Rattling in the Cage: Ions as Probes of Sub-picosecond Water Network Dynamics

We present terahertz (THz) measurements of salt solutions that shed new light on the controversy over whether salts act as kosmotropes (structure makers) or chaotropes (structure breakers), which enhance or reduce the solvent order, respectively. We have carried out precise measurements of the concentration-dependent THz absorption coefficient of 15 solvated alkali halide salts around 85 cm−1 (2.5 THz). In addition, we recorded overview spectra between 30 and 300 cm−1 using a THz Fourier transform spectrometer for six alkali halides. For all solutions we found a linear increase of THz absorption compared to pure water (THz excess) with increasing solute concentration. These results suggest that the ions may be treated as simple defects in an H-bond network. They therefore cannot be characterized as either kosmotropes or chaotropes. Below 200 cm−1, the observed THz excess of all salts can be described by a linear superposition of the water absorption and an additional absorption that is attributed to a rat...