Zhang Yao, Meng Yingfeng, Liu Houbin, Yang Mingli
State Key Laboratory of Oil and Gas Reservoir Geology and Exploration, School of Petroleum and Gas Engineering, Southwest Petroleum University, Chengdu, 610500, China.
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, 610065, China.
J Mol Model. 2016 May;22(5):105. doi: 10.1007/s00894-016-2974-4. Epub 2016 Apr 15.
Knowledge about water desorption is important to give a full picture of water diffusion in montmorillonites (MMT), which is a driving factor in MMT swelling. The desorption paths and energetics of water molecules from the surface of MMT with trapped Li(+), Na(+) or K(+) counterions were studied using periodic density functional theory calculations. Two paths--surface and vacuum desorption--were designed for water desorption starting from a stationary structure in which water bonds with both the counterion and the MMT surface. Surface desorption is energetically more favorable than vacuum desorption due to water-surface hydrogen bonds that help stabilize the intermediate structure of water released from the counterion. The energy barriers of water desorption are in the order of Li(+) > Na(+) > K(+), which can be attributed to the short ionic radius of Li(+), which favors strong binding with the water molecule. The temperature dependence of water adsorption and desorption rates were compared based on the computed activation energies. Our calculations reveal that the water desorption on the MMT surface has a different mechanism from water adsorption, which results from surface effects favoring stabilization of water conformers during the desorption process.
了解水的解吸对于全面了解蒙脱石（MMT）中的水扩散非常重要，而水扩散是蒙脱石膨胀的一个驱动因素。利用周期性密度泛函理论计算研究了含有被困Li(+)、Na(+)或K(+)抗衡离子的蒙脱石表面水分子的解吸路径和能量学。从水与抗衡离子和蒙脱石表面都结合的静止结构开始，设计了两条水的解吸路径——表面解吸和真空解吸。由于水-表面氢键有助于稳定从抗衡离子释放的水的中间结构，表面解吸在能量上比真空解吸更有利。水的解吸能垒顺序为Li(+)＞Na(+)＞K(+)，这可归因于Li(+)的离子半径短，有利于与水分子的强结合。基于计算出的活化能比较了水吸附和解吸速率的温度依赖性。我们的计算表明，蒙脱石表面的水的解吸与水的吸附具有不同的机制，这是由表面效应导致的，表面效应有利于在解吸过程中稳定水的构象异构体。
