Abstract

The coupled processes of (de)hydration and structural changes of a well characterized Na+-exchanged dioctahedral smectite with a low layer charge (0.26 mol(+)/formula unit) were studied by in situ transmission Fourier transform infrared spectroscopy. Therefore, a sample preparation technique based on the tethering by aggregation and growth method is extended to clay minerals to obtain a thin film of montmorillonite on a silicon wafer. After equilibration of the sample at 53% relative humidity, infrared measurements were done with a custom-made, heatable sample holder in an oxygen free atmosphere under N2 purging which allowed subsequent FTIR measurements. Using a very thin film, deviation of the Si–O phonon modes could be simultaneously studied with the vibrational modes of water. The structural changes in the 2:1 layer of the montmorillonite during dehydration can be recognized from the shift of longitudinal optical Si–O phonon mode. The accompanied decrease and shift of the bending vibration of water δ(H–O–H) to lower wavenumbers is correlated to an increase of water–surface attraction. Finally, our experimental results were supported by ab initio thermodynamic simulations that revealed a decrease of the b0 from 9.18 Å to 8.98 Å with decreasing water content.

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