介孔二氧化硅中深冷水的重取向：孔大小依赖性的NMR研究,The Journal of Physical Chemistry B

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介孔二氧化硅中深冷水的重取向：孔大小依赖性的NMR研究
The Journal of Physical Chemistry B ( IF 2.8 ) Pub Date : 2019-02-25 , DOI: 10.1021/acs.jpcb.8b12204
M. Weigler ₁ , M. Brodrecht ₂ , G. Buntkowsky ₂ , M. Vogel ₁

Affiliation

我们应用² H NMR研究介孔二氧化硅中的水重新取向。使用合成的MCM-41和购买的SBA-15材料，我们系统地研究了对孔径的依赖性。为了表征二氧化硅基质的性质，使用扫描电子显微镜和氮气吸附。为了确定承压水的热力学行为，我们利用差示扫描量热法。结合² H自旋晶格弛豫和受激回波实验，在从弱到深冷却的宽温度范围内分析了动力学特性。我们发现，承压水的重新定向受突出的非均质性支配。峰值相关时间τ _p分布的大小与孔径范围在ca左右无关。2-6 nm。在所有研究的围堵中，它都显示了在〜220 K处的动态跨越，包括宽的二氧化硅孔隙，其作用发生在T _m以下，因此在液体和固体水部分共存的两相区域中。上方和下方的水没有人的土地，孔隙尺寸无关的相关时间τ _p的承压水是与大量液体的一致，这表明我们探测一个动态过程，是水的特性特征。我们认为，动力交叉的拟议起源均不与所有观察结果完全一致，并讨论了可能的新方向。

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Reorientation of Deeply Cooled Water in Mesoporous Silica: NMR Studies of the Pore-Size Dependence

We apply ²H NMR to investigate water reorientation in mesoporous silica. Using synthesized MCM-41 and purchased SBA-15 materials, we systematically study the dependence on the pore diameter. For a characterization of the properties of the silica matrices, scanning electron microscopy and nitrogen gas adsorption are employed. To ascertain the thermodynamic behavior of the confined water, we utilize differential scanning calorimetry. The dynamical properties are analyzed in a broad temperature range from the weakly to the deeply cooled regime using a combination of ²H spin-lattice relaxation and stimulated-echo experiments. We find that the reorientation of confined water is governed by prominent heterogeneity. The peak correlation time τ_p of the distribution is independent of the pore diameter in a range of ca. 2–6 nm. It shows a dynamical crossover at ∼220 K in all studied confinements, including wide silica pores where the effect occurs below T_m and, hence, in the two-phase region with coexisting liquid and solid water fractions. Above and below the no-man’s land of water, the pore-size-independent correlation times τ_p of confined water are consistent with those of the bulk liquid, suggesting that we probe a dynamical process that is a characteristic feature of water. We argue that none of the proposed origins of the dynamical crossover is fully consistent with all observations and discuss possible new directions.

更新日期：2019-02-27

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