NMR studies on the coupling of ion and water dynamics on various time and length scales in glass-forming LiCl aqueous solutions

• Published in: The Journal of chemical physics Vol. 149; no. 10; p. 104501
• Main Authors: Schneider, S, Vogel, M
• Format: Journal Article
• Language: English
• Published: United States 14.09.2018
• DOI: 10.1063/1.5047825

We combine H, H, and Li NMR methods to investigate the dynamics of water molecules and lithium ions in LiCl aqueous solutions over wide ranges of time and length scales down to their glass transitions. Structural relaxation times and self-diffusion coefficients reveal that water and lithium dynamics are faster for lower salt content at ambient temperatures, while the differences vanish upon cooling when fractional freezing leads to similar salt concentrations in the remaining liquid phases. Relaxation times and diffusion coefficients of water molecules agree with those of lithium ions in the weakly supercooled regime, indicating that the dynamics are strongly coupled. Furthermore, non-Arrhenius temperature dependence is found and the Stokes-Einstein relation is obeyed in this temperature range. However, we observe various decoupling phenomena for the motion of the constituents and for dynamics on different length scales in the deeply supercooled regime. Most notably, the rotational motion of the water molecules does not follow the glassy slowdown of the studied salt solutions below ∼145 K, but it rather resembles that in nanoscopic confinement, molecular solutions, and high-density amorphous ice at low temperatures. This common low-temperature water dynamics is characterized by large-angle reorientation and Arrhenius temperature dependence.