Polymerisation of aminopropyltrialkoxysilane in the presence of carboxylate: a new layered organosilica mesocomposite built up using intermolecular interactions with LB film-type self-assembly

Abstract

A new layered organosilicate with a multi-layer LB film-like structure, in which the carboxyl groups are connected to each other via a 3-aminopropylsilicate layer, is successfully synthesized by a self-assembly process using alkanoic acid (C_nH_2n+1COOH, n > 10) combined with the co-condensation of 3-aminopropyltriethoxysilane. The structure is elucidated by elemental analysis, X-ray diffraction, ¹³C and ²⁹Si MAS-NMR, infrared spectroscopy and thermogravimetry. The combined results of these techniques demonstrate that the alkyl groups are not packed in a crystalline state, but exist in a liquid-like state in a well-developed lamellar structure with the formula C_nH_2n+1COOHNH₂CH₂CH₂CH₂SiO_1.5. In the extremely thin siloxane layer that is developed in the lamellar structure, nearly all of the silicon atoms are bound to one carbon atom, three -OSi groups and no -OH groups. This structure (one silicon atom bound to one 3-aminopropyl group) possibly contains the most highly-populated aminopropyl group among all of the aminopropylsilicates that have ever been reported. The LB film-like interlayer can be exchanged with other alkanoic acids to form another lamellar structure in the same series that possesses the appropriate layer spacing. A fast delamination (<5 min) process is observed after the addition of Fe³⁺, which implies a rapid diffusion of transition metal cations, and which is also consistent with the liquid-like nature of the alkyl group assembly. The specific amount of Fe³⁺ sorption reaches 5.1 mmol g⁻¹, probably the highest adsorption capacity ever reported among the aminopropylsilicates.