Yao Yang, Fella Verena, Huang Wei, Zhang Kai A I, Landfester Katharina, Butt Hans-Jürgen, Vogel Michael, Floudas George
Max Planck Institute for Polymer Research , D-55128 Mainz , Germany.
Institut für Festkörperphysik , Technische Universität Darmstadt , Hochschulstraße 6 , 64289 Darmstadt , Germany.
Langmuir. 2019 Apr 30;35(17):5890-5901. doi: 10.1021/acs.langmuir.9b00496. Epub 2019 Apr 17.
The crystallization and dynamics of water confined in model mesoporous silica particles (pore diameters ranging from 2.1 to 5 nm; pore length ≈ 1 μm) are studied in homogeneous aqueous suspensions by dielectric spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance (NMR) techniques. We establish the phase diagram ( T vs 1/ d) of confined water covering a broad range of pore diameters. A linear dependence of the heterogeneous and the homogeneous nucleation temperatures on the inverse pore diameter is shown. The two lines converge at a pore diameter of ∼2.6 nm, below which formation of stable crystals is suppressed. By combining dielectric spectroscopy and different NMR techniques, we determine the dynamics of water within mesoporous silica over broad temperature and frequency ranges. Both techniques identify two dynamically distinguishable fractions of confined water coexisting within the pores. We attribute the two fractions to an interfacial water layer at the pore walls and confined water in the pore interior. Two alternative scenarios are proposed to rationalize the coexistence of two dynamically distinguishable water fractions. In the first scenario, two liquid fractions of water coexist under extreme confinement conditions for a range of temperatures; we discuss similarities with the two ultraviscous liquids (high-density liquid and low-density liquid) put forward for supercooled bulk water. In the second scenario, a liquid and a solid phase coexist; we conjecture that highly distorted and unstable crystal nuclei exist under extreme confinement that exhibit reorientation dynamics with time scales intermediate to the surrounding confined liquid and to bulk ice.
通过介电谱、差示扫描量热法和核磁共振（NMR）技术，在均匀水悬浮液中研究了限制在模型介孔二氧化硅颗粒（孔径范围为2.1至5纳米；孔长约1微米）中的水的结晶和动力学。我们建立了涵盖广泛孔径范围的受限水的相图（T对1/d）。结果表明，异质和同质成核温度与孔径倒数呈线性关系。这两条线在孔径约为2.6纳米处相交，低于该孔径时，稳定晶体的形成受到抑制。通过结合介电谱和不同的NMR技术，我们确定了介孔二氧化硅中水分子在很宽的温度和频率范围内的动力学。这两种技术都识别出孔隙内共存的两种动力学上可区分的受限水部分。我们将这两个部分归因于孔壁处的界面水层和孔内部的受限水。提出了两种替代方案来解释两种动力学上可区分的水部分的共存。在第一种方案中，在一系列温度下，两种液态水部分在极端受限条件下共存；我们讨论了与过冷大块水提出的两种超粘性液体（高密度液体和低密度液体）的相似性。在第二种方案中，液相和固相共存；我们推测在极端受限条件下存在高度扭曲和不稳定的晶核，其表现出的重新取向动力学的时间尺度介于周围受限液体和大块冰之间。
