Relaxation dynamics of hydrogen bonds in hydrophobic deep eutectic electrolytes

Abstract

Hydrophobic deep eutectic electrolytes (HDEEs), a subset of deep eutectic solvents, are formed by mixing hydrogen bond donors (HBDs) and acceptors (HBAs) with long alkyl chains. Their low C–H polarization minimizes water interaction, promoting entropy-driven aggregation. However, the dynamic hydrogen-bonding behavior (HBDy), characterized by continuous bond breaking and reformation across timescales, remains largely unexplored. Here, we report the unique shear-thickening (dilatant) behavior of HDEEs, with viscosity increasing from 1.5 to 9.3 ± 0.01 cP as shear stress rises from 0.2 to 4.91 dyn/cm². To elucidate the relaxation dynamics, distribution of relaxation time (DRT) analysis, employing Tikhonov regularization and cross-validation techniques, was performed. This deconvolution analysis identified two distinct relaxation times: 1.137 × 10⁻⁵ s, corresponding to interfacial charge transfer, and 3.43 × 10⁻⁵ s, associated with hydrogen bond breaking and reformation dynamics. The DRT peak frequencies for charge transfer and HBDy processes, measured using a NiFeSi electrode, were identified at 14 kHz and 4.64 kHz, with corresponding polarization impedances of 423 Ω and 402 Ω, respectively. These findings establish DRT-assisted analysis as a powerful tool for probing bulk and interfacial relaxation times in HDEEs, offering key insights for optimizing large-scale production and understanding its ionic properties.