Liquid-Crystalline Nematic Phase in Aqueous Suspensions of a Disk-Shaped Natural Beidellite Clay E. Paineau,* ,† K. Antonova, ‡ C. Baravian, § I. Bihannic, † P. Davidson, | I. Dozov, ‡ M. Impe ´ror-Clerc, | P. Levitz, ⊥ A. Madsen, # F. Meneau, ∇ and L. J. Michot* ,† Laboratoire EnVironnement et Mine ´ralurgie, Nancy UniVersity CNRS-INPL UMR 7569, BP40 54501 VandœuVre Cedex France, Institute of Solid State Physics, Bulgarian Academy of Sciences, BouleVard Tzarigradsko Chaussee 72, Sofia, 1784, Bulgaria, Laboratoire d’Energe ´tique et de Me ´canique The ´orique et Applique ´e, Nancy UniVersity UMR 7563 CNRS-INPL-UHP, 2, AVenue de la Fore ˆt de Haye, BP160 54504 VandœuVre Cedex, France, Laboratoire de Physique des Solides, UMR 8502 CNRS-UniVersite ´ Paris-Sud Ba ˆt 510 91405 Orsay Cedex France, Laboratoire de Physique de la Matie `re Condense ´e, UMR 7643 CNRS-Polytechnique Ecole Polytechnique 91128 Palaiseau Cedex, France, European Synchroton Radiation Facility, BP220, 38043, Grenoble, France, and Synchrotron SOLEIL, BP48, 91192, Gif-sur-YVette Cedex France ReceiVed: August 28, 2009; ReVised Manuscript ReceiVed: October 21, 2009 After size-selection and osmotic pressure measurements at fixed ionic strength, the behavior of aqueous colloidal suspensions of anisotropic disklike beidellite clay particles has been investigated by combining optical observations under polarized light, rheological, and small angle X-ray scattering (SAXS) experiments. The obtained phase diagrams (volume fraction/ionic strength) reveal, for ionic strength below 10 -3 M/L, a first- order isotropic/nematic (I/N) phase transition before gel formation at low volume fractions, typically around 0.5%. This I/N transition line displays a positive slope for increasing ionic strength and shifts toward lower volume fraction with increasing particle size, confirming that the system is controlled by repulsive interactions. The swelling laws, derived from the interparticle distances obtained by SAXS, display a transition from isotropic swelling at low volume fractions to lamellar swelling at higher volume fractions. The liquid-crystal properties have then been investigated in detail. Highly aligned nematic samples can be obtained in three different ways, by applying a magnetic field, an ac electric field, and by spontaneous homeotropic anchoring on surfaces. The birefringence of the fluid nematic phase is negative with typical values around 5 × 10 -4 at a volume fraction of about 0.6%. High nematic order parameters have been obtained as expected for well- aligned samples. The nematic director is aligned parallel to the magnetic field and perpendicular to the electric field. Introduction Suspensions of anisotropic colloidal particles can exhibit phase transitions due to spontaneous self-assembling processes that lead to various liquid-crystalline phases. 1 Nematic ordering in suspensions of rodlike particles was probably first reported by Zocher for V 2 O 5 suspensions. 2 As theoretically rationalized by Onsager, an isotropic (I) to nematic (N) phase transition can occur for rodlike particles as the result of a competition between orientational entropy and the packing entropy governed by excluded-volume interactions. 3 Liquid-crystalline ordering has indeed been observed in numerous colloidal suspensions of rodlike particles and, among these, mineral ones have lately been the focus of a renewed interest. 4-7 Actually, Onsager’s ideas also apply to suspensions of platelike particles, as was later confirmed by computer simulations, 8 and liquid-crystalline order has also been recently reported for suspensions of disklike crystallites, like gibbsite, a system synthesized and investigated in-depth by scientists of the Van’t Hoff Laboratory in Utrecht 9-13 or synthetic monodisperse nickel hydroxide colloids that also leads to columnar phases 14,15 In this context, the case of colloidal aqueous suspensions of swelling clays is less clear. Swelling clay minerals are composed of an octahedral sheet (Al, Fe, Mg) sandwiched between two opposite tetrahedral sheets (Si, Al, Fe). This 2/1 structure carries a negative charge due to isomorphic substitutions that are compensated by exchangeable cations present in the interlayer space. These materials are highly anisotropic with typical aspect ratios between 20 and 1000 and thicknesses below a nanometer. When fully exfoliated in water, they may provide suspensions likely to display liquid-crystalline order. Such a phenomenon was indeed reported by Langmuir for suspensions of California hectorite. 16 However, as he mentioned in his original paper, this observation was not reproducible. Later studies performed with either laponite, a synthetic clay, 17-20 or natural swelling clays 21 have not allowed the detection of a true thermodynamic liquid- crystalline ordering because of the existence at low volume fractions of a sol/gel transition. Nevertheless, nematic textures have been reported for gel samples 21-23 while scattering techniques 24-30 or X-ray microscopy studies 31 have revealed nematic-like orientational order in the gel phase. Recently, Michot et al. studied the phase behavior of aqueous suspensions of a natural clay mineral, namely nontronite, a Fe- * Towhomcorrespondenceshouldbeaddressed.E-mail:laurent.michot@ ensg.inpl-nancy.fr. † Nancy University CNRS-INPL UMR 7569. ‡ Bulgarian Academy of Sciences. § Nancy University UMR 7563 CNRS-INPL-UHP. | UMR 8502 CNRS-Universite ´ Paris-Sud. ⊥ UMR 7643 CNRS-Polytechnique. # European Synchroton Radiation Facility. ∇ Synchrotron SOLEIL. J. Phys. Chem. B 2009, 113, 15858–15869 15858 10.1021/jp908326y CCC: $40.75  2009 American Chemical Society Published on Web 11/11/2009