Abstract
Mixtures of sodium salts with oxygen-containing molecules are useful from the perspective of applications such as sodium ion batteries 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) nuclear magnetic resonance (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 the PFG NMR signal. In the current work, this study is expanded 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 are instead strongly coordinated to the cation. In contrast, Na2B4O7 · 10H2O appears to form a water-rich phase with enhanced sodium mobility.
Highlights
Deep eutectic solvents (DESs) are a class of ionic liquids, which in the last two decades have been used in areas such as materials preparation,[1] gas-separation,[2] electrochemistry,[3] biomass valorization[4] and catalysis.[5]
If the ion-ion or ion-dipole interactions increase, the components will be constricted decreasing the free volume of the liquid and decreasing ionic mobility, since ionic mobility is related to free volume
This study has shown that waters of hydration can play an important role in determining the mobility of ions in eutectic mixtures
Summary
Deep eutectic solvents (DESs) are a class of ionic liquids, which in the last two decades have been used in areas such as materials preparation,[1] gas-separation,[2] electrochemistry,[3] biomass valorization[4] and catalysis.[5] DES properties have been extensively studied focusing on the freezing point,[6] vapour pressure,[7] conductivity,[8] viscosity[9] and more recently diffusion.[10, 11] The knowledge of such properties plays a crucial role in applications and determines the suitability of a particular DES for a specific application. This is because in bulk diffusion measurements there is a net transport of mass
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