Carboxylic acid-directed clustering and dispersion of ZrO₂ nanoparticles in organic solvents: A study by small-angle x-ray/neutron scattering and NMR

The stable dispersion of nanoparticles in organic solvents, essential for their practical application, is often achieved by the grafting of organic dispersants. Good dispersion is usually assumed when a transparent colloidal sol is formed. However, even in such a case, the dispersion may not be in the form of distinct nanoparticles. In this study, zirconia nanoparticles have been modified with a series of carboxylic acids so that they can be dispersed in polar protic/aprotic solvents or nonpolar organic solvents as transparent colloidal sols. The dispersed state in these solvents was characterized by SAXS and SANS. In most cases, the transparent sol consists of soft secondary clusters generated from the ZrO₂ nanoparticles. The size and fractal structure of the dispersed clusters varied with the acid modifier. Only in the linoleic acid-modified case and with chloroform as the solvent was the ZrO ₂ dispersed as primary particles. However, the same modification leads to secondary clusters in other solvents, such as toluene and benzene, with similar solubility parameters. The difference in the dispersion states calls for a molecular-level interpretation. It was proposed that the grafted LOA exists as a swollen and extended brush in benzene and toluene, but as a folded and compact shell in chloroform. The proposed shell structures were then confirmed by the proximity of the chain tail with the middle section observed on 2D NOESY ¹H NMR spectra.