Neutron research shows solvent microstructure influences nanomaterials

Nanoparticles have become an integral part of our everyday lives: they are found in medicines, catalysts, paints, and high-tech materials. Traditionally, solvents have been treated as a uniform background. A new study shows that solvents play a far more active role at the nanoscale than predicted.

Dr Aurel Radulescu positions a sample environment at the small-angle neutron diffractometer KWS-2. © Bernhard Ludewig, FRM II / TUM

Dr Aurel Radulescu positions a sample environment at the small-angle neutron diffractometer KWS-2. © Bernhard Ludewig, FRM II / TUM

An international research team involving Dr Aurel Radulescu from the Jülich Centre for Neutron Science (JCNS) at the Heinz Maier-Leibnitz Zentrum (MLZ) has, for the first time, quantitatively measured how solvent molecules organise at ligand coated nanoparticle surfaces. The surprising result: in certain mixtures, tiny clusters of only a few solvent molecules form, which specifically “attack” the surface of the particles and even change their shape.

Solvents are not a uniform liquid

In the classical view of chemistry, solvents are considered a kind of uniform background. Properties such as polarity or dielectric constant are averaged, and in mixtures it is assumed that the effects simply add up. This works well for large systems in bulk or on a micrometer scale – but apparently not for nanoparticles. This is because on a surface measuring only a few nanometres, each molecule only “sees” a handful of neighbours. Under these conditions, local structures arise: small, short-lived clusters of solvent molecules, only about one nanometre in size.

Schematic presentation of the experimental and data interpretation approach: via combined SANS at KWS-2 and Monte Carlo, a nanoparticle shape transition is observed as solvent composition is tuned. © Forschungszentrum Jülich

Schematic presentation of the experimental and data interpretation approach: via combined SANS at KWS-2 and Monte Carlo, a nanoparticle shape transition is observed as solvent composition is tuned. © Forschungszentrum Jülich

Neutrons make the invisible visible

Using small-angle neutron scattering (SANS) at the MLZ and the Institut Laue-Langevin in Grenoble, alongside complex computer modelling, the researchers studied gold nanoparticles. “This let us characterise the nanoparticle structure and resolve how a solvent behaves at the organic shell, driving nanoparticle shape deformation”, explains Dr Radulescu, instrument scientist at the KWS-2, where the results were obtained.

From perfect tiny spheres to elongated particles

Depending on the solvent mixture, the spherical gold particles changed shape: at specific mixing ratios (around the azeotropic point), they became elongated instead of spherical. Adjusting the mixture reversed the effect. Shape change, increased solvent penetration, and maximum cluster formation always occurred together.

Why this is important

The results challenge fundamental assumptions in nanochemistry. At the nanoscale, solvents can no longer be considered a passive environment: instead they act as active, structure-determining agents. This opens up new possibilities for controlling the shape, stability, and behaviour of nanomaterials through targeted solvent selection.

Editorially shortened version; original text: Angela Wenzik, Forschungszentrum Jülich.