Ethaline: Deep Eutectic Solvent Insights
- Ethaline is a deep eutectic solvent composed of choline chloride and ethylene glycol in a strict 1:2 molar ratio, forming a homogeneous, viscous liquid with an extensive hydrogen-bond network.
- The pronounced melting-point depression (over 100 K below individual components) and glassy transport dynamics highlight its application potential in electrochemistry, biomass processing, and nanoconfinement studies.
- Recent studies reveal its reactive instability via an SN2 decomposition pathway that produces chloromethane and dimethylaminoethanol, challenging its green chemistry profile.
Ethaline is a prototypical deep eutectic solvent (DES) formed by mixing choline chloride (ChCl) and ethylene glycol (EG) in a 1:2 molar ratio, often written equivalently as a 2:1 mixture of EG and ChCl. It is commonly classified as a type-III DES and is among the most widely studied choline-chloride-based eutectics. Its defining feature is a pronounced melting-point depression driven by strong hydrogen bonding between chloride and hydroxyl-bearing species, producing a homogeneous, viscous liquid with an extended hydrogen-bond network near room temperature. That combination has made ethaline central in studies of electrochemistry, biomass processing, transport in glass-forming liquids, hydration effects, and confinement. At the same time, recent work has qualified its “green” status by reporting partial decomposition into chloromethane and dimethylaminoethanol under mild conditions (T. et al., 7 Oct 2025, Yang et al., 2024).
1. Definition, composition, and eutectic character
Ethaline consists of choline chloride, , and ethylene glycol, , in a strict 1:2 molar ratio of ChCl:EG or, equivalently, 2:1 EG:ChCl. In shorthand form,
It is routinely prepared by mixing the components until homogeneous; reported procedures include stirring ChCl and EG at for to obtain a liquid with water, and heating at $60\,^\circ\mathrm{C}$ for until homogenization (Reuter et al., 2019, Jani et al., 2021).
Its eutectic identity is tied to strong, directional hydrogen bonding between chloride and hydroxyl groups. One study states that the eutectic mixture melts near , more than below the individual components, because hydrogen bonding between the quaternary ammonium cation and EG hydroxyls disorders the lattice and depresses the melting point (T. et al., 7 Oct 2025). A force-field study likewise describes the mixture as behaving like a single “liquid salt,” with the melting point dropping to near room temperature because the neutral hydrogen-bond donor interacts strongly and directionally with the hydrogen-bond acceptor chloride (Souza et al., 2021).
Ethaline has often been presented as attractive because of low toxicity, biodegradability, and low cost, and because it is useful in electrochemistry, biomass processing, metal leaching, and carbon capture (T. et al., 7 Oct 2025, Yang et al., 2024). That application profile, however, must be read alongside later evidence for decomposition chemistry, discussed below.
2. Hydrogen-bond network and microscopic structure
At the molecular level, ethaline is characterized by an extensive, percolating hydrogen-bond network in which chloride is the dominant acceptor. X-ray and molecular-dynamics studies cited in the decomposition work report that each 0 typically accepts 1–2 hydrogen bonds from nearby hydroxyl donors, stabilizing the liquid (Yang et al., 2024). A hydrated-state NMR study similarly describes EG–3, EG–EG, and 4–5 contacts as the dominant interactions, yielding a percolating H-bond matrix (Hinz et al., 2024).
More explicit local-structure metrics were reported in the fine-tuned polarizable CL&Pol force-field study. In that model, the chloride–hydroxyl radial distribution functions exhibit first-shell peaks at 6 at 7 for choline hydroxyl protons and 8 at 9 for ethylene-glycol hydroxyl protons. Integration to the first minimum near 0 gives coordination numbers 1 and 2, implying that each chloride accepts on average about two OH%%%%198198%%%%4 bonds from choline and about four from EG (Souza et al., 2021). In the same study, the total X-ray structure factor 5 shows the characteristic charge-alternation peak around 6, corresponding to 7 real-space correlations.
The structural description is not independent of the interaction model. The original CL&Pol formulation produced unphysically strong pre-peaks and anti-peaks at very low 8 (9), interpreted as spurious nano-segregation on 0 length scales. After re-optimizing 1 and 2 against ab initio RDFs of Alizadeh et al., and applying Tang–Toennies damping with 3 and 4, the low-5 features shift to 6, corresponding to a more realistic 7 heterogeneity (Souza et al., 2021).
This structural picture is consistent across several studies: ethaline is not a simple binary liquid but a strongly associated ionic-hydrogen-bonded network in which chloride-centered coordination plays the organizing role. A plausible implication is that small changes in hydrogen-bond donor identity, water content, or confinement can alter dynamics without necessarily destroying the underlying network.
3. Bulk transport, relaxation, and multiscale dynamics
Bulk dynamical studies place ethaline among strongly glass-forming ionic liquids with marked non-Arrhenius transport. Broadband dielectric spectroscopy from 8 to 9 and from 0 down to 1 showed that the dc conductivity decreases by 2–3 decades on cooling and is well described by a Vogel–Fulcher–Tammann law,
4
For one study of neat ethaline, the fitted parameters were 5, 6, and 7; the same work reported 8, 9, 0, fragility index 1, 2, and room-temperature conductivity 3 (Reuter et al., 2019).
The relation between conductivity, viscosity, and dipolar relaxation has been interpreted in two closely related but not identical ways. One dielectric study found 4 over the entire temperature range and described this proportionality in terms of a Debye–Stokes–Einstein-type relation, with a “revolving-door” or “paddle-wheel” mechanism in which dipolar rotations open transient pathways for ion hopping (Reuter et al., 2019). A later rheology-plus-dielectric analysis concluded that, for ethaline, 5 with Walden exponent 6 and 7, and argued that the data can be understood without invoking a revolving-door mechanism, instead as viscosity-controlled translational–rotational coupling (Reuter et al., 2021). The common empirical point is that ionic transport, dielectric 8-relaxation, and viscosity display essentially identical VFT temperature dependences in ethaline.
Neutron scattering and NMR extend that description across shorter length and time scales. A 2025 multiscale study combined pulsed-field-gradient NMR with time-of-flight and backscattering QENS on isotopically labelled samples. On the micrometer scale, the self-diffusion coefficients obey classical hydrodynamics: 9, with $60\,^\circ\mathrm{C}$0–$60\,^\circ\mathrm{C}$1. Using the Stokes–Einstein relation,
$60\,^\circ\mathrm{C}$2
with $60\,^\circ\mathrm{C}$3–$60\,^\circ\mathrm{C}$4 and $60\,^\circ\mathrm{C}$5, the expected ratio $60\,^\circ\mathrm{C}$6–$60\,^\circ\mathrm{C}$7 agrees with experiment (Kamar et al., 23 Sep 2025).
At nanometer scales that hydrodynamic size dependence disappears. The same study found that at $60\,^\circ\mathrm{C}$8,
$60\,^\circ\mathrm{C}$9
indicating dynamically correlated supramolecular units held together by 0 hydrogen bonds (Kamar et al., 23 Sep 2025). The sub-nanometer motions preceding Fickian diffusion were described as jumps of length 1–2 separated by residence times 3–4, with localized motions of amplitude 5 and correlation times 6–7 (Kamar et al., 23 Sep 2025).
A cholinium-selective QENS study on IRIS at ISIS resolved these same ideas with an explicit cage-jump model on 8–9 and 0–1 scales: 2 The jump component was fitted by the Singwi–Sjölander expression,
3
and the mean jump length followed from the Chudley–Elliott relation,
4
For ethaline, 5 increased from 6 to 7 in units of 8 between 9 and 0, while 1 decreased from 2 to 3; 4 remained essentially temperature invariant at 5. Compared with glyceline and reline, ethaline showed the shortest residence times at all temperatures and the highest cholinium self-diffusion coefficients, whereas reline exhibited larger jump lengths and crossed over to higher 6 than glyceline above 7 (T. et al., 7 Oct 2025).
4. Hydration, glassy freezing, and aqueous regimes
Hydration reorganizes ethaline’s phase behavior without immediately destroying its native network. Dielectric studies on mixtures with water mass fraction 8–9 distinguished a “water-in-DES” regime for 00, where the liquid remains macroscopically homogeneous, from a “DES-in-water” regime for 01, where cooling induces phase separation into ice plus a maximally freeze-concentrated DES solution of composition 02 (Jani et al., 2021). A calorimetric phase-diagram study reached the same threshold, reporting 03 as the boundary above which crystallization occurs and below which neat and moderately hydrated mixtures remain glass-forming (Malfait et al., 2022).
In the homogeneous regime, conductivity and dipolar reorientation remain inversely proportional, 04, over the timescale covered by the dielectric study. For 05, the coupling exponent is 06, corresponding to classical Debye–Stokes–Einstein behavior; for 07, 08, indicating fractional decoupling once freeze-concentration and ice domains emerge (Jani et al., 2021). The same study reported that the fragility index decreases from 09 for neat ethaline to 10 at 11, while the stretching parameter inferred from 12 rises from 13 at 14 to 15 for 16.
Component-selective magnetic resonance studies refine that picture. In a sample containing 17 water, stimulated-echo experiments yielded 18 for both 19 and 20 probes, showing no increase in dynamic heterogeneity upon hydration (Hinz et al., 2024). The same work found that water and ethylene glycol display very similar mobilities over 21–22, and that adding 23 24 reduces 25 and 26 by factors of 27–28 near 29–30. It also reported that water acts as an antifreeze, shifting 31 down by 32 at 33 34 (Hinz et al., 2024).
These results support a consistent description of moderate hydration as plasticization rather than demixing. The aqueous ethaline studies explicitly state that, in the water-in-DES regime, water “blends in” with the EG hydrogen-bond network and does not detectably increase heterogeneity (Hinz et al., 2024). A plausible implication is that the principal crossover with increasing water is not immediate local disruption but the onset of phase-separated freeze concentration near the 35–36 threshold.
5. Nanoconfinement, phase behavior, and interfacial thermodynamics
When ethaline is confined in mesoporous silica, neutron scattering indicates substantial structural robustness. In cylindrical mesopores of SBA-15 with 37 and MCM-41 with 38, neutron diffraction found no evidence of core-shell segregation within the pore cross-section. SBA-15 filled with ethaline showed Bragg intensities reduced in proportion to contrast without 39-shift, consistent with uniform filling across the 40 pore radius. In MCM-41, the smaller intensity reduction was reproduced by the homogeneous-filling model if only 41 of the pore volume was filled, again without evidence for core-shell segregation or microphase separation (Nadim et al., 8 Jul 2026).
The confined dynamics preserve the bulk jump-diffusion phenomenology but with longer waiting times between jumps. At 42, the translational linewidths were fitted with
43
Representative values were 44 and 45 in bulk ethaline, 46 and 47–48 in SBA-15, and 49 and 50 in MCM-41 (Nadim et al., 8 Jul 2026). Localized in-cage motion was described through the elastic incoherent structure factor,
51
with 52 and 53 in bulk, 54 and 55 in SBA-15, and 56 and 57–58 in MCM-41 (Nadim et al., 8 Jul 2026). The principal confinement effect is therefore a substantial increase in residence and relaxation times, not a collapse of translational mobility.
Calorimetric studies of hydrated ethaline under confinement add a thermodynamic dimension. For bulk and confined systems alike, 59 marks the threshold above which ice crystallizes and below which glass-forming solutions are observed (Malfait et al., 2022). The melting-point depression in confinement was analyzed using an extended Gibbs–Thomson–Raoult relation,
60
The study reported melting depressions of up to 61, good agreement of the model for bulk and SBA-15 at high water content, and systematic deviation in MCM-41, where measured 62 values lie above the predictions (Malfait et al., 2022). Back-calculated water activities 63 are slightly elevated in SBA-15 relative to bulk and exceed bulk values in MCM-41, even exceeding unity in extreme confinement. Behboudi et al. vapor-pressure data for bulk were reported to agree with calorimetry-derived 64 values (Malfait et al., 2022).
Taken together, the confinement studies indicate that ethaline can remain structurally homogeneous and dynamically recognizable even inside nanometer-scale pores, while its freezing, water activity, and cage lifetimes become strongly geometry dependent.
6. Chemical stability, decomposition pathways, and computational description
The main qualification to ethaline’s benign reputation is chemical instability associated with the choline chloride component. A 2024 study reported partial decomposition of ethaline at room temperature into toxic chloromethane and dimethylaminoethanol, and concluded that choline chloride is susceptible to decomposition in strongly hydrogen-bound mixtures (Yang et al., 2024). Experimentally, isothermal TGA at 65 for 66 showed 67 mass loss. GC–TCD and GC–MS identified chloromethane in the headspace at retention 68, dimethylaminoethanol in the condensed phase at retention 69, with 70 after 71 at 72, together with minor trimethylamine and 2-methoxyethanol (Yang et al., 2024).
The principal pathway was written as an 73 process,
74
initiated by hydrogen-bond fluctuations that bind chloride near reaction sites (Yang et al., 2024). The same work reported a vacuum barrier 75, a “rigid” minimum-energy-pathway barrier of 76, and an average barrier of 77 after 78 solvent relaxation plus 79 NVE AIMD at 80, with fluctuations of 81–82 at the critical coordinate. At the transition state, 83 is held by two strong hydrogen bonds of 84 each (Yang et al., 2024).
That decomposition study also introduced a quantum-chemically accurate workflow based on PBE085-D3 with 86, TZVP-MOLOPT, and CP2K, benchmarked against CCSD(T) for ethylene-glycol ionization potentials. Active learning through “FLARE + OPLS” generated 87 training snapshots spanning intramolecular distortions, intermolecular hydrogen-bond configurations, and reactive 88 geometries; the final Allegro-v2 model reproduced bulk RDFs and thermodynamics at 89 within 90, gave force RMSE 91, and reproduced the 92 barrier within 93 of PBE094-D3 (Yang et al., 2024).
Independent simulation work has focused on equilibrium structure and transport rather than reaction chemistry. The corrected CL&Pol model reproduced density, viscosity, and surface tension at 95 and 96 with close agreement to experiment: 97 versus 98, 99 versus 00, and 01 versus 02. In the same comparison, CL&Pol gave 03, 04, and 05 in units of 06, while experimental values were 07 and 08 (Souza et al., 2021). These results show that accurate modeling of ethaline requires both chemically realistic short-range hydrogen-bond structure and controlled polarization response.
Ethaline therefore occupies a technically important but nontrivial position within the DES literature. It is prototypical in stoichiometry, hydrogen-bond organization, glassy transport, and confinement behavior, yet it also exposes a central limitation of choline-chloride-based design: the same hydrogen-bond environment that produces strong eutectic behavior can stabilize reactive configurations that compromise chemical stability.
