Dinâmica da umidade no preenchimento de meso- e nanoporos em nanosilicatos sintéticos

Dissertação (mestrado)—Universidade de Brasília, Instituto de Física, 2012.A expansão das partículas de argilas esmectitas causam mudanças nas distâncias interplanares (espaçamento-d) como função da temperatura e da umidade relativa. Diferentemente do Sódio Fluorohectorita que possui os estados de hidratação de zero, uma e duas camadas de água, a amostra de Lítio Fluorohectorita, os estados de hidratação estáveis são os de zero, uma, uma e meia, duas e três camadas de água, com saltos discretos no espaçamento-d ocorrendo durante as transições entre esses estados. Mantendo a temperatura fixa e variando a umidade relativa do ambiente, somos capazes de reproduzir essas mudanças no espaçamento dentro dos estados de hidratação. A confiabilidade e a reprodutividade do controle dessa umidade permite-nos usar as distâncias interplanares como uma medida da umidade que circunda localmente as partículas de argila. Queremos usar essas observações para estudar o transporte de umidade ao longo da amostra. Impondo um gradiente de umidade em uma amostra com temperatura controlada quase unidimensional, e usando difração de raios X para coletar as distâncias interplanares, nós poderemos extrair padrões de umidade ao longo da amostra. A evolução temporal desses padrões descreve o transporte de água nos mesoporos dentro da argila. _______________________________________________________________________________________ ABSTRACTThe swelling of layered smectite clay particles consists of a change in the interlayer repetition distance (d-spacing) as a function of temperature and humidity. In this work, a ne scan of the relative humidity under room temperature was done for the synthetic clay Lithium Fluorohectorite. This sample has hydrodynamically stable hydration states with zero, one, one and a half, two and three intercalated monolayers of water which are described in a similar work for the Sodium Fluorohectorite, with discrete jumps in d-spacing at the transitions between the hydration states. These changes are monotonous as a function of relative humidity, and one order of magnitude smaller than the shift in d-spacing that is typical for the transition between two hydration states. The reproducibility and reliability of this relative humidity controlled d-shift enables us to use the interlayer repetition distance d as a measure of the local humidity surrounding the clay particles. We provide an example of application of this observation: imposing a humidity gradient over a quasi-one-dimensional temperature-controlled sample, and using x-ray diffraction to record the d − spacing, we are able to extract pro files of the relative humidity along the sample length. Their time evolution describes the transport of water through the mesoporous space inside the clay

EXAFS and XRD studies in synthetic Ni-Fluorohectorite

In the present work the synthetic clay mineral fluorohectorite was studied by means of an extended X-ray absorption fine structure (EXAFS) in a powder sample with the intention to observe the number of neighboring atoms to the Ni interlayer cation. In addition X-ray diffraction (XRD) was performed in order to follow the hydration states of Ni-fluorohectorite in terms of basal-spacing measurements. The sample conditions were the same for both types of experiments. The EXAFS results show that Ni2 + forms a brucite-like structure in the form of Ni(OH)2, and that this structure coexists with the clay mineral particles. This shows that the Ni atom observed by means of our EXAFS measurements is predominantly the Ni which composes the brucite-like structure and not the interlayer Ni2 + cation. In order to confirm the formation of the brucite-like structure, the EXAFS data from Ni-fluorohectorite were compared to Ni-salt water solutions at various pH

The Impact of Thermal History on Water Adsorption in a Synthetic Nanolayered Silicate with Intercalated Li+ or Na+

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The Impact of Thermal History on Water Adsorption in a Synthetic Nanolayered Silicate with Intercalated Li+ or Na+

For applications benefitting from the swelling properties of nanolayered silicates (clay minerals), it is of paramount importance to understand the hysteresis in the clay–water interaction. In this context, the present work investigates how the thermal history of Na+- and Li+-intercalated fluorohectorite affects the hydration process. By combining X-ray diffraction and thermogravimetric analysis, water adsorption of preheated and non-preheated fluorohectorite was measured and analyzed in terms of the characteristic interlayer distance. The number of water molecules per cation was also inferred. We find that some of the hydration states in preheated samples are suppressed, and transitions to higher hydration states are achieved at higher relative humidity values. This could be due to the initial water content that facilities crystalline swelling. However, the data for Li-fluorohectorite do not exclude the possibility of a low temperature Hofmann–Klemen effect at 150 °C. Our study also provides strong hints that the so-called 1.5 water layer state, observed in previous studies on smectites, is a metastable state. In addition, the impact of a hydrogenous structure in the interlayer space of Li-fluorohectorite on the clay’s hydration behavior is demonstrated. The results, if generalized, would have strong implications on a wide range of applications, where the thermal history of smectites is important

The Impact of Thermal History on Water Adsorption in a Synthetic Nanolayered Silicate with Intercalated Li+ or Na+

For applications benefitting from the swelling properties of nanolayered silicates (clay minerals), it is of paramount importance to understand the hysteresis in the clay–water interaction. In this context, the present work investigates how the thermal history of Na+- and Li+-intercalated fluorohectorite affects the hydration process. By combining X-ray diffraction and thermogravimetric analysis, water adsorption of preheated and non-preheated fluorohectorite was measured and analyzed in terms of the characteristic interlayer distance. The number of water molecules per cation was also inferred. We find that some of the hydration states in preheated samples are suppressed, and transitions to higher hydration states are achieved at higher relative humidity values. This could be due to the initial water content that facilities crystalline swelling. However, the data for Li-fluorohectorite do not exclude the possibility of a low temperature Hofmann–Klemen effect at 150 °C. Our study also provides strong hints that the so-called 1.5 water layer state, observed in previous studies on smectites, is a metastable state. In addition, the impact of a hydrogenous structure in the interlayer space of Li-fluorohectorite on the clay’s hydration behavior is demonstrated. The results, if generalized, would have strong implications on a wide range of applications, where the thermal history of smectites is important