Adapa Sai, Malani Ateeque
Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, 400076, India.
Research and Development Division, Tata Steel Limited, Jamshedpur, 831001, India.
Sci Rep. 2022 Oct 24;12(1):17810. doi: 10.1038/s41598-022-21349-3.
The swelling capacity and stability of clay play a crucial role in various areas ranging from cosmetics to oil extraction; hence change in their swelling behaviour after cation exchange with the surrounding medium is important for their efficient utilisation. Here we focus on understanding the role of different hydration properties of cation on the thermodynamics of clay swelling by water adsorption. We have used mica as the reference clay, Na[Formula: see text], Li[Formula: see text], and H[Formula: see text] ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths [Formula: see text] Å). The disjoining pressure ([Formula: see text]), swelling free energy ([Formula: see text]), and several structural properties of confined water and ions were calculated to perform a thermodynamic analysis of the system. We expected higher water density in H-mica pores ([Formula: see text]) due to the smaller size of [Formula: see text] ions having higher hydration energy. However, the counter-intuitive trend of [Formula: see text] (bulk density) [Formula: see text] was observed due to adsorption energy, where the interaction of water with mica framework atoms was also found to be significant. All three mica systems exhibited oscillatory behaviour in the [Formula: see text] and [Formula: see text] profiles, diminishing to zero for [Formula: see text] Å. The [Formula: see text] for Na-mica is characterised by global minima at [Formula: see text] corresponding to crystalline swelling with significant and multiple barriers for crystalline swelling to osmotic swelling ([Formula: see text] Å). A shift in the location of global minima of [Formula: see text] towards the higher d values and [Formula: see text] becoming more repulsive is observed in the increasing order of hydration energy of [Formula: see text], [Formula: see text], and [Formula: see text] ions. The [Formula: see text] for all d in the H-mica system thus favours osmotic swelling. We found that the Na[Formula: see text] ions hydrate more surface oxygens, act as anchors, and hold the mica pore together (at smaller d), by sharing hydrating water with ions of the opposite side, forming an electrostatically connected mica-Na-water-Na-mica bridge. The Li[Formula: see text] ions do hydrate surface oxygen atoms, albeit in lesser numbers, and sharing of hydration shell with nearby Li[Formula: see text] ions is also minimum. Hydration by surface atoms and water sharing, both, are minimum in the H[Formula: see text] ion case, as they are mostly present in the center of the pore as diffusive ions, thus exerting a consistent osmotic pressure on the mica frameworks, favouring swelling.
黏土的膨胀能力和稳定性在从化妆品到石油开采等各个领域都起着至关重要的作用；因此，其与周围介质进行阳离子交换后膨胀行为的变化对于其有效利用至关重要。在此，我们专注于通过水吸附来理解阳离子不同水化性质对黏土膨胀热力学的作用。我们使用云母作为参考黏土，以Na⁺、Li⁺和H⁺离子作为间隙阳离子，并对宽度为10 Å的云母孔隙中的水吸附进行了巨正则蒙特卡罗模拟。计算了分离压力（Π）、膨胀自由能（ΔG）以及受限水和离子的几种结构性质，以对该系统进行热力学分析。由于水化能较高的H⁺离子尺寸较小，我们预计H - 云母孔隙（d）中的水密度会更高。然而，由于吸附能，观察到了与直觉相反的趋势，即ρ（体密度）d（层间距），其中还发现水与云母骨架原子的相互作用也很显著。所有三种云母体系在Π和ΔG曲线上均表现出振荡行为，对于d = 10 Å时减小至零。Na - 云母的Π曲线在d = 12.4 Å处具有全局最小值，对应于晶体膨胀，从晶体膨胀到渗透膨胀（d = 16 Å）存在显著且多个障碍。随着H⁺、Li⁺和Na⁺离子水化能的增加顺序，观察到Π的全局最小值位置向更高的d值移动，并且ΔG变得更具排斥性。因此，H - 云母体系中所有d值下的ΔG都有利于渗透膨胀。我们发现，Na⁺离子使更多的表面氧原子水化，充当锚点，并通过与另一侧的离子共享水化水将云母孔隙保持在一起（在较小的d值处），形成静电连接的云母 - Na - 水 - Na - 云母桥。Li⁺离子确实会使表面氧原子水化，尽管数量较少，并且与附近Li⁺离子共享水化层的情况也最少。在H⁺离子的情况下，表面原子的水化和水的共享都最少，因为它们大多作为扩散离子存在于孔隙中心，从而对云母骨架施加一致的渗透压，有利于膨胀。
