Marta Gubitosi

Sugar-Bile Acid-Based Bolaamphiphiles: From Scrolls to Monodisperse Single-Walled Tubules.

The introduction of a mannose residue on carbon 3 of lithocholic acid gives rise to an asymmetric and rigid bolaamphiphilic molecule, which self-assembles in water to form elongated tubular aggregates with an outer diameter of about 20 nm. These tubular structures display a temporal evolution, where the average tube diameter decreases with time, which can be followed by time-resolved small-angle X-ray scattering experiments. Cryogenic transmission electron microscopy images collected as a function of time show that at short times after preparation tubular scrolls are formed via the rolling of layers, after which a complex transformation of the scrolls into single-walled tubules takes place. At long time scales, a further evolution occurs where the tubules both elongate and become narrower. The observed self-assembly confirms the tendency of bile acids and their derivatives to form supramolecular aggregates with an ordered packing of the constituent molecules. It also demonstrates that scrolls can be formed as intermediate structures in the self-assembly process of monodisperse single-walled tubules.

Characterization of carbon nanotube dispersions in solutions of bile salts and derivatives containing aromatic substituents.

Bile salts (BS) are known to solubilize high weight fractions of carbon nanotubes (CNTs) in aqueous solutions. Here, the efficiency of derivatives of bile salts (BSDs) containing aromatic substituents in dispersing single-wall CNTs (SWCNTs) has been investigated in order to check whether the presence of aromatic residues, because of their affinity toward carbon nanotube surfaces, determines improvements of the BS dispersion efficiency (DE). Electric arc and CoMoCAT SWCNTs were analyzed. The results, reported for the two surfactant concentrations of 0.06 and 1.0 wt %, show that the DE of BSDs depends on the position, orientation, and structure of the introduced aromatic residues. In the case of the CoMoCAT SWCNTs, at low surfactant concentration a DE improvement is observed in BSDs where the aromatic residue is linked either to carbon 3, located on the rigid four-ring system, or to the side chain. For the latter, this improvement is also enhanced in double-charge derivatives and kept at high surfactant concentration. It was also observed that at low concentrations of surfactant, the DE values of BSs and BSDs are usually larger than those of the more conventional detergent sodium dodecylsulfate.

Tailoring Supramolecular Nanotubes by Bile Salt Based Surfactant Mixtures

An approach for tailoring self-assembled tubular structures is described. By controlling the relative composition of a two-component surfactant mixture comprising the natural bile salt lithocholate and its bolamphiphilic derivative, it was possible to finely tune the nanotube cross-section of the mixed tubular aggregates that self-associated spontaneously in aqueous solution at pH 12. The diameter was found to vary up to 50% when the stoichiometric ratio of the two bile salts was changed. The tuning of supramolecular nanochannels with such remarkable precision is of significant interest for technological applications of these materials.

Multi stimuli response of a single surfactant presenting a rich self-assembly behavior

This work reports on the multi stimuli sensitivity of a cholic acid derivative in a buffer. The molecules self-organize in single walled tubules at room temperature and pH 8.0–9.5. By increasing the pH to 10.0–12.0, these tubules open up and form scrolls, which transform by aging into different tubular structures. A further evolution from scrolls or tubules into ribbons is induced by increasing the temperature. Moreover, a transition into rolled lamellae can be triggered by adding NaCl. All transitions are reversible and accompanied by drastic molecular rearrangements, manifested by spectroscopic and imaging techniques. Such rich multi stimuli responsiveness is usually accomplished by block copolymers or peptides and seldom by pure surfactants. The reported surfactant provides in addition an uncommon variety of structures. The sharpness of the spectroscopic response suggests that the system could be employed in physico-chemical sensors with a rather narrow dynamic range.

On cellulose dissolution and aggregation in aqueous tetrabutylammonium hydroxide

Aqueous tetrabutylammonium hydroxide, TBAH(aq), has been found to dissolve cellulose and to be a potential solvent for chemical processing or fiber spinning. In this paper, we have investigated the dissolution state of cellulose in 40 wt % TBAH(aq) solvent, and present an extensive study of rheology, combined with static light and small-angle X-ray scattering, to correlate cellulose aggregation with changes in the rheological parameters. Two cellulose molecular weights are compared. Microcrystalline cellulose (MCC), with a degree of polymerization of ca. 260, and a dissolving pulp with an approximately ten times higher molecular weight. Scattering data demonstrate that cellulose is molecularly dissolved at lower cellulose concentrations, while aggregates are present when the concentration exceeds a certain value. The onset of the aggregate formation is marked by a pronounced increase in the scattering intensity at low q, shear thinning behavior and violation of the empirical Cox-Merz rule. Additionally, the SAXS data suggest the presence of a solvation shell enriched in TBA(+) ions, compared to the bulk solvent. The results are consistent with the recent suggestion that while native cellulose I may still dissolve, solutions are, above a particular concentration, becoming supersaturated with respect to the more stable crystal form cellulose II.

On the dissolution of cellulose in tetrabutylammonium acetate/dimethyl sulfoxide : a frustrated solvent

We have found that the dissolution of cellulose in the binary mixed solvent tetrabutylammonium acetate/dimethyl sulfoxide follows a previously overlooked near-stoichiometric relationship such that one dissolved acetate ion is able to dissolve an amount of cellulose corresponding to about one glucose residue. The structure and dynamics of the resulting cellulose solutions were investigated using small-angle X-ray scattering (SAXS) and nuclear magnetic resonance techniques as well as molecular dynamics simulation. This yielded a detailed picture of the dissolution mechanism in which acetate ions form hydrogen bonds to cellulose and causes a diffuse solvation sheath of bulky tetrabutylammonium counterions to form. In turn, this leads to a steric repulsion that helps to keep the cellulose chains apart. Structural similarities to previously investigated cellulose solutions in aqueous tetrabutylammonium hydroxide were revealed by SAXS measurement. To what extent this corresponds to similarities in dissolution mechanism is discussed.

Stable, metastable and unstable cellulose solutions

We have characterized the dissolution state of microcrystalline cellulose (MCC) in aqueous tetrabutylammonium hydroxide, TBAH(aq), at different concentrations of TBAH, by means of turbidity and small-angle X-ray scattering. The solubility of cellulose increases with increasing TBAH concentration, which is consistent with solubilization driven by neutralization. When comparing the two polymorphs, the solubility of cellulose I is higher than that of cellulose II. This has the consequence that the dissolution of MCC (cellulose I) may create a supersaturated solution with respect to cellulose II. As for the dissolution state of cellulose, we identify three different regimes. (i) In the stable regime, corresponding to concentrations below the solubility of cellulose II, cellulose is molecularly dissolved and the solutions are thermodynamically stable. (ii) In the metastable regime, corresponding to lower supersaturations with respect to cellulose II, a minor aggregation of cellulose occurs and the solutions are kinetically stable. (iii) In the unstable regime, corresponding to larger supersaturations, there is macroscopic precipitation of cellulose II from solution. Finally, we also discuss strong alkali solvents in general and compare TBAH(aq) with the classical NaOH(aq) solvent.

On the stability of lithocholate derivative supramolecular tubules

The self-assembly of a mannose-labelled bile salt derivative gives rise to a metastable nematic phase of monodisperse nanotubes in aqueous solutions that are characterized by a crystalline order. This work is addressed to study the relative stability of these tubular aggregates in order to have full control of such a system for possible applications. By using a static light scattering method we evaluate both the solubilities of the metastable nanotubes and of stable nanocrystals, demonstrating that these are remarkably lower than the critical micellar concentration of typical bile salts and other ionic conventional surfactants. A partial stability map is developed by combining solubility and calorimetry data, where a nematic nanotube phase region is highlighted below 60–65 °C.

Twisted nanoribbons from a RGD-bearing cholic acid derivative

In light of the biomedical interest for self-assembling amphiphiles bearing the tripeptide Arg-Gly-Gly (RGD), a cholic acid derivative was synthesized by introducing an aromatic moiety on the steroidal skeleton and the RGD sequence on the carboxylic function of its chain 17-24, thus forming a peptide amphiphile with the unconventional rigid amphiphilic structure of bile salts. In aqueous solution, the compound self-assembled into long twisted ribbons characterized by a very low degree of polydispersity in terms of width (≈25nm), thickness (≈4.5nm) and pitch (≈145nm). It was proposed that in the ribbon the molecules are arranged in a bilayer structure with the aromatic moieties in the interior, strongly involved in the intermolecular interaction, whereas the RGD residues are located at the bilayer-water interface. The nanostructure is significantly different from those generally provided by RGD-containing amphiphiles with the conventional peptide-tail structure, for which fibers with a circular cross-section were observed, and successfully tested as scaffolds for tissue regeneration. From previous work on the use of this kind of nanostructures, it is known that features like morphology, rigidity, epitope spacing and periodicity are important factors that dramatically affect cell adhesion and signaling. Within this context, the reported results demonstrate that bile salt-based peptide surfactants are promising building blocks in the preparation of non-trivial RGD-decorated nanoaggregates with well-defined morphologies and epitope distributions.

Structural analysis of Ioncell-F fibres from birch wood

Quite recently, the Ioncell-F process, a novel and promising Lyocell fibre process, has been developed. The ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH]OAc) utilized in this process, was identified as an excellent cellulose solvent for dry-jet wet fibre spinning. Fibres spun from cellulose-[DBNH]OAc solution have shown excellent mechanical properties. Herein, various structural features of these ionic liquid-based fibres were analyzed and correlated with their mechanical properties. The highest slope of tenacity increase of these Ioncell-F fibres (from birch wood) observed at low draw ratios (DRs), while a continuous but slower increase occurs to partly very high draws. The improvements in the mechanical properties do not seem to correlate with changes in the crystallite size or the crystalline orientation based on wide-angle X-ray scattering (WAXS) results. More significant differences were seen for the sample crystallinity, the amorphous orientation (measured by birefringence), the orientation of the voids, the specific surface (measured by small-angle X-ray scattering (SAXS)), and the sorption/desorption properties of the fibres.