Tuning Solvation Dynamics of Electrolytes at Their Eutectic Point Through Halide Identity

Journal Article · Fri May 09 00:00:00 EDT 2025 · Molecules

DOI:https://doi.org/10.3390/molecules30102113· OSTI ID:3028372

Pandian, Rathiesh ^[1]; Hansen, Benworth B. ^[2]; de Araujo Lima e Souza, Giselle ^[3]; Sangoro, Joshua R. ^[2]; Greenbaum, Steven ^[3]; Burda, Clemens ^[1]

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1. Case Western Reserve University, Cleveland, OH (United States)
2. The Ohio State University, Columbus, OH (United States)
3. Hunter College of the City University of New York, NY (United States)

Deep eutectic solvents (DESs) are regarded as highly promising solvent systems for redox flow batteries. DESs, composed of choline halides (ChX, X = F⁻, Cl⁻, Br⁻, I⁻) and ethylene glycol (EG), exhibit distinct physicochemical properties at their eutectic points, including halide-dependent phase behavior, viscosity, polarity, conductivity, and solvation dynamics. In this study, we investigate the effects of the halide identity on the solvation properties of ChX:EG mixtures at varying mol % of ChX salt content. The solvatochromic polarity based on ET(30) measurements indicates higher polarity for larger halides (I⁻ > Br⁻) than for smaller halides (Cl⁻ > F⁻), which exhibit larger compensating solvation shells. The ionic conductivity follows the trend of the solvent fluidity (the inverse of the viscosity), namely ChCl > ChBr > ChI > ChF, influenced by the ion mobility and solvodynamic radii. Measurements of the liquidus temperatures (T_L) reveal that the system with ChCl exhibits the deepest eutectic point (at ~20 mol % ChCl), while ChBr and ChI have shallower minima at ~10 mol % ChBr and ~3 mol % ChI, respectively. ChF does not display a eutectic transition but instead appears to readily supercool at salt concentrations above 30 mol % ChF. Consistent with the phase transition measurements, femtosecond transient absorption spectroscopy shows that in the ChCl system, the solvation dynamics become faster with an increasing salt concentration up to ~16.67 mol %, after which the dynamics slow down with further increases in the salt content. The ChF-based system exhibits similar behavior, though with slower dynamics. In contrast, the solvation dynamics of the systems containing ChBr and ChI monotonously slow down with an increasing salt concentration, in agreement with the phase transition measurements, which show that the eutectic points occur at low salt concentrations. These measurements suggest that the solvent composition and, in particular, the identity of the halide anion play a significant role in the solvation behavior of these ethylene-glycol-based DESs, offering a foundation for tuning the DES properties for specific applications.

• Research Organization: Case Western Reserve University, Cleveland, OH (United States); Hunter College of the City University of New York, NY (United States); The Ohio State University, Columbus, OH (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0019409
• OSTI ID: 3028372
• Journal Information: Molecules, Journal Name: Molecules Journal Issue: 10 Vol. 30; ISSN 1420-3049
• Publisher: MDPI AG
• Country of Publication: United States
• Language: English

Related Subjects

ET(30) polarity
Reichardt’s dye betaine-30
choline halides
deep eutectic solvents
differential scanning calorimetry
femtosecond transient absorption spectroscopy
pulsed-field gradient NMR spectroscopy
self-diffusion
solvation dynamics
solvent structure