On Viscoelastic, Birefringent, and Swelling Properties of Laponite Clay Suspensions:  Revisited Phase Diagram

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.

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 .…”

“…At very low concentration (below 1.0 wt%) the slow gelation process, originating from the attractive interparticle interactions and the relevant clustering, is followed by an extremely slow phase separation between clay-poor and clay-rich phases on the year timescale . At even higher concentration (above 3 wt%) the formation of nematic microdomains was postulated on the basis of birefringence measurements between crossed polarizers (Mourchid et al, 1998). Upon increasing ionic strength above 10 −4 M, the role of electrostatic repulsions decreases in favor of attractive interactions between the oppositely charged edges and faces of the clay platelets.…”

Evidence of the hexagonal columnar liquid-crystal phase of hard colloidal platelets by high-resolution SAXS

“…The first ever observation of this kind was done by Langmuir [21], who reported in 1938 on sols of California bentonite clay particles that, after standing for several 100 hours, separated into two distinct phases -the isotropic and nematic phase. During the last decades, there have been quite a few studies on natural and synthetic clays, investigating their phase behaviour [22][23][24][25][26], rheological [27][28][29] and structural properties [30][31][32][33]. However, in almost all of the studies, clay suspensions were found to gel rather than phase separate (I-N), as was the case with Langmuir's sample.…”

Glass–glass transition during aging of a colloidal clay

“…Among these, Laponite suspensions, widely used in cosmetics, oil industry, paints, have emerged as intriguing systems with peculiar aging dynamics and unexpected phase behaviour [19][20][21][22][23] . Since the pioneering work of Mourchid et al 24 , a notable effort has been devoted to understand their very rich and debated state diagram. Different arrested states such as gels and glasses have been distinguished by varying clay and salt concentration 21,23,25,26 .…”