This lecture describes strategies for synthesizing inorganic and hybrid materials using anisotropic templates based on molecular organogelators. Organogelators are small molecules which, in contact with certain solvents (the nature of the solvent depending on the structure of the gelling molecule), form physical gels at very low concentrations. Organogelation results in the formation of networks of supramolecular assemblies whose anisotropic structures can take on a wide variety of morphologies: platelets, ribbons, fibers, helices, cylinders, bundles of fibers. The forces involved in the construction of these gels are individually weak. They result from hydrogen, donor-acceptor or coordination bonds, Pi, electrostatic, van der Waals or solvophobic (hydrophobic) interactions. The aim of this lesson was to familiarize the public with this relatively unfamiliar field (even for chemists) and to demonstrate both the aesthetics of the structures obtained and the fundamental and applied interest of the resulting materials.
The physical gels resulting from these cooperative supramolecular interactions are thermo-reversible and deformable, and are used in pharmacology, cosmetics, vectorization, tissue engineering, food, hygiene, stabilization of petrols and lubricants, oil spill recovery, restoration of works of art, painting...
These gels can also be impregnated with mineral polymerization precursors under mild chemical conditions. By mineralization, the complex anisotropic structures of these gels can be transcribed (replicated) to form materials with a wide variety of chemical compositions (metal oxides, glasses, chalcogenides, metals, organo-mineral hybrids, etc.). After describing the main families of organogelators (derivatives of amino acids, cyclohexane diamine amides, cholic acid, urea, steroids, etc.) and the solvents for accessing these anisotropic edifices, we illustrated the lesson with more exotic organogelators, based on fullerenes, modified porphyrins and phthalocyanines, drum-structured clusters such as [Bu-Sn(O)O2CR]6 or Strandberg-type polymolybdates [Mo5O15(RPO3)2]2 ). We also described the mechanism of fibrous structure formation based on atomic force microscopy experiments, and demonstrated that a single organogelator molecule disrupts several hundred solvent molecules to form the loose network of the physical gel. We then discussed the inorganic or hybrid mineralization strategies and mechanisms used in the literature to build replicas of silica, organosilica and a wide variety of metal oxides (TiO2, Ta2O5, ZrO2...). These strategies can be broken down into three main approaches:
- post-impregnation, in which the organogel is pre-formed and then impregnated with a mineral phase precursor, a hybrid, or even a polymer;
- in situ co-assembly, in which organogel precursors and mineral or hybrid phase precursors are combined in the first lecture;
- self-assembly of organogels that are already hybrid precursors capable of mineralization by polycondensation. In this case, the gel network is formed simultaneously via non-covalent and covalent bonds
These three routes can be followed by washing and heat treatment, leading to the formation of single fibers, hollow fibers and fiber bundles. In particular, we have shown that by controlling the kinetics of sol-gel silica polycondensation, silicate-organogel interactions and silicate-solvent interactions, we can direct the nucleation of the mineral phase towards the fiber surface or localize it within the solution. These strategies enable us to obtain either isolated thin fibers or tubules, or mesoporous ribbons, thick tubes or bundles of fibers. The compounds obtained have properties of interest to the sciences associated with the environment (enantio-selective catalysts and selective sensors, rheo-commutant photochromic systems) and energy (fibrous nanomaterials for batteries or membranes).
