²³Na NMR T₁ relaxation measurements as a probe for diffusion and dynamics of sodium ions in salt-glycerol mixtures

Mixtures of sodium salts with oxygen-containing molecules are useful from the perspective of applications such as sodium ion batteries but also because they fill the gap between deep eutectic solvents and molten salt hydrates. In a previous work, the physical properties (such as diffusion coefficients, conductivity, viscosity and glass transition temperature) of four salts, namely Na2B4O7.10H2O, NaOAc.3H2O, NaBr, and NaOAc, were measured with glycerol. Pulsed-field gradient (PFG) NMR was also used to measure self-diffusion coefficients of 1H-bearing species. However, the technique was not able to measure diffusion of sodium ions due to the very fast NMR relaxation rate of such species, resulting in loss of PFG NMR signal. In the current work, this study is expanded to using 23Na T1 relaxation measurements, which under certain assumptions, can be translated into diffusion coefficients. Analysis of the physical properties is then correlated with self-diffusion coefficient measurements to elucidate information about structure and ionic mobility. It is shown that NaOAc.3H2O, NaBr and NaOAc fit models for ionic conductivity and diffusion, which are consistent with ionic liquids where charge transport is limited by ionic mobility rather than the number of charge carriers. The waters of hydration of NaOAc.3H2O do not appear to form a separate phase but instead are strongly coordinated to the cation. In contrast, Na2B4O7.10H2O appears to form a water-rich phase with enhanced sodium mobility.