  Concentrated colloidal suspensions of nanorods often exhibit
liquid-crystalline (LC) behavior. The transition to a nematic LC phase, with
long-range orientational order of the particles, is usually well captured by
Onsager's theory for hard rods, at least qualitatively. The theory shows how
the volume fraction at the transition decreases with increasing aspect ratio of
the rods. It also explains that the long-range electrostatic repulsive
interaction occurring between rods stabilized by their surface charge can
significantly increase their effective diameter, resulting in a decrease of the
volume fraction at the transition, as compared to sterically stabilized rods.
Here, we report on a system of ligand-stabilized LaPO4 nanorods, of aspect
ratio around 11, dispersed in apolar medium exhibiting the counter-intuitive
observation that the onset of nematic self-assembly occurs at an extremely low
volume fraction of around 0.25%, which is lower than observed (around 3%) with
the same particles when charge-stabilized in polar solvent. Furthermore, the
nanorod volume fraction at the transition increases with increasing
concentration of ligands, in a similar way as in polar media where increasing
the ionic strength leads to surface-charge screening. This peculiar system was
investigated by dynamic light scattering, Fourier-Transform Infra-Red
spectroscopy, zetametry, electron microscopy, polarized-light microscopy,
photoluminescence measurements, and X-ray scattering. Based on these
experimental data, we formulate several tentative scenarios that might explain
this unexpected phase behavior. However, at this stage, its full understanding
remains a pending theoretical challenge. Nevertheless, this study shows that
dispersing anisotropic nanoparticles in an apolar solvent may sometimes lead to
spontaneous ordering events that defy our intuitive ideas about colloidal
systems.
