Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Ch-Ch 5% Ch-Ch 33% Ch-Cl 5% Ch-Cl 33% Cl-Cl 5% Cl-Cl 33% Figure 6. The structure factors associated with the ionic species obtained from neutron scattering for fully deuterated ChCl/glycerol mixtures. Supplementary Tables 5-9. The derivative representation of the imaginary part of complex permittivity, described in the BDS 29 methods, is also shown in the same plot as the imaginary part of complex permittivity. It is clear from this analysis that more 30 information is obtained by using the derivative representation. 31 Figure 5a in the main text shows the structural relaxation rates from fs-TA, BDS, and DMS as a function of temperature with Vogel-Fulcher-Tammann (VFT) fits. The VFT equation is where τ 0 , A, and T 0 are fitting parameters.

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The data for the real part of complex permittivity of 33mol% ChCl in glycerol at 221 K from Faraone et al. is plotted in the 33 top of Supplementary Figure 13. Data from this paper at 33mol% ChCl at 220 K is plotted as well for reference. In the bottom, 34 the derivative analysis was formed to show that if considered, a secondary, slower process (the sub-α relaxation) in present.

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The electrode polarization occurs at lower frequencies in our data, which could be due to a larger sample thickness used. This 36 unveils even more information for the secondary relaxation.
The parameters for the VFT fits from Figure 5a are in Supplementary Table 10. Data presented in Supplementary Table 12 is done with a force field tuned to simulate ethylene glycol and choline chloride 40 mixtures and is undergoing further refinement to better capture fine, concentration dependent dynamics trends. 41 13/16 Table 11. Coefficients (time in ns) fitted to Equation 5 for the dipole moment rotational dynamics based on CMD simulations at 298 K.  The Stokes-Einstein relation can be written as where D is the self-diffusion coefficient, k b is the Boltzmann constant, T is temperature, η is viscosity, and r is radius of Gly-Gly Oc-Hc 4.8 11.4 5.2 15.3 4.9 11.3 6.7 24.2 5.8 13.6 Oc-Ht 4.8 11.0 2.6 10.5 7.4 15.9 4.0 12.7 4.9 8.9 Ot-Hc 4.3 10.7 2.9 7.3 6.0 13.4 5.8 8.6 4.9 14.3 Ot-Ht 4.8 11.4 3.7 6.3 6.8 9.5 7.2 13.7 4.8 8.3
where ε ∞ is the high frequency plateau, ∆ε is the dielectric strength, τ is the mean relaxation time, σ 0 is the dc conductivity, 47 and γ and β are shape parameters. The HN function is a modified Debye function. A Debye function describes a system 48 of non-interacting systems, and the HN version has added shape parameters to account for broadening in the distribution of 49 relaxation times. Above 5mol% ChCl, the mixtures can be well described by a combination of a Debye function for the slow 50 process, a HN function for the α-relaxation, and a Random Barrier Model (RBM) for the ion dynamics. The RBM describes an 51 ion hopping motion where the ions overcome a potential energy barrier to jump to the next spot. The full equation is: 52 ε * (ω) = ε ∞ + (∆ε) Debye 1 + iωτ Debye + σ 0 τ ion ε 0 (ln(1 + iωτ ion )) + (∆ε) α (1 + (iωτ α ) γ ) β where σ 0 is the dc ionic conductivity and τ ion is the ion hopping time. The subscripts Debye, ion, and α denote the relaxation 53 the parameter is associated with.

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The mechanical spectroscopy data were fit with two Cole-Davidson (CD) modified Maxwell models for the structural 55 relaxation and the slow relaxation associated with hydrogen bonding structures as described in Supplementary Equation 5: 56 η * (ω) = G ∞,slow where G ∞ is the high frequency plateau in G ′ , τ is the relaxation time, γ is the shape parameter, and the subscripts slow and α 57 denote the distinct slow and α relaxations observed in the spectra. A Maxwell model describes a viscoelastic material, and the