Structural Implications of Interfacial Hydrogen Bonding in Hydrated Wyoming-Montmorillonite Clay

Montmorillonite
(MMT) claya layered porous nanomaterial
used as seals in engineered waste containment barriers for spent nuclear
fueladopts discrete hydration/swelling states depending upon
surrounding water and ion activities and confining pressure. The structure
of nanoconfined water and charge-balancing counterions in the clay
mineral interlayers dictate the swelling and mechanical behavior of
MMT, so a molecular model for this clay with high structural fidelity
is required to accurately predict the reliability of long-term nuclear
waste storage. Here, we present a molecular model for MMT that is
based on high resolution transmission electron microscopy of Wyoming-MMT
single crystals. Imaging data unambiguously show a cis-vacant arrangement
of structural hydroxyl groups in the octahedral sheet, whereas existing
molecular models assume a centrosymmetric trans-vacant configuration
for MMT. Using atomistic simulations, we find that the cis-vacant
arrangement of structural hydroxyl groups significantly affects the
structure of adsorbed water yielding a larger population of hydrogen
bonds with bridging oxygens on the tetrahedral sheet and weak hydrogen
bonding between the hydroxyl groups in the octahedral sheet and water
in the clay mineral interlayers. As a result, water adsorbed in the
interlayer is more “ice-like”, with stronger ordering
and lower density, although the diffusivity of the interlayer species
is not significantly diminished. Our improved structural model for
MMT provides insight into the energetics of water adsorption, which
ultimately dictates its pore- to macro-scale swelling, transport,
and fracture properties.

蒙脱石（Montmorillonite, MMT）是一种层状多孔纳米材料，常被用作乏核燃料工程废物阻隔屏障中的密封材料。其离散的水化/溶胀状态取决于周围水环境、离子活度与围压。粘土矿物层间内的纳米限域水与平衡电荷反离子的结构，决定了蒙脱石的溶胀与力学行为，因此亟需具备高结构保真度的该粘土分子模型，以精准预测核废料长期储存的可靠性。本研究提出了一款基于怀俄明型蒙脱石单晶高分辨透射电子显微镜成像的蒙脱石分子模型：成像数据明确显示八面体片层中结构羟基呈顺式空位排布，而现有分子模型均假设蒙脱石为中心对称的反式空位构型。通过原子尺度模拟，我们发现结构羟基的顺式空位排布显著影响吸附水的结构——与四面体片层桥氧形成的氢键数量更多，而八面体片层羟基与层间水之间的氢键作用更弱。由此，层间吸附水呈现更强的“类冰”特性，有序度更高且密度更低，尽管层间物质的扩散性并未显著降低。我们改进的蒙脱石结构模型为水吸附的热力学特性提供了新的认知，而该特性最终决定了材料从孔隙尺度到宏观尺度的溶胀、传输与断裂性能。