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A colloidal suspension of nanosheets displays several liquid-crystalline phases


Aqueous suspensions of nanometric sheets (nanosheets), like clay or graphene oxide particles, are very much used in industry, for applications ranging from cosmetics to public works and oil recovery. A priori, such suspensions could spontaneously organize in liquid-crystalline phases of different kinds. In principle, in the “nematic” phase (Figure 1A), the nanosheets align in the same direction but their positions in the solvent remain random. In the “lamellar” phase (Figure 1B), the nanosheets also form equidistant layers. These two liquid-crystal phases differ not only by their structures but also by their visco-elastic properties, which is very important for industrial processes. Moreover, the lamellar phase allows for the nanometric confinement of liquids, a property used by living organisms, like in cells. However, lamellar phases are usually built by the self-assembly of small organic molecules. For nanosheets, the mere existence of a lamellar phase was still a subject of debate. Indeed, when the nanosheets are large enough, distinguishing the lamellar phase from the nematic one is experimentally quite challenging.

Recently, researchers from CEA (Grenoble) and LPS (Orsay) used the new capabilities of synchrotron beamlines of SOLEIL (Gif-sur-Yvette) and ESRF (Grenoble) to prove the existence of the two phases in a nanosheet suspension and to compare their structures.1 The system considered was an aqueous suspension of synthetic sheets, of chemical formula H3Sb3P2O14, a material used for the solid-liquid extraction of rare-earth elements. The X-ray scattering pattern of the nematic phase is typical because it only shows broad scattering features (Figure 1C). In contrast, the scattering pattern of the lamellar phase displays sharp equidistant peaks, which is typical of a layered organization of the nanosheets (Figure 1D). The nematic phase appears at a nanosheet concentration somewhat lower than that of the lamellar phase. In the latter, each layer is formed by the spontaneous organization of at least a thousand of nanosheets. These results show that such systems should be revisited by theory and numerical simulations which to date only rarely predict the existence of the lamellar phase.

Figure 1. Top : Schematics of the nanosheet organization in the nematic phase (A) and the lamellar phase (B). In the latter, nanosheets belonging to the same layer are shown in the same grey shade, and d is the lamellar period. Bottom row : X-ray scattering patterns in the nematic phase (C) and in the lamellar phase (D). The white arrows point respectively to the nematic diffuse spot and to the lamellar reflections, and the inset shows the (001) reflection.

Reference

Isotropic, nematic, and lamellar phases in colloidal suspensions of nanosheets
Patrick Davidson, Christophe Penisson, Doru Constantin, Jean-Christophe P. Gabriel
PNAS, 115 (26) 6662-6667 (2018)
doi:10.1073/pnas.1802692115

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Patrick Davidson