Adapa Sai, Malani Ateeque
Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, India.
Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, India.
J Colloid Interface Sci. 2021 Oct;599:694-705. doi: 10.1016/j.jcis.2021.04.034. Epub 2021 Apr 20.
The ion exchange capacity of clay plays an important role in many industrial applications ranging from radioactive waste disposal to cosmetics. However, swelling or shrinking of clay platelets due to water and ions adsorption in the interstitial zone is also a well-known phenomenon. For their applications, it is crucial to understand the stability of these layered materials, especially after exchange of interstitial ions with surrounding ions having different properties. Here, we probed the role of cation size on swelling pressure and free energy profile. We used molecular simulations to investigate the stability of mica pore, having K, Rb, and Cs ions. We performed a series of grand canonical Monte Carlo simulations at various pore widths. We probed water adsorption in mica pores from which disjoining pressure, grand potential (swelling free energy), and structural properties of confined water and ions were calculated. While the behavior of these three systems is similar qualitatively because of similar hydration properties of ions, significant differences are observed at the quantitative level due to changes in the hydration structure of cations. The global minimum in swelling free energy is found to be at the smaller pore widths (first minimum) for Rb- and K-mica and at bigger pore widths (second minimum) for Cs-mica pores. We find that ±0.1 Å change in the interstitial cation size leads to a -15 to 5% change in equilibrium loading of adsorbed water and -2 to 35% change in swelling. Our thermodynamic analysis reveals an intricate interplay between enthalpic and entropic contributions caused by the structural change of water in the pores due to the hydration of interstitial cations.
黏土的离子交换容量在从放射性废物处理到化妆品等许多工业应用中都起着重要作用。然而，由于间隙区域中水分和离子的吸附导致黏土薄片的膨胀或收缩也是一个众所周知的现象。对于它们的应用而言，了解这些层状材料的稳定性至关重要，尤其是在间隙离子与具有不同性质的周围离子发生交换之后。在此，我们探究了阳离子大小对膨胀压力和自由能分布的作用。我们使用分子模拟来研究含有钾、铷和铯离子的云母孔隙的稳定性。我们在各种孔隙宽度下进行了一系列巨正则蒙特卡罗模拟。我们探测了云母孔隙中的水吸附情况，并据此计算了分离压力、巨势（膨胀自由能）以及受限水和离子的结构性质。虽然由于离子具有相似的水合性质，这三个系统的行为在定性上是相似的，但由于阳离子水合结构的变化，在定量水平上观察到了显著差异。发现铷云母和钾云母的膨胀自由能全局最小值出现在较小的孔隙宽度处（第一个最小值），而铯云母孔隙的全局最小值出现在较大的孔隙宽度处（第二个最小值）。我们发现间隙阳离子大小变化±0.1 Å会导致吸附水平衡负载变化-15%至5%，膨胀变化-2%至35%。我们的热力学分析揭示了间隙阳离子水合作用导致孔隙中水的结构变化所引起的焓贡献和熵贡献之间的复杂相互作用。
