Revealing Solvation within [l(-)-Menthol : Thymol] Deep Eutectic Solvents via Microfluidity Assessment

Properties of deep eutectic solvents (DESs) can be effectively fine-tuned by judicious selection of the constituents and their relative amounts. The DESs prepared by mixing l(-)-menthol (Men) and thymol (Thy), two nonionic constituents, offer a nonpolar solvation environment desired in many chemical applications. These DESs are shown to exhibit temperature-dependent nonideality and possess complex intermolecular interactions dominated by sterically hindered H-bonded clusters. Key insights into solute solvation within the (Men : Thy) DES system are obtained by employing structurally different fluorescence microfluidity probes that operate based on different mechanisms. The (Men : Thy) DES system at nine (9) different molar ratios, from (5 : 1) to (1 : 5), in the temperature (T) range of 293 to 363 K is investigated using an intramolecular excimer-forming probe, two fluorescence anisotropy probes, and a fluorescence intensity-based microfluidity probe. The logarithm of microviscosity (ln η_micro) estimated from the response of the excimer-forming probe varies linearly with the logarithm of the bulk viscosity (ln η) across all nine compositions in the entire temperature range. Further, excellent linear correlation is observed between the rate constant (k_a) of the intramolecular excimer formation and T/η implying adherence to the Stokes-Einstein relationship. Rotational reorientation times (θ) obtained from the excited-state fluorescence anisotropy decay of the two structurally different probes follow the Perrin formulation (θ varying linearly with η/T) indicating the absence of significant microheterogeneity toward solute rotational diffusion within the (Men : Thy) DES system. The response of the fluorescence intensity (I_F) probe also exhibits a simple Arrhenius-type T dependence with I_F versus η for all DES compositions, fitting well to exponential growth-to-maxima. The solute solvation behavior observed from the responses of different fluorophores indicates a homogeneous solubilization environment afforded by the (Men : Thy) DES system that is independent of the composition and temperature. These findings have significant implications in chemical synthesis and analysis where such nonpolar DES systems have the potential to be effectively employed.