- © 2016 Clay Mineral Society
The potential of near-infrared (NIR) spectroscopy to track the adsorption of water on montmorillonite saturated with different exchangeable cations is demonstrated in the present study. The Na+, K+, Ca2+, and Mg2+ forms of JP montmorillonite (Jelšový Potok, Slovakia) were first dried and then hydrated at 23, 52, 88, and 100% relative humidity (RH). The combination band of water molecules, , allowed the study of the effect of exchangeable cations on the strength of H bonds between water molecules and on the amount of adsorbed water. With increasing ionic potential (IP) of the exchangeable cation, the strength of the H bonds increased and the band was shifted to lower wavenumbers. The area of the band, corresponding to the amount of adsorbed water, was compared with results from gravimetry. The good correlation (R2 > 0.97) between the two independent methods confirmed that the band area reflected reasonably well the amount of H2O in montmorillonite. The peak-fitting analysis of the band allowed differentiation of weakly and strongly H-bonded water molecules. The position of the high-frequency component at 5260–5250 cm−1, related to H2O weakly H-bonded to basal oxygens of the tetrahedral sheets, was influenced only slightly by the exchangeable cations. Two low-frequency components were assigned to the combination modes involving asymmetric (ν3) and symmetric (ν1) stretching vibrations of strongly H-bonded H2O. Only the component (5055–5000 cm−1) showed significant dependence on the type of exchangeable cation and hydration level. Peak-fit analysis revealed a small effect of the type of exchangeable cation on the amount of water molecules weakly H-bonded to the siloxane surface but a pronounced effect on the content of strongly hydrogen-bonded H2O. The amount of weakly H-bonded H2O remained stable after reaching a certain level of hydration, but a gradual increase in the strongly H-bonded water molecules with increasing RH was observed.