Room-temperature ionic liquids (RTILs) are organic salts that are characterized by low melting points. They are considered to possess a homogeneous microscopic structure. We provide the first experimental evidence of the existence of nanoscale heterogeneities in neat liquid and supercooled RTILs, such as 1-alkyl-3-methyl imidazolium-based salts, using X-ray diffraction. These heterogeneities are of the order of a few nanometers and their size is proportional to the alkyl chain length. These results provide strong support to the findings from recent molecular dynamics simulations, which proposed the occurrence of nanostructures in RTILs, as a consequence of alkyl chains segregation. Moreover, our study addresses the issue of the temperature dependence of the heterogeneities size, showing a behavior that resembles the density one only below the glass transition, thus suggesting a complex behavior above this temperature. These results will provide a novel interpretation approach for the unique chemical physical properties of RTILs.

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International audienceWe investigate the dispersion mechanisms of nanocomposites made of well-defined polymer (polystyrene, PS) grafted-nanoparticles (silica) mixed with free chains of the same polymer using a combination of scattering (SAXS/USAXS) and imaging (TEM) techniques. We show that the relevant parameter of the dispersion, the grafted/free chains mass ratio R tuned with specific synthesis process, enables to manage the arrangement of the grafted nanoparticles inside the matrix either as large and compact aggregates (R \textless 0.24) or as individual nanoparticles dispersion (R \textgreater 0.24). From the analysis of the interparticles structure factor, we can extract the thickness of the spherical corona of grafted brushes and correlate it with the dispersion: aggregation of the particles is associated with a significant collapse of the grafted chains, in agreement with the theoretical models describing the free energy as a combination of a mixing entropy term between the free and the grafted chains and an elastic term of deformation of the grafted brushes. At fixed grafting density, the individual dispersion of particles below the theoretical limit of R = 1 can be observed, due to interdiffusion between the grafted and the free chains but also to processing kinetics effects, surface curvature and chains poly dispersity. Mechanical analysis of nanocomposites show the appearance of a longer relaxation time at low frequencies, more pronounced in the aggregated case even without direct connectivity between the aggregates. Correlation between the local structure and the rheological behavior suggests that the macroscopic elastic modulus of the nanocomposite could be described mainly by a short-range contribution, at the scale of the interactions between grafted particles, without significant effect of larger scale organizations

In colloidal systems, the interplay between the short range attraction and long-range repulsion can lead to a low density associated state consisting of clusters of individual particles. Recently, such an equilibrium cluster phase was also reported for concentrated solutions of lysozyme at low ionic strength and close to the physiological pH. Stradner et al. [(2004) Equilibrium cluster formation in concentrated protein solutions and colloids. Nature 432:492-495] found that the position of the low-angle interference peak in small-angle x-ray and neutron scattering (SAXS and SANS) patterns from lysozyme solutions was essentially independent of the protein concentration and attributed these unexpected results to the presence of equilibrium clusters. This work prompted a series of experimental and theoretical investigations, but also revealed some inconsistencies. We have repeated these experiments following the protein preparation protocols of Stradner et al. using several batches of lysozyme and exploring a broad range of concentrations, temperature and other conditions. Our measurements were done in multiple experimental sessions at three different high-resolution SAXS and SANS instruments. The lowionic-strength lysozyme solutions displayed a clear shift in peak positions with concentration, incompatible with the presence of the cluster phase but consistent with the system of repulsively interacting individual lysozyme molecules. Within the decoupling approximation, the experimental data can be fitted using an effective interparticle interaction potential involving short-range attraction and long-range repulsion. dynamic arrested state ͉ macromolecular solutions ͉ protein interactions ͉ small-angle scattering ͉ structure factor T he arrested dynamics of colloidal systems and protein solutions interacting via short-range interactions have been actively studied both theoretically and experimentally in recent years (1-4). The mode coupling theory and molecular dynamics (MD) simulations have successfully unified seemingly dissimilar dynamical arrest scenarios in colloidal systems (4, 5). In addition to the conventional glassy state induced by the packing constraints, the presence of short-range attraction leads to a different glassy behavior. The apparently diverse type of dynamical arrest, such as gelation, jamming, glassification or non-ergodicity transition, etc., found in attractive systems can be unified in terms of this attractive glass transition (3). As competing shortrange attraction and long-range repulsion are introduced, additional features are observed (5). In particular, at intermediate volume fractions, the colloidal particles can form an equilibrium cluster phase, which in turn stabilizes a low-density arrested state (6). This type of particle clustering process at low volume fractions has been observed for various colloidal systems (7-9).Although the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory successfully describes the microstructure and equilibrium phase behavior of charged colloidal systems ov...

We have characterized the steps involved in silk assembly from the protein solution into beta-type fibers by a combination of small-angle and wide-angle X-ray scattering and Raman spectroscopy. The aggregation process was studied in a concentric flow microfluidic cell, which allows mimicking the spinning duct. The fibroin molecule in solution shows an elongated shape with a maximum diameter of 38 nm. During the pH-driven initial assembly step, large-scale aggregates of fibroin molecules with a maximum diameter of about 260 nm are formed. Raman spectroscopy on the dried, fibrous material shows a principally alpha-helical silk I secondary structure, which is transformed gradually into beta-type silk II by increasing immersion times in water. The formation of crystalline beta-sheet domains within the fiber is confirmed by wide-angle X-ray scattering. The assembly process resembles the peptide condensation-ordering model proposed for amyloid cross-beta formation.