Etude des propriétés physico-chimiques de silices lamellaires synthétiques : le rôle de l'eau

Abstract : This study is devoted to the hydration and swelling behavior of a synthetic layered silica: magadiite, which formula can be written as Na2Si14Ü29.9H20. This mineral of unknown crystallographic structure was obtained by hydrothermal synthesis and subsequently exchanged with H+, K+, Mg2+ and Ca2+. Ail the samples were characterized by chemical, thermal and texturai analyses, XRD, 29Si NMR and IR. Magadiite is formed by platelets about 2!J.m in length, which thickness inversely depends on cation polarisability. The layer formed by six-membered rings of silica that do not contain the interlayer cation, is not significantly affected by the nature of the exchangeable species. Except for H-magadiite, the water BET specifie surface area is much higher than the nitrogen one. Hydration was followed by water adsorption gravimetry, XRD and IR (including deuteration experiments) under controlled water pressures, and 29Si NMR and Raman under adjustable vacuum. The hydration of monovalent magadiites occurs in two steps. At low relative pressure, water molecules are in a C1 symmetry. For higher relative pressure ,the second step of hydration leads to water molecules with a C2v symmetry. This scheme remains valid for magnesium but ail the phenomena occur at much higher relative pressure. On the contrary, calcium is located between two Si-O- bonds of the silica framework. Even high water contents do not induce calcium displacement from its initial position.
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Céline Eypert-Blaison. Etude des propriétés physico-chimiques de silices lamellaires synthétiques : le rôle de l'eau. Sciences de la Terre. Institut National Polytechnique de Lorraine, 2001. Français. ⟨NNT : 2001INPL054N⟩. ⟨tel-01775870⟩

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