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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.

Complementary neutron and X-ray small angle scattering results give prominent information on the asphaltene nanostructure. Precise SANS and SAXS measurements on a large q-scale were performed on the same dilute asphaltene-toluene solution, and absolute intensity scaling was carried out. Direct comparison of neutron and X-ray spectra enables description of a fractal organization made from the aggregation of small entities of 16 kDa, exhibiting an internal fine structure. Neutron contrast variation experiments enhance the description of this nanoaggregate in terms of core-shell disk organization, giving insight into core and shell dimensions and chemical compositions. The nanoaggregates are best described by a disk of total radius 32 Å with 30% polydispersity and a height of 6.7 Å. Composition and density calculations show that the core is a dense and aromatic structure, contrary to the shell, which is highly aliphatic. These results show a good agreement with the general view of the Yen model (Yen, T. F.; et al. Anal. Chem.1961, 33, 1587-1594) and as for the modified Yen model (Mullins, O. C. Energy Fuels2010, 24, 2179-2207), provide characteristic dimensions of the asphaltene nanoaggregate in good solvent.

We provide a comprehensive analysis of the microstructure of the porous glass, vycor. Using transmission electron microscopy, small-angle x-ray scattering, molecular adsorption, and the dynamic process of direct energy transfer, a consistent picture of the mass, pore, and interfacial features of this material is presented. From a transmission-electron-microscopy image of an ultrathin section of vycor the material appears to have a homogeneous distribution of mass with no hierarchical organization. The pore interface exhibits a roughness which is probed by both small-angle x-ray scattering and molecular adsorption. The roughness has an upper cutoff of <20 Å which is not resolved in the transmission-electron-microscopy image and is shown to be unimportant to the dynamics of the direct energy transfer process. The dimensionality probed by direct energy transfer is shown to be related to interfacial geometrical crossover from two dimensional to three dimensional, which is characterized by a persistent length of the interface of 45 Å.