No abstract

Relations between thermodynamics, structural, and mechanical properties of Laponite suspensions were recently discussed in the literature. One important issue concerning the liquid/gel transition of the Laponite suspensions is to understand why a mechanical gel appears concomitantly with what appears as an incomplete nematic transition. To get some insight, we first give a more extended characterization of the viscoelastic properties of these suspensions near the liquid/gel transition. For this purpose, stress relaxation experiments are compared to direct determinations of the viscoelastic modulus in the frequency domain. This permits the following of viscoelastic properties, in the linear regime, on a very extended scale, from 10-5 to 102 rad/s. The data show that the relaxation mechanisms are very slow and are compatible with the presence of a large scale structural organization compared to the elementary particle size. The elastic modulus follows the power law:  G‘ = A(C − C 0)α. Only the concentration threshold varies with the ionic strength. In a second part, we compare, on the same system, how the osmotic pressure and the birefringent properties are correlated. As already shown by Gabriel et al., three optical domains can be defined, an isotropic liquid, an isotropic gel, and a birefringence gel, where numerous threadlike defects highly reminiscent of nematic texture are observed. An interesting new result is seen, a line that separates the isotropic and the birefringent gel coincides with the line where the plateau of the osmotic pressure ends up. Recalling that the osmotic plateau starts just at the liquid/solid transition, we propose a more complete phase diagram exhibiting a pseudobiphasic region with no macroscopic phase separation.

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

No abstract