Temperature Dependence of Structural Landscape of Thymol-Coumarin Hydrophobic Deep Eutectic Solvents as Revealed by...

Hydrophobic deep eutectic solvents (HDESs) have gained significant attention as environmentally benign, water-immiscible media poised to transform extraction processes and other solvent-driven applications. In the present study, we explore the temperature dependence of the structural landscape of bulk thymol-coumarin (THY:COU) mixtures with HDES-forming compositions, 2:1 and 1:1, respectively, by employing X-ray scattering experiments and molecular dynamics simulations. The existence of structural intermediate-range ordering or nanoscale heterogeneity is revealed by the presence of a prepeak in the experimental X-ray scattering intensity (IExpt(q)) and its simulated counterpart (ISim(q)) for both compositions that appears to lose its defined feature as the temperature rises. From the simulated partial structure functions, it is revealed that with an increase in temperature, the polar-polar self-correlation and polar-nonpolar cross-correlation have reduced amplitudes in the prepeak region of the simulated total structure function (S(q)), implying a weakening of polar and nonpolar self-segregations and corroborating a temperature-driven shift toward a less heterogeneous structural environment. Using a judicious partitioning of the simulated total S(q), we show that the studied HDESs are characterized by different forms of alternations emanating from density oscillations of a group of carbon atoms of different aromatic moieties, depicting dispersion correlations apart from the polar-nonpolar alternations in the low-q region. Our analysis shows a temperature-induced weakening and shift in key thymol-coumarin and thymol-thymol hydrogen bond contacts, concomitant with a decrease in the average number of hydrogen bond pairs formed per donor molecule. The spatial distribution functions also confirm a discernible weakening of thymol-coumarin and thymol-thymol hydrogen-bonding features at higher temperatures.