Hysteresis at Low Relative Humidity on Vapor Sorption Isotherm of Na-Montmorillonite

The vapor sorption isotherm (VSI) can be employed not only for characterizing the soil properties but also for describing the seepage and deformation phenomena within soils. The adsorption and desorption VSIs of expansive soil exhibit a distinctive hysteresis at low relative humidity (RH). While it is speculated that this hysteresis may be related to the interlayer hydration of expansive minerals, the underlying mechanisms remain poorly understood. In this study, we proposed a procedure that integrates experiments and molecular simulations to analyze this hysteresis. Through this procedure, we successfully reproduced the hysteresis and accurately captured the swelling and shrinkage of Na-montmorillonite (Na-Mt) in molecular simulations. The simulated hysteresis showed good agreement with experimental results. Through the analysis of microphysical states of hydrated montmorillonite, we revealed that this hysteresis mainly arises from the varying hydration states of interlayer cations during adsorption and desorption processes, and cations and hydrogen bonds are key to the stability of additional water molecules in desorption. Based on these findings, we proposed that the hysteresis amount at the 0.3 RH of a soil predominantly composed of Na-Mt is expected to exhibit a linear correlation with its internal specific surface area. This correlation was validated through experimental and numerical data. This study not only reveals the micromechanisms of VSI hysteresis at low RH and provides a potential direction for the utilization of this hysteresis, but also offers a reference for combining molecular simulations with experiments to investigate the water retention of expansive soils.