Dielectric relaxation (DR) measurements in the frequency window 0.2 ≤ ν(GHz) ≤ 50 for deep eutectic solvents (DESs) made of acetamide (CH3CONH2) and urea (NH2CONH2) with the general composition, [f CH3CONH2 + (1 − f) NH2CONH2] at f = 0.6 and 0.7, reveal three distinct relaxation time scales—τ1 ∼ 120 ps, τ2 ∼ 40 ps, and τ3 ∼ 5 ps. Qualitatively similar time scales have been observed for DR of neat molten urea, whereas the reported DR for neat molten acetamide in the same frequency window reflects two relaxation processes with no trace of ∼100 ps time scale. This slowest DR time scale (τ1) resembles closely to the long-time constant of the simulated structural H-bond relaxation (CHB(t)) involving urea pairs. Similarity in activation energies estimated from the temperature dependent DR measurements (335 ≤ T/K ≤ 363) and structural H-bond relaxations indicates that the structural H-bond relaxation overwhelmingly dominates the slowest DR relaxation in these DESs. Simulated collective reorientational correlation functions (Cℓ(t)), on the other hand, suggest that the second slower time scale (∼40 ps) derives contributions from both the single particle orientation dynamics and structural H-bond relaxation, leaving no role for hydrodynamic molecular rotations. The sub-10 ps DR time scale has been found to be connected to the fast reorientation dynamics of the component molecules (acetamide or urea). Fractional viscosity dependence for the longest DR times, τDR∝η/Tp, has been observed for these DESs with the fraction power p = 0.7. Subsequently, the temporal heterogeneity aspects of these media have been investigated by examining the simulated particle motion characteristics and substantiated by estimating the dynamically correlated time scales and length-scales through simulations of four-point susceptibilities and density correlations. These estimated dynamical time scales and length-scales assist in explaining the different inferences regarding solution heterogeneity drawn from different measurements on these DESs.

No abstract

A new room-temperature deep eutectic solvent (DES) composed of glucose, urea, and water has been prepared and its relaxation dynamics explored via temperature-dependent time-resolved fluorescence measurements employing hydrophilic and hydrophobic solute probes. Differential scanning calorimetry measurements indicate a glass transition temperature (T g) of ∼236 K. Measured viscosity coefficients (η) vary from ∼600 to ∼100 cP in the temperature range 318 ≤ T/K ≤ 343 and exhibit Arrhenius-type temperature dependence with an activation energy of ∼65 kJ mol–1. Interestingly, this DES forms a stable liquid at ∼300 K but is too viscous to be accurately measured by us below 318 K. Temperature-dependent dynamic fluorescence anisotropy measurements using hydrophobic and hydrophilic solutes of similar sizes reveal bi-exponential kinetics and Arrhenius-type temperature dependence for solute rotation times (⟨τr⟩) but with significantly decreased activation energies, ∼31 kJ mol–1 (hydrophobic) and ∼21 kJ mol–1 (hydrophilic). Deviation from hydrodynamics is further reflected in the strong fractional viscosity dependence of ⟨τr⟩: ⟨τr⟩ ∝ (η/T) p with p ≈ 0.3–0.5, indicating pronounced temporal heterogeneity in the relaxation dynamics. Dynamic fluorescence Stokes shift measurements (temporal resolution ∼85 ps) produce dynamic shifts of ∼500–700 cm–1, bi-exponential solvation energy relaxation with time constants in the range ∼0.2 ns and ∼4 ns, and estimated missing amplitudes of ∼65–75%. Impact of the density difference between a nonpolar solvent and this DES on the estimated missing amplitudes is explored via measuring the temperature-dependent densities and refractive indices of this DES. Lifetime measurements suggest considerable temperature dependence for the hydrophobic solute but no such dependence for the hydrophilic one. Excitation energy dependence of fluorescence emission of various solutes with widely different lifetimes indicates mild spatial heterogeneity for this DES.

Fluorescence up-conversion (∼250 fs instrumental response) coupled with time correlated single photon counting measurements was performed to explore the complete Stokes shift dynamics of a dipolar solute probe, coumarin 153 (C153), in several ionic acetamide deep eutectic solvents (DESs) that contained lithium nitrate/bromide/perchlorate as electrolyte. Combined measurements near room temperature reflected a total dynamic Stokes shift of approximately 800−1100 cm −1 and triexponential solvation response functions. Interestingly, the average rate of solvation became faster upon successive replacement of bromide by nitrate in these deep eutectics, and a subpicosecond time scale emerged in the measured solvation response when bromide was fully replaced by nitrate. Temperature dependent solute diffusion in these deep eutectics at the single molecule level, monitored by tracking the translational motion of rhodamine 6G (R6G) via fluorescence correlation spectroscopic (FCS) technique, revealed pronounced fractional viscosity dependence of the solute's translational motion. Subsequently, this partial decoupling of solute translation was attributed to the microheterogeneous nature of these ionic DESs after examining the diffusion− viscosity relationship via the FCS measurements of R6G in several normal solvents at room temperature and in a liquid amide solvent at different temperatures.

We explore in this work, after synthesizing and appropriately characterizing an amphiphilic diblock copolymer, its interaction with water molecules and the subsequent aqueous solution dynamics by employing time-resolved fluorescence measurements (TRF) and megahertz–gigahertz dielectric relaxation (DR) experiments. The synthesized amphiphilic diblock copolymer is poly­(2-(((tert-butoxycarbonyl)­alanyl)­oxy)­ethyl methacrylate)-b-poly­(polyethylene glycol monomethyl ether methacrylate) (P­(Boc-l-Ala-HEMA)-b-PPEGMA). Dynamic light scattering measurements of aqueous solutions indicate formation of 14–20 nm particles from a balance between the chain lengths of the hydrophobic (P­(Boc-l-Ala-HEMA) and hydrophilic (PPEGMA) segments. Field-emission scanning electron microscopy, on the other hand, suggests a spherical shape for the dried micelles. The critical micelle concentration of the P­(Boc-l-Ala-HEMA)-b-PPEGMA block copolymer at different block lengths in aqueous media, determined via steady-state fluorescence measurements, is very low (∼4–8 mg/L), and the resultant micellar size has been found to be insensitive to the polymer concentration. Interfacial and bulk aqueous dynamics have been investigated by tracking the solution frictional resistance on rotational motion of dissolved hydrophobic and hydrophilic dipolar solute probes of comparable sizes. TRF anisotropy measurements reflect the biphasic temporal profile for the frictional resistance. Interestingly, the hydrophobic probe, because of its preferential location at the micellar interface, experiences greater frictional resistance than the hydrophilic counterpart, although the latter reports stronger polymer concentration dependence of the frictional retardation than the former. DR measurements at the highest of the polymer concentrations considered suggest presence of aqueous dynamics slower than that for neat bulk water, although evidence for such “slow” dynamics at lower concentrations has not been detected in the present DR measurements.