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, (ν+δ)H2O, 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 (ν+δ)H2O band was shifted to lower wavenumbers. The area of the (ν+δ)H2O 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 (ν+δ)H2O band area reflected reasonably well the amount of H2O in montmorillonite. The peak-fitting analysis of the (ν+δ)H2O 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 (ν1+δ)H2O 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.

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