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

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.