The formation of supramolecular structures initiated by a p‐tert‐butylphenyl‐amide derivative of cholic acid is investigated. The initial spherical vesicles, with a rather low effective bending constant, collapse into necklaces that self‐transform into tubules of small diameter. Finally, molecular tubes are generated (see figure). During the process, the geometrical constraints of fixed surface area and fixed enclosed volume are obeyed.

We report a kinetic study of the supramolecular tubule formation of the bile salt derivative [3 beta, 5 beta, 7 alpha, 12 alpha]-3-(4-t-butylbenzoilamine)-7,12-dihydroxycholan-24-oic acid sodium salt (Na-tbutPhC). At high bicarbonate buffer concentration (pH similar to 10) this salt shows gelator properties. Starting from gels or viscous solutions, the tubule formation is triggered by increasing the temperature beyond the critical value of 34-36 degrees C. For gels, when the process takes place, the transition to sols occurs. The process is easily triggered and can be followed by several techniques. We used static light scattering (SLS), circular dichroism (CD), small angle X-ray scattering (SAXS) along with transmission electron (TEM) and optical microscopies. The CD results show that fibrils with a clockwise arrangement of the bile salt derivative are present in the samples at room temperature. When the tubule formation starts, evolutions of the CD and SLS profiles are observed indicating that the formation process begins with the aggregation of the fibrils accompanied by a simultaneous peculiar reciprocal reorientation of the surfactant molecules. After that, as pointed out by the long time evolution of the curves, a slow transformation towards the final well defined tubules occurs, involving an adjustment of the molecular packing. In the meanwhile, the slow ordering of the tubule walls in well spaced layers takes place, as inferred by SAXS. The TEM images show that short disordered tubules are formed, because of the aggregation of fibrils, in the beginning. Moreover they highlight a final elongation of the tubules taking place without a further aggregation of fibrils. Optical microscopy frames, collected during the process, point out that the tubules grow singly even at quite a high concentration, thus supporting the data interpretation

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The interactions between three beta-cyclodextrin hosts (having 1-3 binding sites) and two adamantyl guests (having 1-2 binding sites) have been studied by ITC, ROESY, static and dynamic light scattering (SLS and DLS), and AFM and TEM techniques. The enthalpy and free energy values (determined from ITC experiments) evidence that the single interaction between one binding site of the guest and one binding site of the host is independent of the number of binding sites of the interacting species. The average values are deltaH degrees = -26.6 +/- 2.3 kJ mol(-1) and deltaG degrees = -30.4 +/- 3.2 kJ mol(-1), indicating that the process is mainly enthalpy driven. In all cases, the experimental molar ratio (from ITC experiments) agrees with the expected one from the number of binding sites of both the host and guest. The formation of polymer-like entities was demonstrated by SLS, DLS, AFM, and TEM measurements. The structure of polymers is linear when both the host and the guest are ditopic entities and dendritic (or Cayley tree type) when the host and the guest have three and two binding sites, respectively.

The three-dimensional structures with nanoscopic dimensions that are yielded by the self-assembly of lipids and surfactants are of particular interest for their applications in nanotechnology. In these applications, the possibility of controlling the charge of the particles allows the regulation of fundamental aspects, such as the ability of the particles to load molecules (drugs, DNA, proteins, etc.), to aggregate, and to penetrate membranes. Within the possible surfactant supramolecular architectures, tubular structures have recently drawn much research interest.[1] The main reason is that micro-and nanoscopic tubules have many interesting potential applications in nanotechnology, involving for example catalysis, selective separations, [2] sensors, electronic, electrochemical, and field emission devices, [3] tissue engineering, [4] and preparations of template nanostructured materials [5] or of prospective nanoscopic networks.[6] Because of this interest, several families of compounds have been studied that selfassemble in tubules, [7] for example phospholipids, [8] glycolipids, [9] peptides, [5,10] polymers, [11] bile salts, [12] and rationally designed amphiphiles.[13]Herein, we report on the preparation of tubules in aqueous solutions of mixed cationic and anionic amphiphiles, whose compositions and charges can be tuned by controlling the stoichiometry of the mixtures. In particular, it was found that a very dilute mixture of anionic (ACD) and cationic (CCD) derivatives of the bile salt sodium cholate (Scheme 1) forms tubules over the whole range of the investigated anionic/cationic surfactant molar ratio. An interval within this wide range was found in which variation of the tubule composition and charge occurs.Mixtures of anionic and cationic surfactants in water have been extensively investigated for many years; [14] however, the behavior described herein has never been observed. Commonly, in conditions similar to those adopted in this work, domains of cationic and anionic rich vesicles and micelles can be recognized in the phase diagram of these systems. Twophase regions and precipitates around the equimolar composition are observed as well. [15] Details of the syntheses of ACD and CCD are given in the Supporting Information. Two sets of mixtures of ACD and CCD were prepared with total surfactant concentrations (c T ) of 0.80 and 0.40 mm, as reported in the Experimental Section. In view of the thermal stability of the previously studied pure ACD tubules, [16] the samples were measured at 40 8C. The mixture composition is reported as molar fraction of each surfactant defined as X i = n i /n tot (where n i and n tot are the number of moles of the component i (CCD or ACD) and of the total surfactant, respectively).The 0.4 and 0.8 mm solutions of neat ACD and CCD are transparent ( Figure S1 in the Supporting Information). Transmission electron microscopy (TEM) images of these samples did not show any aggregate structure even after aging (3 months). Moreover, critical aggregation concentrations of around 0.4 ...