Formation of Double-Helical Structures by Silica Nanotubes Templated by Mixtures of Common Nonionic Surfactants in Aqueous Solutions

Micelles of Pluronic F108 (EO₁₃₂PO₅₀EO₁₃₂)/P104 (EO₂₇PO₆₁EO₂₇) surfactant mixtures swollen with toluene were found to template silica nanotubes that formed double-helical structures under appropriately selected aqueous acidic solution conditions. In particular, the double-helical nanotube structure (DHNTS) formed as a main product at 15 °C for 30-37.5 wt % of Pluronic P104 in a surfactant mixture, with 35 wt % being particularly suitable. The formation of DHNTSs appears to involve a spontaneous wrapping of micelle-templated nanotubes around one another, while a similar structure was known to form only under confinement of anodic alumina pores of appropriate diameter. In addition to DHNTSs, other helical or circular structures, such as a helical nanotube tightly wrapped around a straight nanotube, or nanotube(s) wrapped around a sphere, were observed in many cases as minor components. DHNTSs formed as a major component at a well-defined proportion of silica precursor to surfactant at 15 °C, while the relative amount of the swelling agent and the hydrochloric acid concentration could be varied considerably. The hydrothermal treatment temperature was used to adjust the pore diameter of the DHNTS. However, structures formed without the hydrothermal treatment or with the treatment at a moderate temperature appeared very soft, while the treatment at excessively high temperature resulted in a development of significant gaps in the nanotube walls. Our results establish DHNTS as a well-defined ordered mesoporous silica with ultralarge (∼35 nm) helical mesopores of some degree of diameter adjustability, accessible under aqueous conditions using common nonionic surfactants as templating agents.