Glyceline: Prototypical Deep Eutectic Solvent

• Glyceline is a deep eutectic solvent composed of choline chloride and glycerol (1:2), exhibiting distinctive hydrogen bonding and intermediate ionic/dipolar dynamics.
• Dielectric, rheological, and QENS studies reveal non-Arrhenius behavior with coupled viscosity, structural relaxation, and a glass transition near 175 K.
• Comparative analysis with ethaline and reline highlights glyceline's balanced transport properties, informing DES design to optimize glass transition and dynamic coupling.

Glyceline is the 1:2 molar mixture of choline chloride (ChCl) and glycerol and, in the deep eutectic solvent (DES) literature, is treated as one of the three prototypical DESs alongside ethaline and reline. In that context it is a low-melting, liquid electrolyte-like medium formed through eutectic depression and extensive hydrogen bonding, with both ionic transport and reorientational dipolar dynamics, and it can be supercooled into a regime governed by glassy freezing (Reuter et al., 2019, Reuter et al., 2021, T. et al., 7 Oct 2025). A distinct usage also appears in one meteoritic-organics paper, where the broader, more evocative term “Glyceline” is effectively associated with hemoglycin, a proposed glycine–iron polymer; this nomenclature is separate from the DES usage (McGeoch et al., 2023).

1. Definition, composition, and place in the DES literature

In the DES literature, glyceline is defined specifically as ChCl:glycerol in a 1:2 molar ratio. It is described as a system in which a hydrogen-bond acceptor salt and a hydrogen-bond donor form a low-melting liquid through strong hydrogen-bonding interactions. Because choline and glycerol are both polar, glyceline exhibits both ionic charge transport and dipolar molecular reorientation (Reuter et al., 2019, Reuter et al., 2021).

This formulation makes glyceline a benchmark system for comparing different dynamical channels in DESs. In particular, its behavior has been studied against ethaline and reline to isolate the role of the hydrogen-bond donor in controlling conductivity, structural relaxation, and microscopic diffusion. A central consequence of this role is that glyceline is neither the fastest nor the slowest of the three prototypical DESs; rather, it repeatedly appears as an intermediate case in both macroscopic and microscopic transport measurements (Reuter et al., 2019, T. et al., 7 Oct 2025).

2. Dielectric response, ionic conductivity, and glassy freezing

Broadband dielectric spectroscopy on glyceline has been carried out over roughly 0.1 Hz to 3 GHz, from the low-viscosity liquid into the deeply supercooled regime near the glass transition. In this framework, three observables are tracked: the dc ionic conductivity, the dipolar α-relaxation, and their evolution as the liquid approaches glassy freezing (Reuter et al., 2019).

Glyceline shows pronounced non-Arrhenius conductivity, characteristic of a glass-forming ionic system. Its conductivity increases strongly with temperature, deviates from simple Arrhenius behavior, and is lower than ethaline’s at room temperature while remaining distinct from reline’s. The reorientational dynamics are characterized by a single α-relaxation process attributed to dipolar molecular reorientation, and both conductivity and relaxation slow strongly on cooling in a way described by Vogel-Fulcher-Tammann behavior. The extrapolated glass transition from the relaxation-time criterion gives $T_g \approx 175$ K, in very good agreement with the DSC estimate $T_g \approx 175$ K (Reuter et al., 2019).

A glyceline-specific result is that, at low temperatures, its α-relaxation is much faster than pure glycerol, whereas at high temperatures the relaxation times of glycerol and glyceline become similar. The paper interprets this as a consequence of adding choline chloride to glycerol and thereby modifying the hydrogen-bond network so that reorientation becomes easier than in pure glycerol at low $T$ (Reuter et al., 2019).

3. Rheology, structural relaxation, and coupling of dynamic observables

A complementary characterization comes from rheological measurements extending from the low-viscosity to the high-viscosity supercooled-liquid regime. In glyceline, the shear-mechanical spectra show a pronounced peak in $G''(\nu)$ that shifts strongly to lower frequencies on cooling, signaling the slowing of structural shear relaxation as the system approaches the glass transition (Reuter et al., 2021).

The mechanical compliance spectra can be described well by the random free-energy barrier hopping model (RBM), and time-temperature superposition works well: the compliance spectra can be horizontally shifted onto a master curve, indicating that their shape is nearly temperature invariant over the measured range. The paper also notes that a Cole-Davidson-type fit works well for the mechanical master curve, and that the fit parameter $\beta$ decreases from about 0.4 to 0.29 on cooling, indicating modest broadening of the relaxation spectrum (Reuter et al., 2021).

The dielectric spectra of glyceline, however, cannot be accounted for by ionic hopping alone. A description based only on the RBM is insufficient because the dielectric response contains substantial contributions from dipolar reorientational relaxation. This distinction is central: rheology primarily reflects translational/structural dynamics, whereas dielectric spectroscopy in glyceline contains both conductivity and reorientational α-relaxation (Reuter et al., 2021).

The major synthesis is that, for glyceline, viscosity, structural relaxation time, dielectric relaxation time, and dc resistivity all follow essentially the same non-Arrhenius temperature dependence. When plotted on an Arrhenius-like scale, these quantities can be made to overlap almost perfectly by a simple vertical offset. The authors therefore conclude that glyceline exhibits essentially identical temperature dependences for all dynamic quantities measured, indicating close coupling of ionic translational motion, molecular reorientation, and viscous structural relaxation (Reuter et al., 2021).

4. Microscopic diffusion and the cage-jump picture

Quasielastic neutron scattering (QENS) provides the microscopic transport picture for glyceline. The measurements were performed on the IRIS spectrometer at ISIS, using a PG(002) analyzer in offset mode, an energy-transfer window of about $-0.3$ to $+1$ meV, energy resolution of about 17 $\mu$eV, and an accessible $Q$-range of $0.54$–$T_g \approx 175$0. Temperatures of 300, 315, 330, 355, and 365 K were examined, and deuterated HBDs were used so that the QENS signal was dominated by cholinium-ion dynamics (T. et al., 7 Oct 2025).

The motion is modeled as a two-component diffusion process: jump diffusion of the cholinium ion’s center of mass and localized translation within transient hydrogen-bond cages. This is the paper’s cage-jump picture. To quantify the $T_g \approx 175$1-dependence of the quasielastic width, the authors use the Singwi–Sjölander jump diffusion model,

$T_g \approx 175$2

where $T_g \approx 175$3 is the jump diffusion coefficient and $T_g \approx 175$4 is the residence time (T. et al., 7 Oct 2025).

For glyceline, the reported $T_g \approx 175$5 values are $T_g \approx 175$6, $T_g \approx 175$7, $T_g \approx 175$8, and $T_g \approx 175$9 at 300, 315, 330, and 355 K, respectively. The corresponding residence times are 58.07 ps, 42.90 ps, 20.07 ps, and 10.12 ps. The average jump length is nearly temperature independent, with glyceline: 1.15 $T$0, compared with ethaline: 1.05 $T$1 and reline: 1.72 $T$2 (T. et al., 7 Oct 2025).

These data place glyceline in an intermediate microscopic regime. Relative to ethaline, glyceline is slower because the cholinium ions remain trapped longer. Relative to reline, the comparison is subtler: reline and glyceline are comparable at low temperature, but reline surpasses glyceline above 330 K because reline combines longer residence times with significantly larger jump length (T. et al., 7 Oct 2025).

5. Comparative interpretation and implications for DES design

Across dielectric, rheological, and neutron-scattering studies, glyceline consistently occupies an intermediate position between ethaline and reline. In conductivity it is below ethaline; in microscopic diffusion it shows moderate cage trapping, moderate jump length, and moderate mobility; in glass-transition terms it lies between ethaline: 155 K and reline: 209 K from VFT extrapolation, versus 205 K from DSC, with glyceline: 175 K (Reuter et al., 2019, T. et al., 7 Oct 2025).

This comparative position is important because it constrains interpretation. One possible misconception is that DES conductivity is determined mainly by ion content. The glyceline results do not support that reduction. Instead, the conductivity is strongly constrained by glass-transition temperature, fragility, and translation–rotation coupling, and the microscopic transport is governed by the interplay of residence time, jump length, hydrogen-bond network topology, and temperature (Reuter et al., 2019, T. et al., 7 Oct 2025).

A second misconception is that a special revolving-door mechanism is generally required to explain DES charge transport. For glyceline, the results can be understood without invoking a revolving-door mechanism previously considered as a possible charge-transport mechanism in DESs. Its ionic and molecular dynamics track one another closely and are consistent with a common dependence on viscosity. In this respect glyceline behaves like ethaline and differs from reline, for which ionic conductivity becomes enhanced relative to viscosity-based expectations at low temperature and follows a fractional Walden rule (Reuter et al., 2021).

The design implication drawn explicitly in the dielectric work is that DESs intended for electrochemical use should be optimized by lowering the glass temperature, increasing fragility, and reducing translational–rotational coupling. For glyceline specifically, the data suggest a DES whose transport is balanced rather than extreme: glycerol creates an H-bond environment that yields intermediate cholinium mobility and a clear cage-jump dynamics (Reuter et al., 2019, T. et al., 7 Oct 2025).

6. Alternative usage: hemoglycin as “Glyceline”

A distinct and terminologically separate usage appears in the paper “Fossil and present-day stromatolite ooids contain a meteoritic polymer of glycine and iron” (McGeoch et al., 2023). There, the authors use the name hemoglycin for a proposed meteoritic glycine–iron polymer, while effectively treating this as the object behind the broader, more evocative term “Glyceline.”

In that usage, the key species is a 1494 Da core unit consisting of two antiparallel polyglycine strands linked at both ends by iron atoms. The polymer is proposed to form two- and three-dimensional lattices with an inter-vertex distance of 4.9 nm. The reported signatures include MALDI peaks at $T$3 and $T$4, x-ray fluorescence with a main emission near 480 nm and an absorption dip around 465 nm, high-order diffraction rings fitted by a first-order spacing of $T$5, and FTIR amide I splitting interpreted as an extended antiparallel β-sheet. The material is reported in carbonaceous chondritic meteorites and in fossil and modern stromatolitic ooids (McGeoch et al., 2023).

This usage refers not to the ChCl:glycerol DES, but to a proposed glycine-based, iron-bearing, space-derived polymer. The coexistence of these two meanings makes the term Glyceline context-dependent in the current literature.