Reline: A Model Deep Eutectic Solvent

• Reline is a canonical deep eutectic solvent composed of choline chloride and urea in a 1:2 molar ratio, known for its well-defined hydrogen-bond network and glass-forming behavior.
• It exhibits slow structural and reorientational dynamics, with ionic conductivity that decouples from viscosity and distinctive dielectric responses.
• Neutron scattering reveals that cholinium ions undergo long jump diffusion with a notably larger average jump length, emphasizing unique microscopic transport mechanisms.

Searching arXiv for recent and directly relevant papers on “reline.” arXiv search query: reline choline chloride urea deep eutectic solvent neutron scattering broad dielectric spectroscopy RELINE POI recommendation Reline is the canonical deep eutectic solvent (DES) composed of choline chloride and urea in a $1:2$ molar ratio, commonly written as $\text{ChCl} + 2\,\text{urea}$. In the contemporary DES literature it is treated as one of three archetypal choline-chloride-based systems, alongside ethaline and glyceline, and it is used as a model room-temperature liquid that can be deeply supercooled and ultimately undergo glassy freezing. Across neutron scattering, broadband dielectric spectroscopy, and rheology, reline is notable for a combination of slow structural and reorientational dynamics, modest but clear decoupling of ionic charge transport from viscosity-controlled relaxation, and cholinium motion characterized by relatively long waiting times between jumps but unusually large jump lengths (T. et al., 7 Oct 2025).

1. Chemical identity and canonical formulation

Reline is defined in the cited literature as a DES made from choline chloride (ChCl) and urea in a $1:2$ molar ratio (Reuter et al., 2021). Within the standard trio of prototype choline-chloride DESs, reline uses urea as the hydrogen-bond donor (HBD), whereas glyceline and ethaline use glycerol and ethylene glycol, respectively. The system is scientifically important because it is a room-temperature liquid formed by eutectic melting-point depression, yet it can also be deeply supercooled and ultimately undergo glassy freezing (Reuter et al., 2021).

Preparation details differ somewhat between studies, but the common formulation remains the same. One dielectric study prepared reline by mixing ChCl and urea for $24\ \text{h}$ at $350\ \text{K}$, yielding a colourless liquid without residual crystalline particles; the water content was reported as significantly smaller than $1\ \text{wt}\%$ (Reuter et al., 2019). A rheology-plus-dielectric study prepared reline under dry argon and measured a water content of $0.16\ \text{wt}\%$, emphasizing the strong moisture sensitivity of DES dynamics (Reuter et al., 2021). In the neutron-scattering work, ChCl was mixed with deuterated urea in the same $1:2$ molar ratio and heated to $340\ \text{K}$ until a clear homogeneous liquid formed; after cooling to $300\ \text{K}$, the mixture remained liquid (T. et al., 7 Oct 2025).

These preparation details matter because the cited work treats reline not merely as a eutectic composition, but as a sensitive hydrogen-bonded liquid in which water uptake, isotopic substitution, and thermal history can materially affect transport observables. The use of deuterated urea in particular was chosen so that the neutron signal would be dominated by the protonated cholinium cation rather than by the HBD (T. et al., 7 Oct 2025).

2. Glassy, dielectric, and rheological dynamics

Reline behaves as a glass-forming, supercooled ionic liquid-like system. Broadband dielectric spectroscopy shows a main dipolar $\text{ChCl} + 2\,\text{urea}$0-relaxation, dc ionic conductivity, and, at low temperature, a weak secondary relaxation; differential scanning calorimetry gives a glass transition around $\text{ChCl} + 2\,\text{urea}$1, while dielectric extrapolation using the criterion $\text{ChCl} + 2\,\text{urea}$2 yields $\text{ChCl} + 2\,\text{urea}$3 (Reuter et al., 2019). Among ethaline, glyceline, and reline, the same study reports that reline has the highest glass-transition temperature and the slowest orientational dynamics at a given temperature.

The dielectric spectra require explicit separation of electrode polarization, dc conductivity, dipolar orientational relaxation, and, in reline, a secondary relaxation. The conductivity contribution to dielectric loss is written as

$\text{ChCl} + 2\,\text{urea}$4

and the intrinsic response was fitted using a Cole-Davidson function for the primary $\text{ChCl} + 2\,\text{urea}$5-relaxation and a Cole-Cole function for the secondary relaxation (Reuter et al., 2019). The paper emphasizes that the step in $\text{ChCl} + 2\,\text{urea}$6 is due to reorientational molecular motions and that a translational hopping description such as the random free-energy barrier hopping model (RBM) is not sufficient for the dielectric spectra of reline.

The temperature dependence of both conductivity and relaxation is non-Arrhenius and is described with Vogel-Fulcher-Tammann (VFT) forms. In one dielectric study, the reorientational relaxation fit for reline gave $\text{ChCl} + 2\,\text{urea}$7, $\text{ChCl} + 2\,\text{urea}$8, and fragility $\text{ChCl} + 2\,\text{urea}$9 (Reuter et al., 2019). In a later combined rheology/dielectric analysis, the dielectric relaxation time was fitted with

$1:2$0

while the dc resistivity fit gave

$1:2$1

(Reuter et al., 2021). These are separate study-specific parameterizations rather than a single universal fit.

Rheologically, reline exhibits a broad structural $1:2$2-relaxation and approximate time-temperature superposition in the compliance loss master curve. The compliance function used in one analysis was

$1:2$3

with average structural relaxation time $1:2$4. For the master curve referenced to $1:2$5, the study reported

$1:2$6

while an RBM fit gave

$1:2$7

(Reuter et al., 2021). The key interpretive distinction is that RBM can fit the mechanical compliance master curve, whereas the dielectric spectra require substantial reorientational contributions.

3. Microscopic cholinium transport and jump diffusion

Quasielastic neutron scattering (QENS) was used to resolve the microscopic self-diffusion of cholinium ions in reline on picosecond-to-nanosecond timescales and ångström length scales (T. et al., 7 Oct 2025). Measurements were performed on the IRIS spectrometer at ISIS with the PG(002) analyzer in offset mode, over an energy-transfer window from $1:2$8 to $1:2$9, an energy resolution of about $24\ \text{h}$0, and an accessible momentum-transfer range $24\ \text{h}$1 to $24\ \text{h}$2. Data were collected at $24\ \text{h}$3, $24\ \text{h}$4, $24\ \text{h}$5, $24\ \text{h}$6, and $24\ \text{h}$7, with vanadium used for resolution calibration (T. et al., 7 Oct 2025).

The analysis treated cholinium motion as a two-component process: long-range jump diffusion of the molecular center of mass and localized translational motion of hydrogens within transient hydrogen-bond cages before a cage-to-cage jump occurs. The fitted scattering law was given as a sum of an elastic term and two quasielastic Lorentzian contributions convolved with the instrumental resolution. For the jump component, the Singwi–Sjölander model was used: $24\ \text{h}$8 where $24\ \text{h}$9 is the jump diffusion coefficient and $350\ \text{K}$0 is the mean residence time between successive jumps. The inferred jump length follows the standard relation

$350\ \text{K}$1

For reline, the reline-specific microscopic transport parameters quoted in the study are:

Temperature	$350\ \text{K}$2	$350\ \text{K}$3
$350\ \text{K}$4	$350\ \text{K}$5	$350\ \text{K}$6
$350\ \text{K}$7	$350\ \text{K}$8	$350\ \text{K}$9
$1\ \text{wt}\%$0	$1\ \text{wt}\%$1	$1\ \text{wt}\%$2
$1\ \text{wt}\%$3	$1\ \text{wt}\%$4	$1\ \text{wt}\%$5

The same study states that data were also taken at $1\ \text{wt}\%$6, but no separate reline-specific $1\ \text{wt}\%$7 or $1\ \text{wt}\%$8 value for that temperature is quoted in the supplied text (T. et al., 7 Oct 2025).

The most distinctive microscopic number is the average jump length. The reported nearly temperature-independent values are approximately $1\ \text{wt}\%$9 for ethaline, $0.16\ \text{wt}\%$0 for glyceline, and $0.16\ \text{wt}\%$1 for reline (T. et al., 7 Oct 2025). Thus reline exhibits the largest jump length by a substantial margin, even though its residence times are longer than those of the other two DESs.

4. Comparative transport anomalies and mechanistic interpretation

Reline differs from glyceline and ethaline in two complementary ways. At the microscopic QENS level, cholinium in reline waits longer between jumps than in the other DESs, but each successful jump is much longer; at the macroscopic dielectric and rheological level, structural and orientational dynamics remain closely tied to viscosity, while ionic charge transport becomes modestly decoupled from that common slowdown (T. et al., 7 Oct 2025).

The QENS comparison makes the crossover especially explicit. At $0.16\ \text{wt}\%$2, reline and glyceline have similar cholinium diffusivity, with $0.16\ \text{wt}\%$3 versus $0.16\ \text{wt}\%$4, despite reline’s much longer residence time. At $0.16\ \text{wt}\%$5, reline remains slightly below glyceline in $0.16\ \text{wt}\%$6: $0.16\ \text{wt}\%$7 versus $0.16\ \text{wt}\%$8. By $0.16\ \text{wt}\%$9, reline overtakes glyceline: $1:2$0 versus $1:2$1, and at $1:2$2 the difference widens to $1:2$3 versus $1:2$4 (T. et al., 7 Oct 2025). The interpretation given in the paper is the “interplay between residence time and jump length”: as temperature rises, $1:2$5 decreases strongly in reline, and once jumps occur frequently enough, the large $1:2$6 becomes decisive.

The dielectric and rheological literature reaches a related but not identical conclusion. At the same temperature, reline is the least conductive of the three DESs, largely because it has the highest $1:2$7 and the slowest structural and orientational dynamics (Reuter et al., 2019). However, reline also shows slight fractional coupling rather than strict proportionality. One dielectric study reports

$1:2$8

whereas ethaline and glyceline approximately follow $1:2$9 (Reuter et al., 2019). A later study expresses the same tendency in Walden form: $340\ \text{K}$0 with a fractional Walden exponent $340\ \text{K}$1, and states that at low temperatures the ionic conductivity in reline is enhanced by about one decade compared to expectations based on the temperature dependence of the viscosity (Reuter et al., 2021).

This makes reline atypical in a selective sense. Structural relaxation, dielectric orientational relaxation, and viscosity remain closely coupled, but ionic conduction “peels away” from them on cooling (Reuter et al., 2021). The same study argues that the results for all three DESs can be understood without invoking a revolving-door mechanism. The proposed microscopic alternatives remain unresolved, but the cited clues include weaker charge transfer in reline than in some other choline-chloride DESs, strong hydrogen bonding between chloride and HBD molecules, a supramolecular structure involving one choline ion, one chloride ion, and two urea molecules, and a chloride solvation environment different from that in ethaline (Reuter et al., 2021). A plausible implication is that reline’s charge transport and cholinium hopping are both sensitive to a distinct local aggregation topology, even though the available studies do not provide a unified structural model.

5. Experimental reach, modeling assumptions, and unresolved points

The present description of reline rests on three distinct but complementary experimental windows. Broadband dielectric spectroscopy resolves electrode polarization, conductivity, and reorientational relaxation over a broad frequency range; rheology isolates translational and structural viscoelastic relaxation; and QENS probes stochastic self-motion on molecular length and time scales (Reuter et al., 2019). Their agreement is strongest at the qualitative level: reline is a glass-forming DES with slow collective dynamics and nontrivial transport decoupling.

The QENS methodology has a major strength and a major caveat. Because neutron incoherent scattering is dominated by hydrogen, and because deuterated urea was used, the measured signal comes predominantly from the protonated cholinium cation (T. et al., 7 Oct 2025). This allows relatively direct extraction of cholinium self-motion in a multicomponent liquid. The corresponding limitation is that motions of urea and chloride are not directly resolved; their role enters only indirectly through the local environment that determines cholinium diffusion parameters.

The QENS model is also deliberately simplified. It assumes that center-of-mass jump diffusion and localized cage motion can be represented by separable Lorentzian contributions, and it uses a single mean residence time and an effective jump length. The paper explicitly notes that real DESs likely have distributions of cage lifetimes and jump distances, that no explicit EISF geometric model or reline-specific confinement radius is reported, and that no Arrhenius or VFT analysis of the QENS-derived $340\ \text{K}$2 or $340\ \text{K}$3 is provided in the quoted material (T. et al., 7 Oct 2025). Likewise, the excerpt does not provide direct quantitative comparison of the reline QENS parameters with NMR, conductivity, viscosity, or molecular-dynamics values.

The dielectric and rheological interpretations also have clear scope conditions. In the dielectric work, up to two distributed RC circuits were introduced to model blocking-electrode effects, and the authors argue that modulus-only or RBM-only descriptions are physically incomplete for reline because they neglect the evident dipolar $340\ \text{K}$4-relaxation (Reuter et al., 2019). In the rheology-centered analysis, RBM is instead used successfully for compliance spectra, which supports the narrower claim that mechanical response mainly probes translational and structural dynamics (Reuter et al., 2021). Taken together, these studies do not contradict one another; they delimit different observables.

A common misconception is therefore that reline is either simply “more viscous and slower” than the other prototype DESs, or that one mechanism alone explains all of its transport data. The cited work supports neither reduction. Reline is slower in many macroscopic measures at a fixed temperature, but it also shows enhanced conductivity relative to its viscosity and a cholinium jump length substantially larger than those of glyceline and ethaline (Reuter et al., 2021).

6. Unrelated acronymic usage of “RELINE”

In an unrelated literature, “RELINE” denotes “REcommendations with muLtIple Network Embeddings,” a point-of-interest recommendation model for location-based social networks rather than a deep eutectic solvent (Christoforidis et al., 2019). That model represents user behavior through eight relational graphs—user-user, user-POI, user-time, user-route, POI-POI, POI-user, POI-time, and POI-stay-point—and jointly embeds them in one shared latent space (Christoforidis et al., 2019).

Its final recommendation function is

$340\ \text{K}$5

combining social influence, geographical influence, temporal influence, and preference dynamics (Christoforidis et al., 2019). This usage is acronymic and should be distinguished from chemical reline, the ChCl/urea DES.