Hysteresis at low humidity on vapor sorption isotherm of Ca-montmorillonite: The key role of interlayer cations

IF 5.3 2区 地球科学 Q2 CHEMISTRY, PHYSICAL
Yijie Wang , Zhen-Yu Yin , Pierre-Yves Hicher , Liming Hu
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Abstract

The vapor sorption isotherm (VSI) is significant for studying soil-water interactions. The non-coincidence between the adsorption isotherm and desorption isotherm is termed hysteresis. The VSI of expansive soils exhibits a unique hysteresis at low humidity, whose underlying microscopic mechanism has not yet been fully investigated. In this study, molecular simulations were used to generate the VSI of Ca-montmorillonite (Ca-Mt), successfully reproducing the hydration-induced swelling and dehydration-induced shrinkage in Mt. The specially designed simulation procedure ensured that the simulated hysteresis was consistent with experimental results at the humidity range of 0.1–0.5. We analyzed the variations in external and interlayer water content, as well as the basal spacing with humidity, highlighting the influence of interlayer hydration in the presence of the hysteresis. Through analyses of density, cation hydration state, and hydrogen bond, we found that the cations moved away from the mineral surface during adsorption, providing more space for water retention, and the formation of hydrogen bonds impeded the desorption of newly added water, leading to the hysteresis. Furthermore, we compared the VSI results of Mts with different interlayer cations (Ca2+ and Na+). Within the humidity range of the VSI test, the hydration shell of interlayer Ca2+ comprised 8 water molecules, whereas the hydration shell of interlayer Na+ comprised 4–6 water molecules. In the desorption isotherm simulation, water molecules in the hydration shell of Na+ could be displaced by the mineral surface at low humidity, unlike those of Ca2+, resulting in distinct hysteresis shapes in their VSI results at low humidity.
低湿度下钙蒙脱石蒸汽吸附等温线的滞后:层间阳离子的关键作用
水汽吸附等温线(VSI)对研究土壤与水的相互作用具有重要意义。吸附等温线和解吸等温线之间的不重合称为滞后。膨胀性土壤的 VSI 在低湿度时表现出一种独特的滞后现象,其微观机理尚未得到充分研究。本研究利用分子模拟生成了钙蒙脱石(Ca-Mt)的 VSI,成功地再现了 Mt 的水化诱导膨胀和脱水诱导收缩。我们分析了外部和层间含水量以及基底间距随湿度的变化,突出了层间水合作用对滞后现象的影响。通过对密度、阳离子水合状态和氢键的分析,我们发现阳离子在吸附过程中远离了矿物表面,为水分的保留提供了更大的空间,而氢键的形成阻碍了新加入水分的解吸,从而导致了滞后现象的产生。此外,我们还比较了含有不同层间阳离子(Ca2+ 和 Na+)的 Mts 的 VSI 结果。在 VSI 测试的湿度范围内,层间 Ca2+ 的水合壳由 8 个水分子组成,而层间 Na+ 的水合壳由 4-6 个水分子组成。在解吸等温线模拟中,与 Ca2+ 不同,Na+ 水合壳中的水分子在低湿度时可被矿物表面置换,从而导致它们在低湿度时的 VSI 结果出现明显的滞后形状。
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来源期刊
Applied Clay Science
Applied Clay Science 地学-矿物学
CiteScore
10.30
自引率
10.70%
发文量
289
审稿时长
39 days
期刊介绍: Applied Clay Science aims to be an international journal attracting high quality scientific papers on clays and clay minerals, including research papers, reviews, and technical notes. The journal covers typical subjects of Fundamental and Applied Clay Science such as: • Synthesis and purification • Structural, crystallographic and mineralogical properties of clays and clay minerals • Thermal properties of clays and clay minerals • Physico-chemical properties including i) surface and interface properties; ii) thermodynamic properties; iii) mechanical properties • Interaction with water, with polar and apolar molecules • Colloidal properties and rheology • Adsorption, Intercalation, Ionic exchange • Genesis and deposits of clay minerals • Geology and geochemistry of clays • Modification of clays and clay minerals properties by thermal and physical treatments • Modification by chemical treatments with organic and inorganic molecules(organoclays, pillared clays) • Modification by biological microorganisms. etc...
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