title: Phase diagrams of Wyoming Na-montmorillonite clay. Influence of particle anisotropy.
abstract: Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.
authors.affiliation: Laboratoire Environnement et Minéralurgie, UMR 7569 CNRS-INPL-ENSG, 15 Avenue du Charmois, BP 40 54501 Vandoevre Cedex, France. laurent.michot@ensg.inpl-nancy.fr
authors.forename: Laurent J
authors.initials: LJ
authors.lastname: Michot
authors.forename: Isabelle
authors.initials: I
authors.lastname: Bihannic
authors.forename: Katharina
authors.initials: K
authors.lastname: Porsch
authors.forename: Solange
authors.initials: S
authors.lastname: Maddi
authors.forename: Christophe
authors.initials: C
authors.lastname: Baravian
authors.forename: Julien
authors.initials: J
authors.lastname: Mougel
authors.forename: Pierre
authors.initials: P
authors.lastname: Levitz
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D016880:Anisotropy; D001546:Bentonite / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D000077215:Clay; D009997:Osmotic Pressure; D010316:Particle Size; D044367:Phase Transition; D012212:Rheology / Q000379:methods; D012964:Sodium / Q000737:chemistry*; D014959:Wyoming
chemical_list: D000538:Aluminum Silicates; D001546:Bentonite; D012964:Sodium; D000077215:Clay