Structure, Energetics, and Dynamics of Water Adsorbed on the Muscovite (001) Surface: A Molecular Dynamics Simulation

Molecular dynamics (MD) computer simulations of liquid water adsorbed on the muscovite (001) surface
provide a greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation
in the structural and orientational ordering, hydrogen bond (H-bond) organization, and local density of H2O
molecules at this important model phyllosilicate surface. MD simulations at constant temperature and volume
(statistical NVT ensemble) were performed for a series of model systems consisting of a two-layer muscovite
slab (representing 8 crystallographic surface unit cells of the substrate) and 0 to 319 adsorbed H2O molecules,
probing the atomistic structure and dynamics of surface aqueous films up to 3 nm in thickness. The results
do not demonstrate a completely liquid-like behavior, as otherwise suggested from the interpretation of X-ray
reflectivity measurements and earlier Monte Carlo simulations. Instead, a more structurally and orientationally
restricted behavior of surface H2O molecules is observed, and this structural ordering extends to larger distances
from the surface than previously expected. Even at the largest surface water coverage studied, over 20% of
H2O molecules are associated with specific adsorption sites, and another 50% maintain strongly preferred
orientations relative to the surface. This partially ordered structure is also different from the well-ordered
2-dimensional ice-like structure predicted by ab initio MD simulations for a system with a complete monolayer
water coverage. However, consistent with these ab initio results, our simulations do predict that a full molecular
monolayer surface water coverage represents a relatively stable surface structure in terms of the lowest
diffusional mobility of H2O molecules along the surface. Calculated energies of water adsorption are in good
agreement with available experimental data.

针对吸附于白云母（muscovite）(001)晶面的液态水开展的分子动力学（MD）计算机模拟，极大地深化了人们对该重要层状硅酸盐模型表面处，表面相关效应如何影响水分子的结构与取向有序性、氢键（H-bond）排布以及局域密度的空间变化的原子级细节认知。
本研究针对一系列模型体系开展了恒温恒容（统计力学NVT系综）下的MD模拟，模型体系由两层白云母薄板（对应基底的8个晶体学表面原胞）与0至319个吸附水分子构成，用以探究厚度最高达3 nm的表面水膜的原子级结构与动力学特性。
研究结果并未展现出完全类液行为——这与此前X射线反射率测量的诠释以及早期蒙特卡洛模拟的结论相悖。相反，我们观测到表面水分子呈现出更强的结构与取向受限特性，且该结构有序性的延伸距离远超此前预期，可覆盖距表面更远的区域。
即便在所研究的最高表面水覆盖度下，仍有超过20%的水分子结合于特定吸附位点，另有约50%的水分子保持着相对于表面的强烈择优取向。这种部分有序的结构，也不同于从头算分子动力学（ab initio MD）模拟针对完整单层水覆盖体系所预测的高度有序二维类冰结构。
不过，与上述从头算结果一致的是，本模拟同样预测，以水分子沿表面的扩散迁移率最低为衡量标准，完整分子单层的表面水覆盖度对应着相对稳定的表面结构。计算得到的水吸附能与已公开的实验数据吻合良好。