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Interplay of anisotropy in shape and interactions on phase behavior and dynamics of charged platelet suspensions


Colloidal suspensions of plate-like nanoparticles such as swelling clays, disc-like mineral crystallites or exfoliated nano-sheets are ubiquitous in Nature, and can all be envisioned as dispersions of charged platelets. While the geometry of colloids thus seems simple, the corresponding phase behaviour has so far remained elusive : it markedly differs in otherwise seemingly closely related systems. Some clay suspensions indeed form arrested states at low densities, whereas others exhibit an equilibrium isotropic-nematic transition instead.

Without electrostatic interactions, the physics is clear : hard platelets undergo an isotropic-nematic transition as a result of the competition between orientational and positional entropy, a phenomenon predicted by Onsager. The main question that arises is to understand how electrostatic interactions influence the isotropic-nematic transition, and to attempt a classification of new phases that may emerge.

To address the influence of the interplay between electrostatic interactions and colloidal shape, a team from the Laboratory of Theoretical Physics and Statistical Models (LPTMS), started a collaboration with colleagues from Laboratoire de Physique Théorique (LPT Orsay), Laboratoire de Physique des Solides (LPS Orsay), and Laboratoire PECSA (Paris VI). A first principle derived orientation-dependent effective pair potential was used to investigate the phase behaviour by means of Monte-Carlo simulations.


 

The angular dependence of the effective pair potential has an unusual angular form, that induces an asymmetry between the states of parallel disks in co-planar, and stacked configurations (see the left part of the Figure).

This work shows that the original intrinsic anisotropy of the electrostatic potential leads to a rich phase behaviour (see Figure), that not only rationalizes generic features of the complex phase diagram of charged colloidal platelets, but also predicts the existence of novel structures. Furthermore, studying the dynamics as a function of density provides evidence of slowing-down in the orientationally disordered states. This points to the possible formation of arrested states in such highly charged systems. The present investigation thereby opens the way to understanding the equilibrium features of charged colloidal platelet suspensions, and charts out the regions in parameter space (salinity, density) where non-equilibrium features may be prevalent.

This work was supported by Triangle de la Physique and IEF Marie-Curie fellowships.

Contacts :
Patrick Davidson
Sara Jabbari-Farouji

Reference :
On phase behavior and dynamical signatures of charged colloidal platelets
Sara Jabbari-Farouji, Jean-Jacques Weis, Patrick Davidson, Pierre Levitz & Emmanuel Trizac
Scientific Reports 3, 3559 (2013).