Time-resolved terahertz–Raman spectroscopy reveals that cations and anions distinctly modify intermolecular interactions of water

• Published in: Nature chemistry 2022-09, Vol.14 (9), p.1031-1037
• Main Authors: Balos, Vasileios, Kaliannan, Naveen Kumar, Elgabarty, Hossam, Wolf, Martin, Kühne, Thomas D., Sajadi, Mohsen
• Format: Article
• Language: English
• Subjects: 639/638/440/527/1821; 639/638/440/94; 639/638/440/948; Analytical Chemistry; Anions; Biochemistry; Biopolymer denaturation; Bonding strength; Cations; Charge density; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Denaturation; Energy dissipation; Energy transfer; Hydrogen; Hydrogen bonding; Hydrogen bonds; Inorganic Chemistry; Ion charge; Ion concentration; Ions; Molecular dynamics; Monitoring; Organic Chemistry; Perturbation; Physical Chemistry; Raman spectroscopy; Rotational spectra; Saline solutions; Salts; Solvation; Spectroscopy; Spectrum analysis; Spectrum Analysis, Raman; Surface charge; Terahertz frequencies; Thermodynamic properties; Translation; Water; Water chemistry
• ISSN: 1755-4330; 1755-4349
• DOI: 10.1038/s41557-022-00977-2

The solvation of ions changes the physical, chemical and thermodynamic properties of water, and the microscopic origin of this behaviour is believed to be ion-induced perturbation of water’s hydrogen-bonding network. Here we provide microscopic insights into this process by monitoring the dissipation of energy in salt solutions using time-resolved terahertz–Raman spectroscopy. We resonantly drive the low-frequency rotational dynamics of water molecules using intense terahertz pulses and probe the Raman response of their intermolecular translational motions. We find that the intermolecular rotational-to-translational energy transfer is enhanced by highly charged cations and is drastically reduced by highly charged anions, scaling with the ion surface charge density and ion concentration. Our molecular dynamics simulations reveal that the water–water hydrogen-bond strength between the first and second solvation shells of cations increases, while it decreases around anions. The opposite effects of cations and anions on the intermolecular interactions of water resemble the effects of ions on the stabilization and denaturation of proteins. Ions in salt solutions perturb the hydrogen bonding between the surrounding water molecules, altering the properties of water, but how ion polarity affects this is not fully understood. By monitoring the dissipation of terahertz energy in salt solutions, it has now been shown that intermolecular rotational-to-translational energy transfer is enhanced by highly charged cations and reduced by highly charged anions.