Water dynamics in divalent and monovalent concentrated salt solutions

Area of Science:

• Physical Chemistry
• Spectroscopy
• Solution Chemistry

Background:

• Understanding water hydrogen bond dynamics is crucial for various chemical and biological processes.
• Previous studies on concentrated salt solutions primarily focused on anion effects on water dynamics.

Purpose of the Study:

• To investigate the roles of both cations and anions in influencing water hydrogen bond dynamics in concentrated salt solutions.
• To elucidate how ion identity affects vibrational properties and structural evolution of water.

Main Methods:

• Polarization-selective infrared (IR) pump-probe spectroscopy.
• 2D IR vibrational echo spectroscopy on the OD hydroxyl stretching mode of HOD in H2O/salt solutions.
• Fourier-transform infrared (FT-IR) spectroscopy.

Main Results:

• Both cations and anions shift the OD stretch absorption frequency, with effects pronounced for larger, polarizable anions or smaller, high charge-density cations.
• Vibrational lifetime is locally dependent on the anion and hydrogen bonding environment, with a minor cation influence.
• Orientational relaxation is a concerted process influenced by both cation and anion identity, indicating a single dynamic ensemble.
• Spectral diffusion measurements reveal that structural dynamics are dependent on both cation and anion identity.

Conclusions:

• Contrary to previous beliefs, both cations and anions significantly impact water hydrogen bond dynamics in concentrated solutions.
• Ion identity plays a critical role in determining water's vibrational lifetime, orientational relaxation, and structural evolution.
• These findings highlight the complex interplay between ions and water structure at a molecular level.