Water adsorption isotherms on soil external particle surface by molecular simulation

Water adsorption isotherm describes soils' ability in retaining water under given relative humidity and isothermal conditions, and is frequently utilized to characterize soils' engineering properties. Fundamentally, the water adsorption isotherm on soil external particle surface is the macroscopic manifestation of atomistic scale soil-water interaction mechanisms, including van der Waals, surface hydration, cation hydration, and electrical double layer. Yet, the quantitative link between these mechanisms and water adsorption isotherms remains obscure, and it still lacks effective approaches to probe these mechanisms. Here, a general framework based on Grand Canonical Monte Carlo simulation was developed to directly obtain water adsorption isotherms on soil external particle surface from the interatomic potentials. The simulated adsorption isotherm agrees well with existing experimental and numerical results, confirming the validity of the proposed framework. The proposed framework facilitates a quantitative assessment of the impact of atomistic scale soil-water interaction mechanisms on the macroscale water adsorption isotherms. It reveals that the atomistic scale soil-water interaction mechanisms dictate the shape of adsorption isotherms, i.e., an 'S' shape by prevailing cation hydration and a concave shape by prevailing surface hydroxyl hydration; and divalent cations exhibit a much more intensive interaction with water molecules than univalent cations, increasing the adsorption capacity up to 9.5 times.