Schnetzer Florian, Johnston Cliff T, Premachandra Gnanasiri S, Giraudo Nicolas, Schuhmann Rainer, Thissen Peter, Emmerich Katja
Competence Center for Material Moisture, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
Department of Agronomy, Crop, Soil and Environmental Sciences, Purdue University, 915 West State Street, West Lafayette, Indiana 47907-2054, United States.
ACS Earth Space Chem. 2017 Dec 21;1(10):608-620. doi: 10.1021/acsearthspacechem.7b00091. Epub 2017 Oct 3.
Several 2:1 layer silicates comprising di- and trioctahedral smectites of different layer charge between 0.2 and 0.4 per formula unit and a trioctahedral vermiculite were studied by an method that allowed Fourier transform infrared spectroscopy (FTIR) spectra and water vapor sorption isotherms to be obtained simultaneously. The particle size and shape of the selected materials were determined using X-ray diffraction and gas adsorption analyses, which provided an estimate of the particle size with resulting edge site proportion. The aim of this study was to elucidate the hydration mechanism in 2:1 layer silicates during desorption and adsorption of water vapor. Domains in the desorption and adsorption of water vapor of the smectite samples with a slightly increasing slope were explained by a heterogeneous layer charge distribution, which enables the coexistence of different hydration states even under controlled conditions. Whereas hysteresis was observed over the entire isothermal range of the smectites, the isotherm of the vermiculite sample only showed hysteresis in the transition from the monohydrated state (1W) to the bihydrated state (2W). We also revealed that hysteresis is a function of the layer charge distribution, the achieved water content, and the particle size with resulting edge site contribution. Increasing the edge site proportions led to an increased hysteresis. The findings from the experimental FTIR/gravimetric analysis showed that the transition from 2W to 1W and backward is visible using infrared spectroscopy. The shifting of (H-O-H) was influenced by the layer charge and octahedral substitutions. As a final point, we use water as a sensor molecule to describe the OH groups of the octahedral sheet and show that the observed shifts result from a change in the tilting angle. Our experimental results were supported by thermodynamic simulations that revealed the different shifting behavior of (H-O-H) and (M -OH-N ) related to the differences in surface charge density and octahedral compositions.
研究了几种二八面体层状硅酸盐，包括每分子式单元层电荷在0.2至0.4之间的二八面体蒙脱石和三八面体蛭石，采用一种方法同时获得了傅里叶变换红外光谱（FTIR）和水蒸气吸附等温线。使用X射线衍射和气体吸附分析确定了所选材料的粒径和形状，从而估计了粒径及边缘位点比例。本研究的目的是阐明二八面体层状硅酸盐在水蒸气解吸和吸附过程中的水合机制。蒙脱石样品水蒸气解吸和吸附中斜率略有增加的区域可通过非均匀层电荷分布来解释，这使得即使在受控条件下也能共存不同的水合状态。蒙脱石在整个等温范围内都观察到滞后现象，而蛭石样品的等温线仅在从单水合状态（1W）转变为二水合状态（2W）时出现滞后。我们还发现滞后现象是层电荷分布、达到的含水量以及粒径和边缘位点贡献的函数。边缘位点比例增加导致滞后现象加剧。FTIR/重量分析实验结果表明，利用红外光谱可以观察到从2W到1W以及反向的转变。（H - O - H）的位移受层电荷和八面体取代的影响。最后，我们用水作为传感分子来描述八面体片层的OH基团，并表明观察到的位移是由倾斜角的变化引起的。我们的实验结果得到了热力学模拟的支持，该模拟揭示了（H - O - H）和（M - OH - N）与表面电荷密度和八面体组成差异相关的不同位移行为。
