No abstract

The excitement about self-assembled molecular structures results from their potential capabilities to mimic various biological systems and their potential for use in the fabrication of structures [1,2] for advanced materials and nanoscience applications. Both fundamentally and materially, the pioneering work of Schnur and co-workers [3] has produced experimental evidence that phospholipid tubule formation is based on chiral packing of the lipids. [4,5] In that context, the control of the monodispersity and orientation distribution of the nanostructures is of utmost importance. It has been previously reported [6,7] that dispersing lithocholic bile acid (LCA) in sodium hydroxide aqueous solutions leads to the formation of organic nanotubes (NaLC) with well-defined diameters on the nanometer scale. Cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS) were used as complementary techniques to characterize the structures of the nanotubes and their intermediary species. The tubes, of 52 nm diameter, are uniform and have an internal cavity of 49 nm diameter. The NaLC tubes studied by SAXS were shown to keep their structural features up to 57°C. The ionic strength and type of counterion may alter the aggregation properties in self-assembled ionic systems. Cryo-TEM measurements have shown that the NaLC structures were unaffected by high NaCl contents. In the present study, we investigate the role of the counterion (NH 4 + versus Na + ) in the tube formation and the orientation distribution of NH 4 LC tubes in aqueous suspensions. A theoretical model [8] describes the formation of tubes through unstable precursors like helical ribbons in a process independent of the electrostatic interactions. Nevertheless, other experiments with tubules of amphiphiles having charged head groups (e.g., acidic diacetylenic phospholipids) [9] have shown that the nature of the head group, as well as that of the counteranion, affect the tubule diameters. NH 4 LC suspensions appear with a decreased consistency, and this intriguing behavior motivates a deeper consideration of the electrostatic context. A previous study of the kinetics of tube formation [7] in NaOH has suggested that thin fibrils can act as precursor species of helical ribbons, finally closing into tubules. Considering the lowered consistency of ammonia suspensions of LCA, a decrease of the average contour length of the 1D species (fibrils, ribbons, tubes) is suspected. Self-assembled aggregates are in thermal equilibrium and their statistical length can also be varied with the equilibration temperature. The present work investigates the ability of LC systems to develop nanotubes with well-defined cross sections that can be easily oriented by adjustment of the type of counterion (from Na + to NH 4 + ) and temperature. It is shown that a strikingly well-ordered hexagonal organization of the NH 4 LC tubes is obtained. The observation is made at higher concentrations than those mentioned in the previous cryo-TEM studies (ca. 2.5 versus 0.1 wt %) [6...