Liquid-solid transition of Laponite suspensions at very low ionic strength: Long-range electrostatic stabilisation of anisotropic colloids

Abstract

We study suspensions of synthetic clay Laponite at very low ionic strength. We show the existence, for these charged disk-like particles, of a liquid-soft solid transition mainly driven by electrostatic repulsive interactions. Such a process defines a re-entrant transition line in the phase diagram. Location of this line is predicted using basic arguments. The structure is characterized by ultra-small-angle X-ray scattering (USAXS). Soft-solid suspensions show a correlation peak compatible with long-range electrostatic stabilization. Such a result strongly contrasts with the evolution of the scattering spectra for solid-like suspensions of Laponite at high ionic strength (above 10 −4 M). Close inspection of this correlation peak reveals that individual particle distribution is not homogeneous in space.Colloidal systems undergo phase transitions such as liquid-solid, order-disorder, sol-gel or glass transition [1-6], which are of important technical and scientific interest. Phase transitions involving, for example, purely repulsive interactions (the so-called Kirkwood-Alder transition [3]) are now well established [3][4][5][6] for spherical charged particles (latex, SiO 2 ). This is not the case for suspensions of charged disk-like colloids where the origin and the status of the so-called phase diagram are still under debate [7][8][9][10][11][12]. A demonstrative example is the case of Laponite, a synthetic smectite clay which can be considered, on average, as a hard disc having a thickness of 1nm, an average diameter of 300 Å and a bulk density of 2.65 • 10 6 kg/m 3 . This material has recently generated a good deal of experimental and theoretical studies [8][9][10][11][12][13][14][15][16][17][18][19]. Above an ionic strength of 10 −4 M, several authors have observed a transition line from a liquid to a soft solid when the solid concentration C (expressed as the mass of solid over the

Aging dynamics in a colloidal glass

“…These repulsions originate from the strong surface charges at the faces of the colloidal disks, making that the effective volume fraction is very high. Recent experiments have shown that the location of the "glass transition" line in the (volume fraction / electrolyte concentration) phase diagram is consistent with this assumption [17].…”

Rheology of Laponite-scleroglucan hydrogels

“…Numerous investigations have been addressed to define the state diagram of aqueous Laponite dispersions in the ionic strength vs clay concentration plane, i.e. to individuate the different regions of isotropic liquids, disordered (gels and glasses), ordered (nematic phases), flocculated states (Cummins, 2007;Gabriel, Sanchez, & Davidson, 1996;Jabbari-Farouji, Tanaka, Wegdam, & Bonn, 2008;Levitz, Lécolier, Mourchid, Delville, & Lyonnard, 2000;Mourchid, Lécolier, Van Damme, & Levitz, 1998;Mourchid, Delville, Lambard, Lécolier, & Levitz, 1995;Mongondry, Tassin, & Nicolai, 2005;Ruzicka, Zulian, & Ruocco, 2004;Ruzicka, Zulian, & Ruocco, 2006;Tanaka, Meunier, & Bonn, 2004;Tanaka, Jabbari-Farouji, Meunier, & Bonn, 2005). The various contradictions emerging from the comparison of the proposed Laponite dispersion state diagrams are partly apparent and can be mainly ascribed to different aging times and, secondarily, to different protocols of samples preparation or Laponite type .…”

Cluster, Glass, and Gel Formation and Viscoelastic Phase Separation in Aqueous Clay Suspensions

“…Further evidence for such a Wigner glass at low electrolyte concentrations came from the rheological measurements of Rand et al 44 and from many other experiments. 10,13,25,45 By contrast, it has recently been hypothesized that Laponite suspensions could behave like an attractive glass. 17 A comprehensive discussion of different possible glassy states in Laponite suspensions, including comparison with experimental data, was recently given by Mongondry et al 46 and Tanaka et al 15,17 The majority of clay gelation studies have been done with Laponite suspensions due to their relatively high longitudinal monodispersity in comparison with natural clays.…”