How frictional response during solute solvation controls solute rotation in naturally abundant deep eutectic solvent (NADES)? A case study with amino acid derivative containing DES

Abstract

We explored frictional response of a room temperature naturally abundant deep eutectic solvent made of betaine, urea and water ([Bet:Ure:Wat (11.7:12:1 weight ratio)]) via temperature dependent measurements of solute-centered dynamics, and characterized the coupling between the relaxation rates and medium friction. Time-resolved fluorescence studies with this medium at different temperatures revealed fractional viscosity dependencies for solute rotation and solvation rates, suggesting strong temporal heterogeneity in the medium. Intriguingly, such a substantial viscosity decoupling was found to occur at a temperature ~90–130 K above the measured glass transition temperature (Tg~218 K) of this amino acid derivative containing DES. Estimated activation energies from dynamic fluorescence anisotropies of hydrophobic and hydrophilic solutes were nearly half (~30 kJ mol−1) of that estimated from temperature dependent viscosity measurements (~57 kJ mol−1). Both rotational and solvation energy relaxations for these solutes were found strongly non-exponential yet the friction that controlled solute's rotation in this biodegradable deep eutectic solvent did not fully map the resistance that medium particles experienced while solvating the solute. This is new, and contrasting to results found for several complex systems that include ionic deep eutectics, ionic liquids, and confined water. This observation should be reexamined via more sophisticated measurements. Temperature dependent densities and refractive indices were measured to understand the dipolar solvation response in this medium, particularly the correctness of the missing components estimated from a well-known approximate method developed for conventional room temperature polar solvents.