Biosurfactant-mediated one-step synthesis of hydrophobic functional imogolite nanotubes

Abstract

Imogolites are aluminosilicate nanotubes that attract a particular interest due to their insulating properties, surface reactivity and shape-selective properties in catalysis. Although being available in nature, synthetic protocols have been developed since the 70's to obtain large amounts of this particular mineral without purification processes. In particular, surface functionalized imogolites are currently studied for their application as a dispersant in hydrophobic matrixes like polymers. With respect to known synthetic processes, natural imogolite is produced by using either milli- or deci-molar amounts of aluminium salts in solution while surface functionalized imogolites are systematically prepared in two steps: imogolite synthesis followed by surface functionalization mainly using phosphonic acids. Here, we propose a new one-step synthesis involving a cheap, widely-used, non-toxic surfactant, glycerol α-monolaurate (MG) and centimolar aluminium concentrations. In the presence of Al³⁺ cations, MG forms an emulsion, and the subsequent addition of tetraethoxysilane leads to the formation of imogolite nanotubes within 3 days at neutral pH under hydrothermal conditions. An interesting, new, lamellar long-range order of the nanotubes can then be achieved after 10 days. Under similar conditions, blank tests performed on MG-free systems result in the formation of proto-imogolite. Based on the combination of highly selective ²⁹Si, ²⁷Al and ¹H solid-state NMR experiments and additional synthesis using lauric acid (LA) and glycerol, a mechanism of formation is proposed based on MG dissociation and LA binding on the outside surface of the nanotubes. As a result, our process provides imogolite with unprecedented hydrophobic surface properties that offer new perspectives for these materials as nanoscale fillers in, e.g., polymeric media.