水化Na-蒙脱石和Na/Mg-蒙脱石的分子动力学模拟

利用分子动力学方法(MD)研究了Na-蒙脱石和Na/Mg-蒙脱石层间的补偿阳离子和水分子的结构及扩散性质. 模拟结果表明, 在一定水含量范围内Na-蒙脱石和Na/Mg-蒙脱石表现出不同的膨胀形式, 特别是层间水分子数目在48～72之间时, Na/Mg-蒙脱石的层间距比Na-蒙脱石有较为明显的增大. Na/Mg-蒙脱石两层水化物的层间水分子与Mg2+形成了明显的两层水合壳; 而与Na+只形成了一层平面的水合壳. 在Na/Mg-蒙脱石中, Na+和 Mg2+的扩散方式不同, Na+的扩散范围相对更广, 自扩散系数更大. Na/Mg-蒙脱石比相同水含量下的Na-蒙脱石层间水的自扩散系数小. 由于Mg2+和Na+对层间结构的强烈影响, 从而使有少量Mg2+取代Na+的Na/Mg-蒙脱石与Na-蒙脱石表现出不同的膨胀性质和层间物质的扩散性质.

Molecular Dynamics Simulation of Na-montmorillonite and Na/Mg-montmorillonite Hydrates

Molecular dynamics (MD) is used to investigate the structural and diffusive properties of interlayer species, namely water and counterion speciations of Na-montmorillonite and Na/Mg-montmorillonite. It is found that Na/Mg-montmorillonite exhibits a higher swellability compored with that of Na-montmorillonite for given water content, especially with 48～72 water molecules of interlayer, the expansion of layer spacings of Na/Mg-montmorillonite is greater than those of Na-montmmorillonite. Interlayer water molecules have a stronger tendency to solvate Mg2+ compared to that of Na+. Inner-sphere complexes and outer-sphere complexes of Mg2+ with water are observed, whereas only planar inner-sphere complexes of Na+ with water are found for two-layer Na/Mg-montmorillonite hydrates. The results also indicate the different diffusion modes of Na+ and Mg2+ for Na/Mg montmorillonite: Na+ remains close to the surfaces of the sheets, whereas Mg2+ is easily hydrated and is located in the middle of the interlayer. Comparatively, Na+ has a wider diffusion range and higher self-diffusion coefficient than Mg2+. Moreover, self-diffusion coefficient of interlayer water is higher for Na-montmorillonite than that for Na/Mg-montmorillonite at the same water content. Most of the results obtained in this study can be understood in terms of the strong solvation of Mg2+ with water molecules, and Mg2+ and Na+ have different influences on the structure of interlayer. It can therefore be assumed that the replacement of a relatively small amount of interlayer Na+ cations by Mg2+ in Na-montmorillonite during the initial stage can substantially change the swelling and diffusion properties of interlayer species.