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1.
Phys Chem Chem Phys ; 23(17): 10498-10508, 2021 May 05.
Artigo em Inglês | MEDLINE | ID: mdl-33899859

RESUMO

Topochemical reactions involving ionic exchange have been used to assess a large number of metastable compositions, particularly in layered metal oxides. This method encompasses complex reactions that are poorly explored, yet are of prime importance to understand and control the materials' properties. In this work, we embark on investigating the reactions involved during the ionic exchange between a layered Na-titanate (lepidocrocite-type structure) and an acidic solution (HCl), leading to a protonic (H3O+) titanate (trititanate structure). The reactions involve an ionic exchange provoking a structural change from the lepidocrocite-type to the trititanate structure as shown by real-space refinements of ex situ pair distribution function data. Mobile Na+ ions are exchanged by hydronium ions inducing high proton mobility in the final structure. Moreover, the reaction was followed by ex situ23Na and 1H solid-state MAS NMR which allowed, among other things, confirming that the Na+ ions are in the interlayer space and specifying their local environment. Strikingly, the ionic exchange reaction induces progressive exfoliation of the Na-titanate particles leading to 2-5 nm thin elongated crystallites. To further understand the different steps associated with the ionic exchange, the evolution of the electrolytic conductivity, using conductimetric titration, has been monitored upon HCl addition, enabling characterization of the intercalation(H+)/de-intercalation(Na+) reactions and assessing kinetic parameters. Accordingly, it is hypothesized that the exfoliation of the particles is due to the accumulation of charges at the particle level in relation to the rapid intercalation of protons. This work provides novel insights into ionic exchange reactions involved in layered oxide compounds.

2.
Environ Sci Technol ; 45(7): 2850-5, 2011 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-21381672

RESUMO

Within the wider context of water behavior in soils, and with a particular emphasis on clays surrounding underground radioactive waste packages, we present here the translational dynamics of water in clays in low hydrated states as studied by coupling molecular dynamics (MD) simulations and quasielastic neutron scattering experiments by neutron spin echo (NSE). A natural montmorillonite clay of interest is modeled by a synthetic clay which allows us to understand the determining parameters from MD simulations by comparison with the experimental values. We focus on temperatures between 300 and 350 K, i.e., the range relevant to the highlighted application. The activation energy Ea experimentally determined is 6.6 kJ/mol higher than that for bulk water. Simulations are in good agreement with experiments for the relevant set of conditions, and they give more insight into the origin of the observed dynamics.


Assuntos
Silicatos de Alumínio/química , Modelos Moleculares , Silicatos/química , Argila , Hidrodinâmica , Espectrometria de Massas , Nêutrons , Proteção Radiológica/métodos , Solo/química , Temperatura
3.
J Phys Chem B ; 113(25): 8670-4, 2009 Jun 25.
Artigo em Inglês | MEDLINE | ID: mdl-19485401

RESUMO

The purpose of this study is to predict the electrical conductivity of an electrolyte solution containing several simple ionic species. Explicit equations of the MSA-transport theory for the electrical conductivity in this complex solution are given. The theoretical conductivity of simple salts is first compared to experimental results of the literature to deduce the sizes of the ions. These sizes allow us to calculate the conductivity for a mixture of several ionic species without any additional parameter. We have also measured the electrical conductivity of solutions of LiCl, NaCl, and KCl and of KBr and MgCl(2) at 25 degrees C. A very good agreement between theoretical calculations and experimental values is obtained for each studied system.

4.
J Colloid Interface Sci ; 271(2): 261-9, 2004 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-14972601

RESUMO

The specific surface area of synthetic goethite depends on the preparation: the Fe(III):OH ratio, the rate of base titration of Fe salt, and the temperature and time of crystallization. The crystals also have different morphologies as determined by SEM or TEM. Carbon coating is used to improve the quality of SEM images of nonconducting specimens. We show here that needle-like goethite particles become substantially thicker in the course of standard carbon coating, and the length-to-width ratio obtained for carbon-coated particles is lower than that for the original goethite particles. The morphology of the goethite particles was also studied by tapping mode AFM.

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