Novel mechanism for nanotube formation from vesicle: A transition induced by the rearrangement of molecular pairs

It has been reported that vesicles formed from two simple, achiral chemicals, 2-phenylbenzimidazole 5-sulfonic acid sodium salt (PBSS) and cetyltrimethylammonium bromide (CTAB), could transform to thin nanotubes spontaneously. But the transition process was not well understood. In this paper, the transition was studied in detail using turbidity, UV-adsorption, NMR and TEM methods. As a result, a novel mechanism is presented based on the consideration of the shape of PBSS molecule and its rotation. According to the mechanism, a flat PBSS molecule has different packing parameter at its different side, and further consists molecular pairs with CTAB with different configurations, cylinder-like and cuplike, respectively. While the molecular pairs are randomly arranged, they will form large cuplike units and construct a vesicle. But while the cylinder-like molecular pairs are arranged side by side, they construct a structure with low curvature close to zero at lengthwise, and high curvature at crosswise, so, a thin and long nanotube is formed. Therefore, the transformation from vesicle to nanotube is substantially the configuration change of the molecular pairs. The driving force of the transition could be related to the extenuation of the rotation of the PBSS molecules due to the hindering of the neighbour molecules.