Unraveling the Coupled Processes of (De)hydration and Structural Changes in Na+-Saturated Montmorillonite

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(...