Résumé
The phase diagram of colloidal suspensions of electrically charged nanosheets, such as clays, in spite of their many industrial uses, is not yet understood either experimentally or theoretically. When the nanosheet diameter is very large (~ 100 nm – 1 µm), distinguishing the lamellar liquid-crystalline phase from a nematic phase with strong stacking local order, often called “columnar” nematic, is quite challenging. We show here that newly upgraded small-angle x-ray scattering (SAXS) beamlines at synchrotron radiation facilities provide high-resolution measurements which allow identifying both phases unambiguously, provided that single domains can be obtained. We investigated dilute (~ 0.1 vol%) aqueous suspensions of synthetic Sb3P2O143- nanosheets,1,2 with aspect ratio > 100, that self-organize into two distinct liquid-crystalline phases, sometimes coexisting in the same sample.3 Close examination of their x-ray diffraction patterns (Figure 1) and their reflection profiles in the directions perpendicular to the director demonstrates that these two mesophases are respectively a columnar nematic and a lamellar phase. In the latter, the domain size reaches up to ~ 20 µm, which means that each layer is made of ~ 600 to tens of thousands nanosheets depending on their size. Since the lamellar phase was only rarely predicted in suspensions of charged disks, our results show that these systems should be revisited by theory or simulations. The unexpected stability of the lamellar phase also suggests that the rims and faces of Sb3P2O143- nanosheets may have different properties, giving them a “patchy” particle character. Figure 1: SAXS patterns of the nematic (A) and lamellar phase (B). White arrows point to the nematic diffuse ring in A) and the lamellar reflections in B) where the inset shows the (001) reflection.[1] P. Davidson, J-C. P. Gabriel., Current Opinion Coll. Inter. Sci. 9, 377 (2005).[2] J.-C. P. Gabriel et al., Nature 413, 504 (2001) 508.[3] J.C.P. Gabriel et al, to be submitted (2018).* Author for Correspondence: patrick.davidson@u-psud.fr; jean-christophe.gabriel@cea.fr