Infrared Spectra in the 1000–100 cm−1 Region Combined with 4000–3000 cm−1 Region to Evaluate the States of Water

In this study, we evaluated the state of water by performing infrared (IR) spectroscopic analysis in the 4000–100 cm −1 region. The effects of ions on the structure of water molecules were investigated by analyzing specific IR bands of salt solutions in the 1000–100 cm −1 region. Chloride solutions of Li, Na, K, Cs, Ba, and Ca were prepared at different concentrations, and their IR spectra were recorded by the attenuated total reflection method. The isosbestic point was observed in the 1000–100 cm −1 region, and the position was related to the ratio of the Stokes radius and effective ionic radius of each ion. Two bands were identified at around 660 and 400 cm −1 by curve fitting, and the intensity ratio increased linearly with a decrease in water activity. Thus, this demonstrates the potential of the 1000–100 cm −1 region as a marker for the evaluation of water structure subjected to ions. Moreover, it is possible to evaluate different states of water simultaneously by combining this with the band in the 4000–3000 cm −1 region. These results successfully demonstrate the effectiveness of the spectra in the 1000–100 cm −1 region to evaluate the state of water in ionic solutions.