Synthesis of bridged-organosilica nanotubes with widely adjustable inner diameter based on temperature-dependent templating by swollen mixed surfactant micelles

The mixture of Pluronic F108 (EO132PO50EO132) and P104 (EO27PO61EO27) surfactants combined with toluene (swelling agent) templates ethylene-bridged organosilica nanotubes, whose size is widely adjustable by changing the initial synthesis temperature. While nanotubes of inner diameters ∼11 nm formed at room temperature, the temperature decrease led to the size increase and afforded nanotubes of particularly large inner diameters (from 25 to ∼35–40 nm) and thick walls at 7.5–9.25 °C, with outer diameters reaching ∼80 nm. The thick walls make these nanotubes resistant to collapse and suppress the nanotube aggregation. The stirring rate during the initial stage of the synthesis was an important factor in determining the uniformity and size of the ultra-large-diameter nanotubes. Slow stirring may lead to thin-walled organosilica nanotubes with outer diameters above 80 nm, showing that such structures can form, but their content and diameter uniformity were low. Our results suggest that the tubular soft-templating system transitions from the swollen micelle regime to the microemulsion regime, in which the inner core consists of the swelling agent only. In the latter case, the inner diameter of the surfactant-templated nanotubes, which is expected to be related to the size of the hydrophobic domain, may be enlarged to 35–40 nm (for good-quality products) or even beyond 40 nm (for low-purity products). Methylene-bridged organosilica nanotubes with tunable diameters, including very large ones, were also obtained, suggesting a broader scope of our pore diameter adjustment approach. Ethenylene- and xylylene-bridged organosilica nanotubes were also synthesized, indicating a wide applicability of our templating system.