BACKGROUND Many important geochemical and biogeochemical reactions occur in the mineral/formation water interface of the highly abundant mineral, goethite [α-Fe(OOH)]. Ab initio molecular dynamics (AIMD) simulations of the goethite α-FeOOH (100) surface and the structure, water bond formation and dynamics of water molecules in the mineral/aqueous interface are presented. Several exchange correlation functionals were employed (PBE96, PBE96 + Grimme, and PBE0) in the simulations of a (3 × 2) goethite surface with 65 absorbed water molecules in a 3D-periodic supercell (a = 30 Å, FeOOH slab ~12 Å thick, solvation layer ~18 Å thick). RESULTS The lowest energy goethite (100) surface termination model was determined to have an exposed surface Fe3+ that was loosely capped by a water molecule and a shared hydroxide with a neighboring surface Fe3+. The water molecules capping surface Fe3+ ions were found to be loosely bound at all DFT levels with and without Grimme corrections, indicative that each surface Fe3+ was coordinated with only five neighbors. These long bonds were supported by bond valence theory calculations, which showed that the bond valence of the surface Fe3+ was saturated and surface has a neutral charge. The polarization of the water layer adjacent to the surface was found to be small and affected only the nearest water. Analysis by density difference plots and localized Boys orbitals identified three types of water molecules: those loosely bound to the surface Fe3+, those hydrogen bonded to the surface hydroxyl, and bulk water with tetrahedral coordination. Boys orbital analysis showed that the spin down lone pair orbital of the weakly absorbed water interact more strongly with the spin up Fe3+ ion. These weakly bound surface water molecules were found to rapidly exchange with the second water layer (~0.025 exchanges/ps) using a dissociative mechanism. CONCLUSIONS Water molecules adjacent to the surface were found to only weakly interact with the surface and as a result were readily able to exchange with the bulk water. To account for the large surface Fe-OH2 distances in the DFT calculations it was proposed that the surface Fe3+ atoms, which already have their bond valence fully satisfied with only five neighbors, are under-coordinated with respect to the bulk coordination. Graphical abstract All first principle calculations, at all practically achievable levels, for the goethite 100 aqueous interface support a long bond and weak interaction between the exposed surface Fe3+ and water molecules capping the surface. This result is supported by bond valence theory calculations and is indicative that each surface Fe3+ is coordinated with only 5 neighbors.

中文翻译：

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针铁矿（100）表面/水界面处的弱结合水结构，键价饱和度和水动力学：从头算动力学模拟。

背景技术许多重要的地球化学和生物地球化学反应发生在高度丰富的矿物针铁矿[α-Fe（OOH）]的矿物/地层水界面中。提出了针铁矿α-FeO​​OH（100）表面以及矿物/水界面中水分子的结构，水键形成和动力学的从头算分子动力学（AIMD）模拟。在3D周期超晶胞（a = 30Å，FeOOH平板〜12Å）中模拟具有65个吸收水分子的（3×2）针铁矿表面时，采用了几种交换相关函数（PBE96，PBE96 + Grimme和PBE0）厚，溶剂化层厚约18Å）。结果确定最低能量的针铁矿（100）表面终止模型具有暴露的表面Fe3 +，该表面被水分子和与相邻表面Fe3 +共享的氢氧化物松散地覆盖。发现在有和没有进行Grimme校正的情况下，覆盖表面Fe3 +离子的水分子在所有DFT水平上均松散结合，表明每个表面Fe3 +仅与五个邻域配位。这些长键得到键价理论计算的支持，这表明表面Fe3 +的键价饱和并且表面带有中性电荷。发现与表面相邻的水层的极化很小，并且仅影响最近的水。通过密度差图和局部Boys轨道进行的分析确定了三种类型的水分子：与表面Fe3 +松散结合的氢分子，与表面羟基结合的氢原子和具有四面体配位的大体积水。男孩的轨道分析表明，吸收较弱的水的向下旋转的孤对轨道与向上旋转的Fe3 +离子相互作用更强。发现这些弱结合的表面水分子使用解离机制与第二水层快速交换（〜0.025交换/ ps）。结论发现与表面相邻的水分子仅与表面弱相互作用，因此易于与大量水交换。为了在DFT计算中考虑到较大的表面Fe-OH2距离，提出了表面Fe3 +原子（其键价已完全满足五个邻域的要求）相对于本体配位不足。图形摘要在所有实际可达到的水平上，所有第一原理计算，针铁矿100的水界面支持长键和暴露的表面Fe3 +与覆盖表面的水分子之间的弱相互作用。该结果得到键合价理论计算的支持，并表明每个表面Fe3 +仅与5个邻域配合。