Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Université Montpellier II, Montpellier, France.
J Colloid Interface Sci. 2013 Apr 1;395:269-76. doi: 10.1016/j.jcis.2012.12.050. Epub 2013 Jan 3.
Important structural modifications occur in swelling clays upon water adsorption. The multi-scale evolution of the swelling clay structure is usually evidenced by various experimental techniques. However, the driving force behind such phenomena is still not thoroughly understood. It appears strongly dependent on the nature of the interlayer cation. In the case of montmorillonites saturated with alkaline cations, it was inferred that the compensating cation or the layer surface could control the hydration process and thus the opening of the interlayer space, depending on the nature of the interlayer cation. In the present study, emphasis is put on the impact of divalent alkaline-earth cations compensating the layer charge in montmorillonites. Since no experimental technique offers the possibility of directly determining the hydration contributions related to interlayer cations and layer surfaces, an approach based on the combination of electrostatic calculations and immersion data is developed here, as already validated in the case of montmorillonites saturated by alkaline cations. This methodology allows to estimate the hydration energy for divalent interlayer cations and therefore to shed a new light on the driving force for hydration process occurring in montmorillonites saturated with alkaline-earth cations. Firstly, the surface energy values obtained from the electrostatic calculations based on the Electronegativity Equalization Method vary from 450 mJ m(-2) for Mg-montmorillonite to 1100 mJ m(-2) for Ba-montmorillonite. Secondly, considering both the hydration energy for cations and layer surfaces, the driving force for the hydration of alkaline-earth saturated montmorillonites can be attributed to the interlayer cation in the case of Mg-, Ca-, Sr-montmorillonites and to the interlayer surface in the case of Ba-montmorillonites. These results explain the differences in behaviour upon water adsorption as a function of the nature of the interlayer cation, thereby allowing the macroscopic swelling trends to be better understood. The knowledge of hydration processes occurring in homoionic montmorillonites saturated with both the alkaline and the alkaline-earth cations may be of great importance to explain the behaviour of natural clay samples where mixtures of the two types of interlayer cation are present and also provides valuable information on the cation exchange occurring in the swelling clays.
在吸水时，膨胀粘土会发生重要的结构变化。通常可以通过各种实验技术来证明膨胀粘土结构的多尺度演化。然而，这些现象背后的驱动力仍未被完全理解。它似乎强烈依赖于层间阳离子的性质。在被碱性阳离子饱和的蒙脱石的情况下，有人推断，补偿阳离子或层表面可以控制水合过程，从而打开层间空间，这取决于层间阳离子的性质。在本研究中，重点放在补偿蒙脱石中层电荷的二价碱土金属阳离子的影响上。由于没有实验技术能够直接确定与层间阳离子和层表面相关的水合贡献，因此这里开发了一种基于静电计算和浸没法数据相结合的方法，该方法在被碱性阳离子饱和的蒙脱石的情况下已经得到了验证。该方法允许估算二价层间阳离子的水合能，从而为理解发生在被碱土金属阳离子饱和的蒙脱石中的水合过程的驱动力提供新的视角。首先，基于电负性均衡法的静电计算得到的表面能值从 Mg-蒙脱石的 450 mJ m(-2)变化到 Ba-蒙脱石的 1100 mJ m(-2)。其次，考虑到阳离子和层表面的水合能，Mg-、Ca-、Sr-蒙脱石的水合驱动力归因于层间阳离子，而 Ba-蒙脱石的水合驱动力则归因于层间表面。这些结果解释了在水吸附过程中，由于层间阳离子的性质不同而导致的行为差异，从而可以更好地理解宏观膨胀趋势。了解在同时被碱和碱土金属阳离子饱和的同晶蒙脱石中发生的水合过程对于解释天然粘土样品的行为非常重要，因为在天然粘土样品中存在两种类型的层间阳离子的混合物，并且还提供了关于在膨胀粘土中发生的阳离子交换的有价值的信息。
