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Fonctionnalisation et cristallochimie de talcs synthétiques submicroniques

Abstract : Synthetic talc is a material of great industrial interest because of its submicronic size, its large specific surface area (> 300 m²/g), and its hydrophilic properties, offering a wide range of applications in the fields of polymers, cosmetics, papers, paints, or medecine. In this thesis, the adsorption capacity of the material was tested towards different organic and/or inorganic compounds of desired physicochemical properties (coloured and/or fluorescent compounds, metallic nanoparticles), to elaborate new mineral fillers with functional and innovative characteristics (e.g. fluorescent synthetic talc particles). The results show that synthetic talc particles possess a much higher adsorption capacity towards organic and inorganic compounds, compared to natural plurimicron-sized talc particles. Research on the adsorption mechanisms occuring at the " mineral - adsorbate " interface was followed by the study of the crystal-chemistry of the material, using a combination of several characterization techniques such as X-Ray Diffraction (XRD), Nuclear Magnetic Resonance spectroscopy (NMR), or X-Ray Absorption spectroscopy (XAS). Beyond the fact that a great progress was made on the overall crystal-chemistry of the particles, the main results show that " synthetic minerals " should not be considered in the same way as " natural minerals " due to their nanometric size which necessarily disturbed the observed signals. As an example, the methods conventionally used in XRD to determine the interplanar spacings should not be applied on nanomaterials because of their very low particle stacking and numerical simulations shall be used instead. Similarly, our study shows that NMR signals are also impacted by the nanometric size of the particles because of a spectral differentiation of the signals coming from the bulk of the particles versus the ones coming from the external surfaces of the particles. These results were highlighted by combining an experimental NMR approach and a theoretical DFT (Density Functional Theory). Those external surfaces, which contain crystalline defects, are probably responsible for the high adsorption capacity of the material, and may explain some of the crystallogenesis processes that have been observed by XAS spectroscopy. To conclude, nano-sized lamellar synthetic materials represent a great tool to investigate the surface crystal-chemistry of natural minerals, as they revealed new contributions which were previously undetectable in larger-sized minerals.
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Mathilde Poirier. Fonctionnalisation et cristallochimie de talcs synthétiques submicroniques. Minéralogie. Université Paul Sabatier - Toulouse III, 2019. Français. ⟨NNT : 2019TOU30237⟩. ⟨tel-02955577⟩

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