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Abstract--Sepiolite from Vallecas-Vichlvaro, Spain, contains 1.3% fluorine. Laser microprobe mass spectrometry of this sepiolite suggests the presence of fragments of (SiO2),OMgF and (SiO~),OMgOH, which are typical of the sepiolite structure. During thermal dehydroxylation, the fluorine in this sepiolite is removed simultaneously with OH groups at about 750~ Nuclear magnetic resonance spectroscopy (NMR) of ~gF indicates that the fluorine is located in the interior of the sepiolite structure, probably substituting for OH groups, and is homogeneously distributed. In the Vallecas-Vic~lvaro sepiolite, about one of every four OH groups bound to Mg 2 § is substituted by fluorine. The kinetics of extraction of Mg 2+ and F-ions by acid treatment (1 N HC1) shows a more rapid extraction of Mg 2 § with a monotonous decrease of the Mg/F ratio as the extent of extraction increases. These results support the internal location of the fluorine, as suggested by the NMR data.

Surface modification reactions on needle-like sepiolite using alkyl and functional silanes have been carried out in the form of aqueous gels. In contrast with modifications in organic solvents, reactions in water make it possible to modify the surface of almost-individual sepiolite fibers and produce either a continuous coating or a nanotexturization of the sepiolite fiber surface, depending on the reaction conditions. This clean procedure substitutes advantageously organic solvent surface modifications and allows the tuning of surface properties such as specific surface area, wetting behavior, and chemical functionalization. A consequence of such tuning is, for example, the excellent dispersion of modified sepiolite nanofibers in a great variety of polymers by routine compounding and processing techniques.

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The preparation of large quantities of heterogeneous materials containing non-agglomerated and monodispersed nanoparticles is becoming one of the bottlenecks that hinders the development of commercial devices. Here we describe a method to prepare monodispersed metallic (Cu, Ag, Au, Ni, Co, and Fe) nanoparticles in a silicate matrix (sepiolite) by means of a reduction process of metallic cations associated with a dehydration process of the matrix. This process is characterized by the huge amount of monodispersed metallic nanoparticles that it produces. Additionally, these nanoparticles have been revealed to be remarkably stable against oxidation because the transformed sepiolite matrix becomes a diffusion barrier for oxygen. Furthermore, the nanoparticles present suitable properties to be used for optical and magnetic applications.

The adsorption isotherm of methylene blue (MB) on sepiolite gels is of the Langmuir type, indicating a great affinity of the MB towards the sepiolite; the adsorption is quantitative up to adsorption of MB of 0.1 mmol/g sepiolite. The differences observed in the absorption spectrum in the region 500-650 nm for different MB Ioadings are interpreted in terms of aggregation of MB molecules on the sepiolite surface with the formation of MB dimers, trimers and higher aggregates. The progressive coverage of the sepiolite surface by MB produces a sharp decrease in the viscosity of the gels, and the suspension becomes peptized for an MB loading of 0.21 mmol/g which corresponds to the adsorption of MB dimers. These results indicate that MB avoids particle-to-particle interactions. The viscosity decrease is parallel to the perturbation of the Si-OH groups on the sepiolite surface as observed by IR spectroscopy, suggesting that these silanol groups can also play an important role in the theological behaviour of aqueous sepiolite dispersions.