Probing the surfaces of core-shell and hollow nanoparticles by solvent relaxation NMR

Probing Selective Adsorption in Cationic-Polymer Induced Aggregation of Binary Anionic Particulate Dispersions Using Solvent Relaxation NMR

NMR solvent relaxation has been used to characterize the surfaces present in binary anionic particle dispersions, before and after exposure to a cationic polymer. In the polymer-free case, it is shown that the measured specific relaxation rate of the solvent is a population-weighted average of all surfaces present, enabling preferential adsorption to be explored. The addition of the oppositely charged polymer led to phase separation, which was accelerated by gentle centrifugation. The measured relaxation rates and the equilibrium particle concentrations indicate that the cationic hydroxyethylcellulose polymer (HEC LR) exhibited no significant preference for either latex or laponite in binary blends with silica, but a strong preference for TiO2. This study illustrates the versatility of solvent relaxation to probe surface area, surface type and dispersion composition in complex formulations.

Green earth pigments dispersions: Water dynamics at the interfaces

HYPOTHESIS

The objective is to elucidate the multiscale dynamics of water within natural mixtures of minerals, green earth pigments that are mainly composed of phyllosilicates containing large amount of iron. In particular, the interaction of water with the different kinds of surfaces has to be probed. One issue is to examine the influence of surface type, basal or edge, on the dispersion quality.

EXPERIMENT

The study was carried out using ¹H variable field NMR relaxometry on various green earth pigment dispersions and concentrations. To analyse the data, a new analytical model was developed for natural phyllosilicates containing large amount of paramagnetic centres.

FINDING

The proposed theoretical framework is able to fit the experimental data for various samples using few parameters. It allows to determining water diffusion and residence times in complex phyllosilicate dispersions. Furthermore, it makes it possible to differentiate the contribution of the basal and edge surfaces and their respective surface area in interaction with water. Moreover, NMR relaxation profile reveals to be highly sensitive to the structural aspect of the phyllosilicates and to the accessibility of water to iron, hence allowing to discriminate clearly between two very similar phyllosilicates (glauconite and celadonite) that are difficult to distinguish by standard structural methods.