Steinrücken Elisa, Weigler Max, Kloth Sebastian, Vogel Michael
Institute for Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany.
J Chem Phys. 2024 Jul 7;161(1). doi: 10.1063/5.0215451.
We investigate water dynamics in mesoporous silica across partial crystallization by combining broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR), and molecular dynamics simulations (MDS). Exploiting the fact that not only BDS but also NMR field-cycling relaxometry and stimulated-echo experiments provide access to dynamical susceptibilities in broad frequency and temperature ranges, we study both the fully liquid state above the melting point Tm and the dynamics of coexisting water and ice phases below this temperature. It is found that partial crystallization leads to a change in the temperature dependence of rotational correlation times τ, which occurs in addition to previously reported dynamical crossovers of confined water and depends on the pore diameter. Furthermore, we observe that dynamical susceptibilities of water are strongly asymmetric in the fully liquid state, whereas they are much broader and nearly symmetric in the partially frozen state. Finally, water in the nonfreezable interfacial layer below Tm does not exhibit a much debated dynamical crossover at ∼220 K. We argue that its dynamics is governed by a static energy landscape, which results from the interaction with the bordering silica and ice surfaces and features a Gaussian-like barrier distribution. Consistently, our MDS analysis of the motional mechanism reveals a hopping motion of water in thin interfacial layers. The rotational correlation times of the confined ice phases follow Arrhenius laws. While the values of τ depend on the pore diameter, freezable water in various types of confinements and mixtures shows similar activation energies of Ea ≈ 0.43 eV.
我们通过结合宽带介电谱（BDS）、核磁共振（NMR）和分子动力学模拟（MDS）来研究介孔二氧化硅在部分结晶过程中的水动力学。利用BDS以及NMR场循环弛豫测量和受激回波实验不仅能在宽频率和温度范围内获取动态磁化率这一事实，我们研究了熔点(T_m)以上的完全液态以及该温度以下共存的水和冰相的动力学。研究发现，部分结晶导致旋转相关时间(\tau)的温度依赖性发生变化，这是除先前报道的受限水的动态转变之外发生的，并且取决于孔径。此外，我们观察到水的动态磁化率在完全液态时强烈不对称，而在部分冻结状态时则更宽且几乎对称。最后，低于(T_m)时不可冻结界面层中的水在约220K处未表现出备受争议的动态转变。我们认为其动力学受静态能量景观支配，这是由与相邻二氧化硅和冰表面的相互作用导致的，并且具有类似高斯的势垒分布。一致地，我们对运动机制的MDS分析揭示了薄界面层中水的跳跃运动。受限冰相的旋转相关时间遵循阿伦尼乌斯定律。虽然(\tau)的值取决于孔径，但各种类型的受限和混合状态下的可冻结水显示出相似的活化能(E_a\approx0.43)eV。
