Louis Cot

Synthesis and characterization of inorganic gels in a lyotropic liquid crystal medium. Part 2.—Synthesis of silica gels in lyotropic crystal phases obtained from cationic surfactants

Microporous silica materials with tailored porosity are synthesized by the sol–gel process using lyotropic liquid-crystal phases as templates. The starting isotropic sol is obtained by mixing tetramethoxysilane, water and an alkyltrimethylammonium bromide. The polymerization of the silica network and the formation of the amphiphilic mesophase are simultaneous and cooperative. After thermal elimination of the surfactant molecules, an ordered porous texture is maintained in the silica material. The pore size is related to the size of the templating unit and modulated by the length of the alkyl chain of the used surfactant. In this paper, the various steps of the synthesis are studied and the porous texture of the final material is characterized and discussed in terms of templating effects.

Sol–gel systems with controlled structure formed in surfactant media

The structure and mechanisms of formation of silica and zirconia gels produced by alkoxide hydrolysis [tetramethylorthosilicate (TMOS) and Zr (OPri)4)] in different surfactant media have been investigated using small-angle neutron scattering and other techniques. Inverse micelle and lamellar phase systems have been formed with non-ionic octylphenyl polyether alcohol surfactants, C8ΦEx, where x, is 5 and 10, respectively. In these three-phase systems (water–surfactant–organic solvent) a large proportion of water is bound to the hydrophilic polyoxyethylene chains of the surfactant molecules. This has an important role in controlling the formation of the gel and its structure. Inverse micelles act as nucleation sites for the formation of very small oxide particles (⩽3 nm), which subsequently aggregate to give a fractal structure (D≈ 2.3). The lamellar phases exist as microdomains (0.1–1 µm), and alkoxide hydrolysis occurs predominantly in the surrounding zones, to give fractal aggregates (D≈ 1.9) corresponding to diffusion-limited cluster aggregation (DLCA). Under shear, such lamellar phases are aligned and then produce oriented silica gels with an anisotropic structure. Cylindrical micelles can be formed with the cationic n-alkyl trimethyl ammonium surfactant. With these systems hydrolysis of TMOS occurs in the aqueous phase surrounding the micelles. Such relations can induce the organisation of the micelles to a hexagonal phase. Subsequent elimination of surfactant results in silica xerogels containing cylindrical pores of controlled size.

Synthesis and characterization of porous ferrimagnetic membranes

Abstract Nanoporous ferrimagnetic membranes based on maghemite (γ-Fe2O3) and cobalt ferrite (Fe2CoO4) were prepared by the sol–gel route using colloidal ferrofluids. These membranes were used for the separation of paramagnetic and diamagnetic gaseous species at room temperature. Their magnetic properties were analyzed using a SQUID magnetometer, and their structures and porous textures were studied by X-ray diffraction, scanning electron microscopy and nitrogen adsorption–desorption analysis. O2–N2 adsorption and air separation experiments were carried out in order to study the magnetic interactions in static and dynamic conditions, respectively. A slight effect of the magnetic field on the selectivity of these membranes was observed. However, the measured selectivity values αN2/O2 were always close to 1. Although an optimization of the membranes remains possible, it seems difficult to reach the high selectivity values required for technological applications.

Ferrimagnetic membranes for separation of paramagnetic and diamagnetic species

For using as separative membranes based on magnetic selectivity, nanoporous ferrimagnetic membranes of maghemite (gamma-Fe2O3) and cobalt ferrite (Fe2CoO4) were prepared by the sol-gel route from ferrofluid colloidal solutions. Their magnetic properties were examined by superconductor quantum interference device (SQUID), and their structures and porous textures were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption-desorption analyses. O-2-N-2 adsorption and air separation experiments were carried out in order to evidence magnetic interactions in static and dynamic conditions, respectively. A small effect of an external magnetic field on the selectivity of these membranes was observed.