The Journal of Physical Chemistry C, 2017, vol 121, 13, pp. 7387-7396
DOI:10.1021/acs.jpcc.7b01715
Abstract
The aim of this study is to scrutinize the mechanism behind aggregation, i.e., tactoid formation of nanostructures with the shape of a platelet. For that purpose, the clay minerals Laponite and montmorillonite have been used as model systems. More specifically, we are interested in the role of: the platelet size, the electrostatic interactions, and adsorbing polymers. Our hypothesis is that the presence of PEG is crucial for tactoid formation if the system is constituted by small nanometric platelets. For this purpose, SAXS, USAXS, Cryo-TEM, and coarse-grained molecular dynamics simulations have been used to study how the formation and the morphology of the tactoids are affected by the platelet size. The simulations indicate that ion–ion correlations are not enough to induce large tactoids solely if the platelets are small and the absolute charge is too low, i.e., in the size and charge range of Laponite. When a polymer is introduced into the system, the tactoid size grows, and the results can be explained by weak attractive electrostatic correlation forces and polymer bridging. It is shown that when the salt concentration increases the long-ranged electrostatic repulsion is screened, and a free energy minimum appears at short distances due to the ion–ion correlation effects. When a strongly adsorbing polymer is introduced into the system, a second free energy minimum appears at a slightly larger separation. The latter dominates if the polymer is relatively long and/or the polymer concentration is high enough.
