Nematic textures in colloidal dispersions of Na-fluorohectorite synthetic clay

Abstract

We have studied stable strata of gravity-induced phase separation in suspensions of synthetic Na-fluorohectorite clay in saline solutions. We have observed how the strata depend on clay concentration as well as on salt content. The mass distribution and density variation at the isotropic-nematic interface indicate that existing models and assumptions in existing simulations are able to relatively well account for the observed behavior. We suggest that discrepancies could be due to the high polydispersity and the irregular shape of our Na-fluorohectorite particles, as well as diffusive double-layer effects, which could result in a competition between nematic ordering and gelation. The dependence on ionic strength displays three main regimes irrespective of clay concentration. At low ionic strength (approximately 0.1-5 mM NaCl), the Debye screening length is longer than the van der Waals force range. In this regime, the particles repel each other electrostatically and entropy-driven Onsager-type nematic ordering may occur, although gelation effects could also play a role. For ionic strengths above about 5 mM, we believe that the van der Waals force comes into play and that particles attract each other locally according to the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) model of colloid interactions, resulting in a small-domain regime of attractive nematiclike ordering. In the third regime, for ionic strengths above approximately 10 mM, the clay particles aggregate into larger assemblies, due to the dominant van der Waals force, and the observed birefringency is reduced. We have studied the nematic phase in detail between crossed polarizers and have found textures showing nematic Schlieren patterns. By rotating the polarizers as well as the samples, we have observed examples of disclinations of strengths -1, -1/2, and +1.

Isotropic/nematic and sol/gel transitions in aqueous suspensions of size selected nontronite NAu1

“…It was shown that at low ionic strengths (4 5.10 À3 M/L), all the interactions in the system were repulsive and that as a first approximation, most rheological features of the suspensions could be explained using arguments based on excluded volume effects. Liquid crystal behaviour was also observed recently for synthetic fluorohectorite samples (De Azevedo et al, 2007;Hemmen et al, 2009;Miyamoto et al, 2010;Ringdal et al, 2010) and for beidellite (Paineau et al, 2009), although the former samples were not fully delaminated.…”

Aspect ratio and polydispersity dependence of isotropic-nematic transition in discotic suspensions

“…9 show that the exponent (n) slightly depends on polydispersity. Therefore, it becomes interesting to see why similar colloidal systems, such as clay, having a large size polydispersity and large aspect ratio (ξ ∼ 0.01), commonly do not exhibit a nematic, but rather a gel phase, where some orientational order should be present [41,62,64,95]. In this paper, we observed that it was polydispersity that widened φ I−N .…”

Aspect-ratio-dependent phase transitions and concentration fluctuations in aqueous colloidal dispersions of charged platelike particles

“…For instance, the clear-cut pieces of evidence for isotropicnematic [20,[34][35][36][37][38][39], isotropic-lamellar [21,40], and a sequential transition involving isotropic-nematic and nematiclamellar transitions [26] with increasing φ p at a particular r asp (in the range satisfying 1.1 × 10 −3 r asp 7.1 × 10 −2 ) have already been obtained by using the scattering method. However, due to the difficulty in preparing a wide variety of size-controlled particles, the experimental studies attempting to elucidate systematically the r asp dependence of the phase behavior are rather minor [22,23,[34][35][36][37][38].…”