Solvation dynamics of an ionic probe in choline chloride-based deep eutectic solvents.

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Deep eutectic solvents (DES) are novel solvents with tunable properties.
• Understanding ion solvation is crucial for DES applications.
• The thiocyanate ion (SCN-) is a common model anion.

Purpose of the Study:

• To investigate the solvation structure and dynamics of the thiocyanate ion in three different DES.
• To elucidate the hydrogen bonding interactions between SCN- and DES components.
• To explore the relationship between DES structure and ion dynamics.

Main Methods:

• Linear and Two-Dimensional Infrared (2D IR) spectroscopy.
• Photon-echo vibrational spectroscopy.
• Molecular dynamics (MD) simulations and ab initio calculations.

Main Results:

• Linear FTIR confirmed hydrogen bonding of SCN- through its sulfur atom, with the nitrile end remaining free.
• 2D IR and photon-echo spectroscopy revealed two distinct picosecond-timescale dynamics (in-place and diffusional motions) in the SCN- solvation shell.
• Theoretical modeling indicated alcohol-based DES are more disordered than amide-based DES.
• Simulations and calculations supported experimental findings on solvation and dynamics.

Conclusions:

• The solvation shell dynamics of SCN- in DES are characterized by both localized and diffusive motions.
• Alcohol-based DES exhibit greater structural disorder compared to amide-based DES.
• The observed structural organization in DES is insufficient to fully explain bulk properties like density via simple defect models.