Self-assembled nanoribbons and nanotubes in water: energetic vs entropic networks

We present a comparative investigation of two opposite classes of self-assembled fibrillar networks. Ribbons and tubes having cross-sectional dimensions in the nanoscale can be formed in aqueous solutions of steroids derived, respectively, from deoxycholic (DC) and lithocholic (LC) acids. Rheological features distinguish energetic networks of DC ribbons rigidly fixed in cylindrical bundles and entropic transient networks of LC tubes weakly interacting in shear-sensitive suspensions. The two classes are characterized by their frequency sweep profiles, viscoelastic linear domains, scaling laws of the elastic shear modulus vs concentration, kinetics of formation of the networks, and their optical birefringence aspects. A theoretical context for networks of rigid fibers is used to account for the scaling exponents α in the G' (and σ ) ∝Cα laws (α =2.0 and 1.0, respectively, for DC and LC). The evolution observed in DC gels from ribbons to cylindrical fibers with monodisperse sections made up with four ribbons is an indication of an equilibrated balance between face-to-face attractions and untwisting elastic processes of the constitutive ribbons

Direct visualization of lipid aggregates in native human bile by light- and cryo-transmission electron-microscopy

AbstractThe evolution of microstructures present in human gallbladder and hepatic bile was observed simultaneously by video-enhanced light microscopy (VELM) and transmission electron microscopy of vitrified specimens (cryo-TEM), as a function of time after withdrawal from patients. Fresh centrifuged gallbladder bile samples contained small (6 nm) spherical micelles in coexistence with vesicles (40 nm). Out of the seven bile samples investigated four contained, in addition, two types of elongated aggregates that have not been previously described. Uncentrifuged gallbladder bile also contained a mixture of ribbon- and plate-like crystals seen by VELM, but not by cryo-TEM. In aged (3–6-week-old) gallbladder bile samples VELM also revealed spiral and helical crystal structures. No such crystals were present in hepatic bile samples, although microcrystals, not observable by VELM were seen by cryo-TEM in addition to micelles and vesicles. The similarity of these observations to those observed in bile models lends strong support for the validity of the model systems. Furthermore, the presence of microcrystals in hepatic bile samples, apparently devoid of crystals by light microscopy, indicates that under certain conditions the common criterion of ‘nueleation time’ (NT), based on light microscopy, does not represent the real time of nucleation. In the human bile samples investigated in this study the dissociation between NT and the time of observation of microcrystals was seen in hepatic but not in gallbladder bile samples. Hence, crystal growth may be rate limiting only in dilute biles

Extraction of Boron Nitride Nanotubes and Fabrication of Macroscopic Articles Using Chlorosulfonic Acid

Due to recent advances in high-throughput synthesis, research on boron nitride nanotubes (BNNTs) is moving towards applications. One future goal is the assembly of macroscopic articles of high aspect ratio, pristine BNNTs. However, these articles are presently unattainable because of insufficient purification and fabrication methods. We introduce a solution process for extracting BNNTs from synthesis impurities without sonication or the use of surfactants and proceed to convert the extracted BNNTs into thin films. The solution process can also be used to convert assynthesized materialwhich contains significant amounts of hexagonal boron nitride (h-BN) into mats and foams with controllable structure and dimension. The scalable solution extraction method, combined with further advances in synthesis and purification, contributes to the development of all-BNNT macroscopic articles, such as fibers and 3-D structures

Enhanced ordering in length-polydisperse carbon nanotube solutions at high concentrations as revealed by small angle X-ray scattering

Carbon nanotubes (CNTs) are stiff, all-carbon macromolecules with diameters as small as one nanometer and few microns long. Solutions of CNTs in chlorosulfonic acid (CSA) follow the phase behavior of rigid rod polymers interacting via a repulsive potential and display a liquid crystalline phase at sufficiently high concentration. Here, we show that small-angle X-ray scattering and polarized light microscopy data can be combined to characterize quantitatively the morphology of liquid crystalline phases formed in CNT solutions at concentrations from 3 to 6.5 % by volume. We find that upon increasing their concentration, CNTs self-assemble into a liquid crystalline phase with a pleated texture and with a large inter-particle spacing that could be indicative of a transition to higher-order liquid crystalline phases. We explain how thermal undulations of CNTs can enhance their electrostatic repulsion and increase their effective diameter by an order of magnitude. By calculating the critical concentration, where the mean amplitude of undulation of an unconstrained rod becomes comparable to the rod spacing, we find that thermal undulations start to affect steric forces at concentrations as low as the isotropic cloud point in CNT solutions

Cellulose-stabilized oil-in-water emulsions: structural features, microrheology, and stability

Cellulose-based oil-in-water (O/W) emulsions were studied by diffusing wave spectroscopy (DWS) regarding the effect of the cellulose concentration and mixing rate on the average droplet size, microrheological features and stability. Furthermore, the microstructure of these emulsions was imaged by cryo-scanning electron microscopy (cryo-SEM). The micrographs showed that cellulose was effectively adsorbed at the oil-water interface, resembling a film-like shell that protected the oil droplets from coalescing. The non-adsorbed cellulose that was observed in the continuous aqueous medium, contributed to the enhancement of the viscosity of the medium, leading to an improvement in the stability of the overall system. Generally, the higher the cellulose concentration and mixing rate, the smaller the emulsion droplets formed, and the higher was their stability. The combination of both techniques, DWS and cryo-SEM, revealed a very appealing and robust methodology for the characterization and design of novel emulsion-based formulations.FCT: PTDC/ASP-SIL/30619/2017, UIDB/05183/2020 / CEECIND/01014/2018info:eu-repo/semantics/publishedVersio