Swelling Pressure of Montmorillonite Layers Versus H-O-H Bending Frequency of the Interlayer Water

The in-depth perturbation of vicinal water by the surfaces of montmorillonite layers was investigated by relating the swelling pressure, Π, of the montmorillonite layers to the H-O-H bending frequency, ν 2 , of the interlayer water. For this purpose, an oriented montmorillonite gel was deposited on a porous filter in an environmental chamber. On its underside the filter was in contact with a solution maintained at atmospheric pressure. By admitting nitrogen gas at a known pressure to the environmental chamber, water was squeezed from the gel into the solution until equilibrium was reached and Π equalled the applied pressure. Then the gel was divided into 2 parts. One part was used for the gravimetric determination of the water content, m w / m c . It was possible, therefore, to determine m w / m c as a function of Π. The other part of the sample was transferred to an FTIR spectrometer where the ν 2 of the water within it was measured by attenuated total reflectance. Thus, the same samples were used to determine the dependence of both Π and ν 2 on m w / m c . It was found that Π and ν 2 were both exponential functions of m c / m w and so a linear relation was found between ln(Π + 1) and ln(ν 2 /ν 2 °), where ν2° is the H-O-H bending frequency of bulk water. These results strongly support the conclusion that the in-depth perturbation of the water by the surfaces of the montmorillonite layers is primarily responsible for both the development of Π and the departure of ν 2 from ν 2 °.