Flocculated Laponite-PEG/PEO Dispersions with Multivalent Salt: A SAXS, Cryo-TEM, and Computer Simulation Study

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.

Microstructure and Soft Glassy Dynamics of an Aqueous Laponite Dispersion

“…However, beyond a certain molar mass of PEO, Mongondry et al 155 propose bridging between Laponite particles, which leads to the formation of clusters. Such bridged Laponite-PEO structure has also been reported by Thuresson et al 154 , Loizou et al 159 and Baghdadi et al 160 The delay in formation of the arrested state in Laponite-PEO dispersion has been monitored using rheology, 153,160 DLS 60,161,162 and SANS studies. 155,157,158 Atmuri et al 163 studied the effect of molecular weight and concentration of PEO on aging behavior of Laponite dispersion.…”

“…To best of our knowledge, there is only one report on the effect of multivalent salt on aging dynamics of Laponite dispersion. Thuresson et al 154 Interestingly the salts that contain anions with a valency greater than 3 such as tetrasodium pyrophosphate, sodium tripolyphosphate, sodium tetraphosphate and sodium hexametaphosphate have been observed to retard the process of aggregation. 156 Mongondry et al 155 studied the influence of tetrasodium pyrophosphate on aggregation of Laponite dispersions using rheology and light scattering.…”

“…Out of various salts, most of the studies on Laponite dispersions investigate the effect of NaCl on the same. ,,,,, As a result, most of the discussion in this article involves the effect of NaCl on the microstructure as well as aging behavior of Laponite dispersions. Other than NaCl, the impact of other monovalent as well as multivalent salts on Laponite dispersions has been studied in the literature . It has been observed that the incorporation of most of the salts (except those containing anions with a valency of >3) in a Laponite dispersion leads to the acceleration of the aging dynamics.…”

2D Nanoclay for Biomedical Applications: Regenerative Medicine, Therapeutic Delivery, and Additive Manufacturing

“…Na + ) result in the largest "d-spacing" between nanoclay sheets (~3.6 nm). [35] Use of higher valency cations, such as divalent and trivalent, reduces the d-spacing between nanoclay. Structural analysis via SAXS indicates that nanoclay exposed to multivalent cations results in higher number of particles present in the solution.…”

“…Structural analysis via SAXS indicates that nanoclay exposed to multivalent cations results in higher number of particles present in the solution. [35] Experimental results often report the physical-chemical relationship between nanoclay shape and chemical composition . The edges of nanoclay are positively charged due to cleaved Mg 2+ or Li 2+ exposure to an aqueous environment, while the face of nanoclay is negatively charged because of the presence of oxygen on the surface.…”