Interplay of ethaline and water dynamics in a hydrated eutectic solvent: Deuteron and oxygen magnetic resonance studies of aqueous ethaline.

This article focuses on the component-selective investigation of molecular dynamics in hydrated ethaline, a deep eutectic solvent (DES) composed of a 2:1 molar mixture of ethylene glycol (EG) and choline chloride (ChCl), with about 17 wt.% water added. Using isotope-edited nuclear magnetic resonance (NMR) techniques—specifically deuteron (^2H) and oxygen-17 (^17O) spin relaxation and stimulated-echo spectroscopy—alongside shear mechanical rheology and dielectric spectroscopy, the study reveals that the cholinium cation reorients roughly twice as slowly as EG, while water and EG molecules exhibit similar mobilities across a wide temperature range. The findings indicate that hydration enhances molecular dynamics without increasing solvent heterogeneity and that water integrates smoothly into the hydrogen-bond network of ethaline up to this hydration level, preserving the native DES structure in the "water-in-DES" regime. Additionally, no significant isotope effects were detected, and the temperature-dependent correlation times of the components follow a consistent pattern well described by Waterton's law.