Reuhl Melanie, Monnard Philipp, Vogel Michael
Institute of Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany.
J Chem Phys. 2022 Feb 28;156(8):084506. doi: 10.1063/5.0082406.
We perform nuclear magnetic resonance, broadband dielectric spectroscopy, and differential scanning calorimetry studies to ascertain the dynamical behaviors of aqueous ethylene glycol (EG) solutions in silica pores over broad temperature ranges. Both translational and rotational motions are analyzed, and the pore diameter (2.4-9.2 nm) and the EG concentration (12-57 mol. %) are varied, leading to fully liquid or partially crystalline systems. It is found that the translational diffusion coefficient strongly decreases when the diameter is reduced, resulting in a slowdown of nearly three orders of magnitude in the narrowest pores, while the confinement effects on the rotational correlation times are moderate. For the fully liquid solutions, we attribute bulk-like and slowed down reorientation processes to the central and interfacial pore regions, respectively. This coexistence is found in all the studied pores, and, hence, the range of the wall effects on the solution dynamics does not exceed ∼1 nm. Compared to the situation in the bulk, the concentration dependence is reduced in confinements, implying that the specific interactions of the molecular species with the silica walls lead to preferential adsorption. On the other hand, bulk-like structural relaxation is not observed in the partially frozen samples, where the liquid is sandwiched between the silica walls and the ice crystallites. Under such circumstances, there is another relaxation process with a weaker temperature dependence, which is observed in various kinds of partially frozen aqueous systems and denoted as the x process.
我们进行了核磁共振、宽带介电谱和差示扫描量热法研究，以确定在较宽温度范围内二氧化硅孔隙中乙二醇（EG）水溶液的动力学行为。对平动和转动运动都进行了分析，改变了孔径（2.4 - 9.2纳米）和EG浓度（12 - 57摩尔%），从而得到完全液态或部分结晶的体系。研究发现，当孔径减小时，平动扩散系数会大幅下降，在最窄的孔隙中导致近三个数量级的减慢，而对转动相关时间的限制效应则较为适中。对于完全液态的溶液，我们分别将类似本体的和减慢的重取向过程归因于孔隙的中心区域和界面区域。在所有研究的孔隙中都发现了这种共存现象，因此，壁面对溶液动力学的影响范围不超过约1纳米。与本体情况相比，在受限体系中浓度依赖性降低，这意味着分子物种与二氧化硅壁的特定相互作用导致了优先吸附。另一方面，在部分冻结的样品中未观察到类似本体的结构弛豫，其中液体夹在二氧化硅壁和冰晶之间。在这种情况下，存在另一种温度依赖性较弱的弛豫过程，在各种部分冻结的水体系中都能观察到，并被称为x过程。
