Abstract The mobility of confined water at clay mineral surfaces underpins many of their functional properties as industrial minerals. Oriented thin films of the sodium (Na+) and calcium (Ca2+) forms of beidellite (Bd) and charge-reduced montmorillonite (Mt), equilibrated at 55% relative humidity using D2O, were studied using high-resolution neutron spectroscopy to determine the overall mobility of the OH groups and interlayer water, as well as the effect of layer charge location and total layer charge on anisotropy of collective motions at clay mineral surfaces. An elastic fixed window approach was applied to follow temperature (T) and scattering vector (Q) dependent changes in the elastic scattering signal at two different film orientations with respect to the instrument configuration. The mean square displacement associated with interlayer D2O was greater when the film (and layer) plane was aligned perpendicular with the instrument's average scattering Q vector. A minor displacement was observed near T = 100 K and assigned to localised isotropic rotational contributions of structural OH. While the anisotropic effect at higher T was minor, Q-dependence was present in all samples and dominated by incoherent scattering from interlayer D2O. The Ca2+-form of reduced charge Mt. had larger correlation lengths in the direction perpendicular to the lattice plane than in the parallel direction. While no obvious direct effect of anisotropy due to total layer charge was observed, higher layer charge resulted in greater overall mobility of both structural OH and interlayer D2O. Despite the challenges of studying smectite thin films by neutron spectroscopy, this study confirmed that hydrated interlayer cations mobilised water above 190 K.

中文翻译：

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高分辨率中子能谱探测粘土矿物层间水各向异性和结构OH动力学的层电荷效应

摘要 粘土矿物表面的承压水的流动性是它们作为工业矿物的许多功能特性的基础。钠 (Na+) 和钙 (Ca2+) 形式的贝得石 (Bd) 和电荷减少的蒙脱石 (Mt) 的定向薄膜，使用 D2O 在 55% 的相对湿度下平衡，使用高分辨率中子能谱研究以确定整体OH 基团和层间水的迁移率，以及层电荷位置和总层电荷对粘土矿物表面集体运动各向异性的影响。应用弹性固定窗口方法来跟踪温度 (T) 和散射矢量 (Q) 相关的弹性散射信号在相对于仪器配置的两个不同薄膜方向上的变化。当薄膜（和层）平面与仪器的平均散射 Q 向量垂直对齐时，与夹层 D2O 相关的均方位移更大。在 T = 100 K 附近观察到微小位移，并分配给结构 OH 的局部各向同性旋转贡献。虽然在较高 T 下的各向异性效应很小，但 Q 依赖性存在于所有样品中，并且主要是来自层间 D2O 的非相干散射。减少电荷的 Ca2+ 形式 与晶格平面垂直的方向上的相关长度大于平行方向上的相关长度。虽然没有观察到由于总层电荷引起的各向异性的明显直接影响，但层电荷越高，结构 OH 和层间 D2O 的整体迁移率越大。