Salt Effects on the Structure of Water Probed by Attenuated Total Reflection Infrared Spectroscopy and Molecular Dynamics Simulations

We study aqueous solutions of alkaline chlorides (NaCl, KCl, RbCl and CsCl) with a combination of attenuated total reflection infrared (ATR-IR) spectroscopy and molecular dynamics (MD) simulations using the TIP4P-Ew water model (Horn et al., J. Chem, Phys. 2004, 120, 9665), covering concentration ranges between 0.1 and 6 M. Spectral modifications in the OH stretch region are evaluated and correlated to the various salts and salt concentrations. By taking the difference spectra between the spectral line shapes of aqueous salt solutions and those of pure water, we specifically focus on the small quasi-"free OH" band appearing the highest wavenumbers in the spectra. This free-OH feature is found constantly at 3650 cm(-1) for all salts and salt concentrations, but it shows a characteristic intensity depending on the chosen cation. In the order from Na+ to Cs+, the free OH intensity decreases compared to that of pure water. To interpret the experimental results, we performed MD simulations for similar salt solutions. The experimentally observed effects can be correlated with structural alterations indicated by differences between the site-site pair correlation functions of water in aqueous salt solutions and those of pure water.