分子動力学法によるモンモリロナイト層間中の水とイオンの物性評価─拡散モデルへの反映─

抄録

Montmorillonite clay plays an important role as a key component in bentonite buffer materials used for the safe geological disposal of radioactive waste. Molecular dynamics (MD) simulations were performed to investigate the physical properties of water and relevant cations within the montmorillonite interlayer nanopores. We discuss the application of these results to a diffusion model in compacted montmorillonite. The swelling behavior and hydration states were evaluated as functions of interlayer cations (Na, K, Cs, Ca and Sr) and layer charge (0.2–0.75 e/unit cell) by comparing the basal spacing, immersion enthalpy and partial molar volumes. These were controlled by the hydration properties of the interlayer cations and by electrostatic interaction with the montmorillonite layer. The diffusion coefficients of water and cations in the interlayer nanopores decreased compared to those in bulk water and became close to those in bulk water as the basal spacing increased. The diffusion behavior was correlated to the hydration energy and hydration radius of the interlayer cations. The viscosity coefficients of the interlayer water, estimated from the common relationship between diffusion and viscosity coefficients in bulk water, indicated a significant viscoelectric effect for both the 1- and 2-layer hydration states which increased in montmorillonites with higher layer charge. These trends derived from the MD calculations are consistent with existing measured data and previous MD simulations. In addition, the parameters related to viscoelectric effect used in the diffusion model were refined, based on comparison between the MD simulations and measurements. This series of MD calculations provides an atomic level understanding for the development and improvement for the diffusion model of compacted montmorillonite.