvent is arguably of equal importance as the ion in determining the overall behaviour at the surface and definitely cannot be ignored," explains Chris Howard, from UCL. “This research challenges the models that are written into undergraduate textbooks. These models work reasonably well for some systems but notably fail in others, and they're crucial to our understanding of systems such as supercapacitors and batteries."

The understanding gained from this research paves the way for better devices. For example, they saw that the ions close to the surface are ignored by the liquid (a process known as desolvation), which would allow for more energy to be stored in a supercapacitor.

To further this research, Chris and Tom have a joint PhD student who will be studying supercapacitors, using in situ monitoring on NIMROD.

The full paper can be found at DOI: 10.1038/s41565-025-01865-9​