Partial Viscosity Decoupling of Solute Solvation, Rotation, and Translation Dynamics in Lauric Acid/Menthol Deep Eutectic Solvent: Modulation of Dynamic Heterogeneity with Length Scale

Abstract

Deep eutectic solvents (DESs) are new-generation media that can be fine-tuned to have desired properties circumventing economic and environmental issues. Typically, these are ionic, and only recently, nonionic DESs, having interesting properties, are being explored. In this report, we examined the structure and dynamics of a nonionic lauric acid/menthol (LA/Men) DES through steady-state emission, solvation dynamics, time-resolved fluorescence anisotropy, and translational diffusion dynamics. The zero shift in the emission spectra of coumarin 153 (a solvatochromic dye) as a function of the excitation wavelength suggests that LA/Men DES is spatially homogenous. Decoupling (p = 0.63) of the average solvation time, ⟨τs⟩, from medium viscosity suggests the presence of mild dynamic heterogeneity in the system. Rotational time, ⟨τr⟩, which reflects the nature of the first solvation shell, shows little decoupling (p = 0.81), suggesting it to be fairly dynamically homogeneous at a shorter length scale. An Arrhenius-type analysis also proves that rotation is mainly controlled by medium viscosity. Translational diffusion time, ⟨τD⟩, which provides information at a larger length scale, is strongly decoupled from medium viscosity (p = 0.29). This indicates that at a larger length scale, the DES is quite dynamically heterogeneous. The slow component of solvation time, which is believed to originate at a larger length scale, correlates well with the translational diffusion timescale having similar activation energies. This suggests that their origin is same. Expectedly, for the long component of solvation time, the decoupling is quite strong (p = 0.30). Overall, our result demonstrates the structure and dynamics of the nonionic LA/Men DES, and the existence of length scale-dependent heterogeneity has been proposed.

An Overview of Structure and Dynamics Associated with Hydrophobic Deep Eutectic Solvents and Their Applications in Extraction Processes

“…The non‐Gaussian parameters shown in Figure 16(a,b) show the movements of the HDESs molecules possess dynamic heterogeneity. Earlier experimental studies also predicted the existence of dynamic heterogeneity in menthol‐based HDESs [101] . Figure 16(a) depicts that the magnitude of the non‐Gaussian parameter value increases with an increase in the acid‘s alkyl chain, highlighting an increase in the dynamic heterogeneity of DLM molecules.…”

“…The simulated and experimental investigations support the fact that urea plus acetamide based DES possess lesser dynamic heterogeneity than the DLMbased HDESs. [101,103,104] Figure 16(c) shows that the deviation in the self-van Hove correlation function from its ideal counter part (Gaussian distribution) is more at the shorter time scales in DLM-C6 HDESs, implying enhanced dynamic heterogeneity at shorter times. With increasing chain length of the acid, the selfvan Hove correlation functions depicted enhanced caging which can be visualized from the simulation trajectories shown in Figure 17.…”

“…Sen and co-workers studied the translational, rotational, and solvation dynamics associated with DLM-C12 (2 : 1) HDES by employing various fluorescence techniques. [101] Since HBD in DLM-C12 is a long alkyl chain, the HDES is expected to have micelle-like domains supported by high solvation time. The authors found evidence of mild dynamic heterogeneity due to decoupling of viscosity from the average solvation time.…”

“…Earlier experimental studies also predicted the existence of dynamic heterogeneity in menthol-based HDESs. [101] Figure 16(a) depicts that the magnitude of the non-Gaussian parameter value increases with an increase in the acid's alkyl chain, highlighting an increase in the dynamic heterogeneity of DLM molecules. The simulated and experimental investigations support the fact that urea plus acetamide based DES possess lesser dynamic heterogeneity than the DLMbased HDESs.…”

Deep Eutectic Solvents: A New Class of Versatile Liquids

“…A number of studies report results from theory (Hayyan, Tuinier) , as well as classical and ab initio molecular dynamics simulations (Atilhan, Aparicio, Kashyap, Hung, Chaumont, Banerjee, Pádua, Kirchner, Zahn, Paul, and Balasubramanian) − to shed light on the structure, thermodynamics, and dynamics of DESs. X-ray and neutron scattering studies, sometimes in conjunction with molecular simulations, are reported to better understand DES structure (Triolo, Russina, Lewandowski, Brown, Atkin, Maginn, and Edler). − Various spectroscopic techniques are used, sometimes in conjunction with simulations, to study solvation structure, dynamics, and diffusion (Mitra, Sen, Biswas, Pandey, Kuroda, Gurkan, and Baker). − Other studies have focused on catalysis (Liu, Zhang), the effect of water on properties (Gutiérrez, Panzer), , electrodeposition (Magagnin), and fundamental thermodynamics and phase behavior (Coutinho, Marrucho). , …”