Anovitz Lawrence M, Mamontov Eugene, ben Ishai Paul, Kolesnikov Alexander I
Chemical Sciences Division, MS 6110, P.O. Box 2008, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6110, USA.
Chemical and Engineering Materials Division, MS 6473, P.O. Box 2008, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6473, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Nov;88(5):052306. doi: 10.1103/PhysRevE.88.052306. Epub 2013 Nov 8.
The properties of fluids can be significantly altered by the geometry of their confining environments. While there has been significant work on the properties of such confined fluids, the properties of fluids under ultraconfinement, environments where, at least in one plane, the dimensions of the confining environment are similar to that of the confined molecule, have not been investigated. This paper investigates the dynamic properties of water in beryl (Be(3)Al(2)Si(6)O(18)), the structure of which contains approximately 5-Å-diam channels parallel to the c axis. Three techniques, inelastic neutron scattering, quasielastic neutron scattering, and dielectric spectroscopy, have been used to quantify these properties over a dynamic range covering approximately 16 orders of magnitude. Because beryl can be obtained in large single crystals we were able to quantify directional variations, perpendicular and parallel to the channel directions, in the dynamics of the confined fluid. These are significantly anisotropic and, somewhat counterintuitively, show that vibrations parallel to the c-axis channels are significantly more hindered than those perpendicular to the channels. The effective potential for vibrations in the c direction is harder than the potential in directions perpendicular to it. There is evidence of single-file diffusion of water molecules along the channels at higher temperatures, but below 150 K this diffusion is strongly suppressed. No such suppression, however, has been observed in the channel-perpendicular direction. Inelastic neutron scattering spectra include an intramolecular stretching O-H peak at ~465 meV. As this is nearly coincident with that known for free water molecules and approximately 30 meV higher than that in liquid water or ice, this suggests that there is no hydrogen bonding constraining vibrations between the channel water and the beryl structure. However, dielectric spectroscopic measurements at higher temperatures and lower frequencies yield an activation energy for the dipole reorientation of 16.4 ± 0.14 kJ/mol, close to the energy required to break a hydrogen bond in bulk water. This may suggest the presence of some other form of bonding between the water molecules and the structure, but the resolution of the apparent contradiction between the inelastic neutron and dielectric spectroscopic results remains uncertain.
流体的性质会因其受限环境的几何形状而发生显著改变。虽然针对此类受限流体的性质已有大量研究工作，但超受限环境下流体的性质尚未得到研究。在超受限环境中，至少在一个平面上，受限环境的尺寸与被限制分子的尺寸相近。本文研究了绿柱石（Be(3)Al(2)Si(6)O(18)）中水分子的动力学性质，其结构包含平行于c轴的直径约为5 Å的通道。我们运用了三种技术，即非弹性中子散射、准弹性中子散射和介电谱，在覆盖约16个数量级的动态范围内对这些性质进行了量化。由于能够获得大尺寸的绿柱石单晶，我们得以量化受限流体动力学中与通道方向垂直和平行的方向变化。这些变化具有显著的各向异性，而且有点违反直觉的是，结果表明平行于c轴通道的振动比垂直于通道的振动受到的阻碍要大得多。在c方向上振动的有效势比垂直方向上的势更难。有证据表明，在较高温度下，水分子会沿着通道进行单列扩散，但在150 K以下，这种扩散会受到强烈抑制。然而，在垂直于通道的方向上并未观察到这种抑制现象。非弹性中子散射谱在约465 meV处包含一个分子内拉伸O-H峰。由于这个峰与自由水分子的峰几乎重合，且比液态水或冰中的峰高约30 meV，这表明通道中的水与绿柱石结构之间不存在限制振动的氢键。然而，在较高温度和较低频率下的介电谱测量得出，偶极子重新取向的活化能为16.4±0.14 kJ/mol，这与在大量水中打破一个氢键所需的能量相近。这可能表明水分子与结构之间存在某种其他形式的键合，但非弹性中子散射和介电谱结果之间明显矛盾的解决仍不确定。
