Correlations in the Solute–Solvent Dynamics Reach Beyond the First Hydration Shell of Ions

Abstract

While the real-space structure of solvation shells has been explored for decades, a dynamical perspective that directly relies on changes in the H-bond network became accessible more recently mainly via far-infrared (THz) spectroscopies. A remaining key question is how many hydration shells are affected by ion-induced network perturbations. We disclose that theoretical THz difference spectra of aqueous salt solutions can be deciphered in terms of only a handful of dipolar auto- and cross-correlations, including the second solvation shell. This emphasizes the importance of cross-correlations being often neglected in multicomponent models. Analogously, experimental THz responses of simple ions can be deciphered in a similar way. Dramatic intensity cancellations due to large positive and negative contributions are found to effectively shift intensity maxima. Thus, THz spectroscopy provides an unprecedented view on the details of hydration dynamics, which can be understood by a combination of experiment and theory.

Evidence for Reduced Hydrogen-Bond Cooperativity in Ionic Solvation Shells from Isotope-Dependent Dielectric Relaxation

“…We find that the experimentally determined kinetic depolarization is much smaller than the kinetic depolarization that follows from commonly used models. This discrepancy is due to the assumption that the solvating water has the same dielectric response as bulk water, which is incorrect since the ions locally disrupt the hydrogen-bond structure of water [36][37][38][39][40][41][42][43][44].…”

Towards Intense THz Spectroscopy on Water: Characterization of Optical Rectification by GaP, OH1, and DSTMS at OPA Wavelengths

“…The THz spectrum of pure liquid water at T = 20 °C is shown in Figure 1. In order to fit the band centered at ~20 THz shown in Figure 1b, at least two hindered rotational (or librational) modes are needed [24][25][26]. The infrared active rotations of a single water molecule have been assigned to the "rocking" and the "wagging", while the "twisting" should be exclusively active in Raman experiments [26].…”

“…Intermolecular translations or "network stretching" modes are centered at ca. 6 THz, while librational modes are found between~10 and~20 THz [24][25][26]. (b) Equilibrium absorption coefficient of pure water at T = 20 • C [1][2][3][4].…”

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations

“…The structure and dynamics of liquid water are significantly altered on solute dissolution, which, in turn, determine the extent of solvation of a given solute. While pure water is known to undergo rapid evolution in its hydrogen bond network through breaking and formation of hydrogen bonds, both the structure and dynamics of hydrogen bond network may get considerably influenced by the presence of solutes in many cases . A common classification has been made where solutes, which tend to enhance the hydrogen bonded network, are called “structure makers” and those disrupting the same are called “structure breakers” .…”

“…While pure water is known to undergo rapid evolution in its hydrogen bond network through breaking and formation of hydrogen bonds, both the structure and dynamics of hydrogen bond network may get considerably influenced by the presence of solutes in many cases. [4][5][6][7] A common classification has been made where solutes, which tend to enhance the hydrogen bonded network, are called "structure makers" and those disrupting the same are called "structure breakers". [8] Further, monatomic ions dissolved in liquid water offer relatively simpler systems for an in-depth investigation.…”