Changes in the Si−O Vibrations of Smectite Layers Accompanying the Sorption of Interlayer Water

The frequencies of Si−O stretching, ν Si - O, in layers of montmorillonite and of H−O−H bending, ν 2, in the interlayer water were investigated by infrared spectroscopy. Oriented gels of Li- and Na-montmorillonite equilibrated at different water contents, m w / m c, were obtained by using a miniature pressure membrane apparatus. One part of each gel was used for the gravimetric determination of m w / m c . The other part was transferred to an FTIR spectrometer where the spectrum of the gel was measured by attenuated total reflectance. Thus, the dependence of both ν Si - O and ν 2 on m w / m c was determined in the same samples. The absorption bands for Si−O stretching were resolved by using a curve-fitting technique to determine the frequency, area, and height of the component peaks. Two prominent peaks, labeled peak III and peak IV, were selected for analysis. It was found that ν Si - O for each peak and the ratio of the areas (or heights) of the two peaks were all exponential functions of m c / m w . Moreover, a linear relation was found between ln(ν Si - O ) and ln(ν 2 ). These results were interpreted to mean that the Si−O stretching vibrations in the montmorillonite layers and the H−O−H bending vibrations in the interlayer water are coupled and that both change as the layers come closer together.