Chemical, Biological and Macromolecular Sciences (CBMS) , S. N. Bose National Centre for Basic Sciences , JD Block, Sector III, Salt Lake , Kolkata 700106 , India.
J Phys Chem B. 2019 Nov 7;123(44):9378-9387. doi: 10.1021/acs.jpcb.9b06783. Epub 2019 Oct 25.
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 () of ∼236 K. Measured viscosity coefficients (η) vary from ∼600 to ∼100 cP in the temperature range 318 ≤ /K ≤ 343 and exhibit Arrhenius-type temperature dependence with an activation energy of ∼65 kJ mol. 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 (⟨τ⟩) but with significantly decreased activation energies, ∼31 kJ mol (hydrophobic) and ∼21 kJ mol (hydrophilic). Deviation from hydrodynamics is further reflected in the strong fractional viscosity dependence of ⟨τ⟩: ⟨τ⟩ ∝ (η/) with ≈ 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, 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.
一种由葡萄糖、尿素和水组成的新型室温深共熔溶剂（DES）已被制备，并通过使用亲水性和疏水性探针的温度相关时间分辨荧光测量来探索其弛豫动力学。差示扫描量热法测量表明玻璃化转变温度（）约为 236 K。在所研究的温度范围内（318 ≤ /K ≤ 343），测量的粘度系数（η）从约 600 到约 100 cP 变化，表现出阿伦尼乌斯型的温度依赖性，活化能约为 65 kJ mol。有趣的是，这种 DES 在约 300 K 下形成稳定的液体，但在 318 K 以下粘度太大以至于我们无法准确测量。使用大小相似的疏水性和亲水性探针进行的温度依赖的动态荧光各向异性测量揭示了溶质旋转时间（⟨τ⟩）的双指数动力学和阿伦尼乌斯型温度依赖性，但活化能显著降低，分别为约 31 kJ mol（疏水性）和约 21 kJ mol（亲水性）。偏离流体力学进一步反映在 ⟨τ⟩对分数粘度的强烈依赖性中：⟨τ⟩∝（η/），其中 ≈0.3-0.5，表明弛豫动力学中存在明显的时间不均匀性。动态荧光斯托克斯位移测量（时间分辨率约 85 ps）产生约 500-700 cm 的动态位移，具有约 0.2 ns 和约 4 ns 时标范围的双指数溶剂化能量弛豫，以及估计的缺失幅度约为 65-75%。通过测量该 DES 的温度相关密度和折射率，探索了非极性溶剂和该 DES 之间密度差对估计缺失幅度的影响。寿命测量表明疏水溶质有相当大的温度依赖性，但亲水溶质则没有。具有广泛不同寿命的各种溶质的荧光发射激发能依赖性表明该 DES 具有轻度的空间不均匀性。
