NMR studies on the temperature-dependent dynamics of confined water.
NMR studies on the temperature-dependent dynamics of confined water.
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DOI：
10.1039/c4cp02057j
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发表时间：
2014-07
期刊：
影响因子：
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通讯作者：
M. Sattig;S. Reutter;F. Fujara;M. Werner;G. Buntkowsky;M. Vogel
中科院分区：
文献类型：
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作者：
M. Sattig;S. Reutter;F. Fujara;M. Werner;G. Buntkowsky;M. Vogel
我们用（2）H NMR研究了过冷水在不同直径的二氧化硅孔中的旋转运动，特别是在MCM-41材料C10，C12和C14中。结合自旋晶格弛豫，线形，和受激回波分析，使我们能够在很宽的时间和温度范围内确定相关时间。对于所研究的孔径，2.1-2.9 nm，我们发现两个交叉的温度依赖的相关时间的液体水冷却后。在220-230 K，第一个扭结的温度依赖性是伴随着一部分的承压水的固化，这意味着所观察到的交叉是由于从散装的界面为主的水动力学的变化，而不是一个液-液相变。此外，结果提供的证据表明，在交叉温度以上探测到α过程样动力学，而在下面观察到β过程样动力学。在180-190 K，我们发现了温度依赖性的第二个变化，这类似于在玻璃化转变期间过冷液体的β过程的报道，表明界面影响的液态水的Tg值为185 K。在高温区（T > 225 K），较小的C10孔中的水重取向温度依赖性比较大的C12和C14孔中的弱，在较大的C12和C14孔中，水重取向温度依赖性更像块体，表明SiO2约束对水在2.1 nm约束下的α过程有显著影响。相比之下，水重取向的温度依赖性在很大程度上与限制尺寸无关，并且在低温范围（T < 180 K）内由阿克里尼乌斯定律描述，活化能Ea = 0.5 eV，表明限制尺寸对水的β过程起次要作用。
We use (2)H NMR to study the rotational motion of supercooled water in silica pores of various diameters, specifically, in the MCM-41 materials C10, C12, and C14. Combination of spin-lattice relaxation, line-shape, and stimulated-echo analyses allows us to determine correlation times in very broad time and temperature ranges. For the studied pore diameters, 2.1-2.9 nm, we find two crossovers in the temperature-dependent correlation times of liquid water upon cooling. At 220-230 K, a first kink in the temperature dependence is accompanied by a solidification of a fraction of the confined water, implying that the observed crossover is due to a change from bulk-like to interface-dominated water dynamics, rather than to a liquid-liquid phase transition. Moreover, the results provide evidence that α process-like dynamics is probed above the crossover temperature, whereas β process-like dynamics is observed below. At 180-190 K, we find a second change of the temperature dependence, which resembles that reported for the β process of supercooled liquids during the glass transition, suggesting a value of Tg ≈ 185 K for interface-affected liquid water. In the high-temperature range, T > 225 K, the temperature dependence of water reorientation is weaker in the smaller C10 pores than in the larger C12 and C14 pores, where it is more bulk-like, indicating a significant effect of the silica confinement on the α process of water in the former 2.1 nm confinement. By contrast, the temperature dependence of water reorientation is largely independent of the confinement size and described by an Arrhenius law with an activation energy of Ea ≈ 0.5 eV in the low-temperature range, T < 180 K, revealing that the confinement size plays a minor role for the β process of water.