Soft Matter Theory and Simulations Group, Computational Biology Center, IBM Watson Research Center, Yorktown Heights, New York 10598, USA.
J Am Chem Soc. 2012 Feb 15;134(6):3042-53. doi: 10.1021/ja208894m. Epub 2012 Feb 1.
The dynamics of water confined to mesoporous regions in minerals such as swelling clays and zeolites is fundamental to a wide range of resource management issues impacting many processes on a global scale, including radioactive waste containment, desalination, and enhanced oil recovery. Large-scale atomic models of freely diffusing multilayer smectite particles at low hydration confined in a silicalite cage are used to investigate water dynamics in the composite environment with the ReaxFF reactive force field over a temperature range of 300-647 K. The reactive capability of the force field enabled a range of relevant surface chemistry to emerge, including acid/base equilibria in the interlayer calcium hydrates and silanol formation on the edges of the clay and inner surface of the zeolite housing. After annealing, the resulting clay models exhibit both mono- and bilayer hydration structures. Clay surface hydration redistributed markedly and yielded to silicalite water loading. We find that the absolute rates and temperature dependence of water dynamics compare well to neutron scattering data and pulse field gradient measures from relevant samples of Ca-montmorillonite and silicalite, respectively. Within an atomistic, reactive context, our results distinguish water dynamics in the interlayer Ca(OH)(2)·nH(2)O environment from water flowing over the clay surface, and from water diffusing within silicalite. We find that the diffusion of water when complexed to Ca hydrates is considerably slower than freely diffusing water over the clay surface, and the reduced mobility is well described by a difference in the Arrhenius pre-exponential factor rather than a change in activation energy.
水在蒙脱石和沸石等矿物的中孔区域中的动力学对于广泛的资源管理问题至关重要，这些问题影响着全球范围内的许多过程，包括放射性废物处理、海水淡化和提高石油采收率。使用 ReaxFF 反应力场，对在低水合状态下被限制在硅质沸石笼中的膨胀粘土的多层自由扩散粒子进行了大规模原子模型模拟，以研究在 300-647 K 温度范围内的复合环境中的水动力学。该力场的反应能力使包括层间钙水合作用中的酸碱平衡和粘土边缘以及沸石外壳内表面上的硅醇形成在内的一系列相关表面化学作用得以显现。退火后，得到的粘土模型表现出单层和双层水化结构。粘土表面的水化作用重新分布，导致硅质沸石的水负荷增加。我们发现，水动力学的绝对速率和温度依赖性与中子散射数据以及来自相关 Ca-蒙脱石和硅质沸石样品的脉冲场梯度测量结果非常吻合。在原子反应的背景下，我们的结果将层间 Ca(OH)(2)·nH(2)O 环境中的水动力学与流过粘土表面的水以及在硅质沸石内扩散的水区分开来。我们发现，当水与 Ca 水合物络合时，水的扩散速度比在粘土表面上自由扩散的水慢得多，而降低的迁移率可以很好地通过 Arrhenius 前指数因子的差异而不是活化能的变化来描述。
