Lyotropic Smectic B Phase Formed in Suspensions of Charged Colloidal Platelets

Abstract

Here, we present the first observation of a smectic B (Sm B ) phase in a system of charged colloidal gibbsite platelets suspended in dimethyl sulfoxide (DMSO). The use of DMSO, a polar aprotic solvent, leads to a long range of the electrostatic Coulomb repulsion between platelets. We believe this to be responsible for the formation of the layered liquid crystalline phase consisting of hexagonally ordered particles, that is, the Sm B phase. We support our finding by highresolution X-ray scattering experiments, which additionally indicate a high degree of ordering in the Sm B phase. ■ INTRODUCTIONSelf-organization in colloidal suspensions leads to a fascinating range of colloidal crystal and liquid crystalline (LC) phases. 1,2 Initially attention of both experiments and simulations was focused on spherical particles interacting through hard-core repulsion. 3−5 Subsequently, these studies were extended to colloids with anisotropic shapes, such as rods and plates, 6,7 and also to a variety of colloidal interactions. 8 Attraction (depletion, 9−12 van der Waals, 13,14 or Coulomb 13,15 ) and/or repulsion (steric 16 or Coulomb 17 ) were applied to govern the colloidal self-assembly process. Additionally, recognition mechanisms based on particles with complementary shapes 18 or on Watson−Crick attraction between DNA strands 19−22 have led to an extended control over the self-organization process.LC phase formation in suspensions of hard colloidal discs has been studied theoretically, 23,24 experimentally 25 and using computer simulations. 26 Already in the 1940s Lars Onsager qualitatively predicted a transition from a disordered isotropic (I) to an orientationally ordered nematic (N) phase in suspensions of hard platelet-like particles. 27 At higher colloidal concentrations, platelets can also form a columnar (C) phase, with orientational and 2D positional ordering. Experimentally, both LC phases (N and C) were found only in a single hard platelet system, namely, that of sterically stabilized gibbsite (γ-Al(OH) 3 ) platelets. 25 Charged colloidal platelets have been studied mostly experimentally. In addition to the typical for platelets I−N−C phase sequence, a columnar nematic 28 and lamellar 29−32 phases were observed. Moreover, even chiral liquid crystals were found in aqueous suspensions of graphehe oxide sheets. 33 The enrichment of the phase diagram of charged platelets is clearly due to the electrostatic repulsion between the platelets. Recently, Morales-Anda et al. 34 presented Monte Carlo computer simulation results for a model system of charged colloidal platelets. While the authors predict a columnar nematic phase characterized by interpenetrating columns, no evidence for any layered LC phase, such as smectic or lamellar, in charged platelet suspensions was found.In this article, we show that increasing the range of the electrostatic Coulomb repulsion leads to the formation of a new and unexpected LC phase in colloidal platelet suspensions. Specifically, we study monodisperse rigid gibbsite platelets ...

Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots

“…Recently, high-quality computer simulations on systems of charged colloidal platelets have been reported [63], which support the transitions from a discotic nematic phase (N D ), via a columnar nematic (N C ) to a columnar phase (C). However, the SmB phase observed by Kleshchanok et al [62] was not detected in these simulations. …”

Formation of liquid crystalline phases in aqueous suspensions of platelet-like tripalmitin nanoparticles

“…Recently, also a smectic B phase has been reported for suspensions of Gibbsite platelets. 9 For the Gibbsite suspensions it was found that on a microscopic level the formation of the nematic phase is preceded by the formation of nematic droplets (tactoids) in the isotropic phase. Finally these tactoids coalesce at sufficiently high concentrations to a nematic phase on a macroscopic scale.…”

Particle shape effects in colloidal crystals and colloidal liquid crystals: Small-angle X-ray scattering studies with microradian resolution

“…Interestingly, SAXS experiments revealed that the nematic phases at high salt concentrations or low platelet concentrations are of the usual (discotic) nematic type, while those at low salt/high platelet concentrations are of the columnar nematic (N C ) type, where increased lateral correlations indicate the formation of stacks of platelets. Kleshchanok et al [39] dispersed charged gibbsite platelets in dimethyl sulfoxide (DMSO), a polar aprotic solvent leading to a long range of the electrostatic Coulomb repulsion between platelets, and did report a smectic-B, a layered liquid-crystalline phase consisting of hexagonally ordered particles (without inter-layer correlations).…”