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Conventional conceptual mechanisms for the hysteresis of soil-water retention are the ink-bottle pore neck and the solid–liquid–air-contact angle. However, these mechanisms fail to explain hydraulic hysteresis for matric suction greater than 10 MPa. A conceptual model, based on hydration-water retention, is provided in this paper. Two hydration mechanisms, namely, particle-surface hydration and crystalline cation hydration, are distinguished to explain hydraulic hysteresis. The former is mainly involved in water retention by anions of oxygen and/or hydroxyls on particle surface, leading to reversible water adsorption and desorption. By contrast, cation hydration is controlled by both exchangeable cations and the intermolecular forces such as Coulomb attraction and London dispersion, leading to hysteretic water-retention behavior. Based on this hysteresis model, the highest total suction for any soil can be identified. From the isotherms of various soils at 25°C, it is found that the highest total suction varies from 475 to 1,180 MPa. This value depends on soil types and can be uniquely related to the BET adsorption constant, which represents the energy needed to change soil water from gas phase to liquid phase.
Lu et al. (Thu,) studied this question.