Unsaturation triggers specific adsorption performance of water films in the clay nanopores

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2024-08, Vol.694, p.134186, Article 134186
• Main Authors: Li, Xiong, Zhang, Taoying, Jiang, Shuaiheng, Sun, Kai, Zhang, Jianguo
• Format: Article
• Language: English
• Subjects: adsorption; clay; Clay particle; diffusivity; hydrogen; Ions; Molecular dynamics; nanopores; relative humidity; soil; soil mechanics; Unsaturation; Water film; water vapor
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2024.134186

Soil is typically unsaturated, with water films covering the soil particles playing a crucial role in soil mechanics, structure, and element transport. However, the unusual behavior of these water films and the underlying mechanisms are not yet fully understood. This study, which used molecular dynamics methods, sheds light on the distinct interactions between water films and clay surfaces under unsaturation. At the water-mineral interface, water molecules formed a layered structure, while they rearranged as a meniscus as the unsaturation extent increased. On the other hand, unsaturation had a significant impact on the location of water molecules at the water-vapor interface, but only a limited effect at the water-mineral interface. The water’s orientational ordering and hydrogen bonding interactions were significantly affected by changes in relative humidity (RH). Based on the distance-dependent diffusion coefficient, the water molecules within the water film were classified into three types: WT1 (< 5 Å) had faster diffusivity at higher RH, WT2 (5–6 Å) had comparable diffusivity at any RH, and WT3 (> 6 Å) had faster diffusivity at intermediate RH. This classification was supported by the retention dynamics analysis. The specific behaviors mentioned above, which are associated with unsaturation, are closely linked to Na+ hydration. This is because the interaction between water and Na+ is more pronounced than the interaction between water and minerals or Cl−. The results indicate that unsaturation triggers specific adsorption behaviors of water films that cover the clay surfaces. These behaviors should be considered in interfacial processes and environmental engineering. [Display omitted] •Wettability and adsorption are significantly different in clay particles under non-saturation.•Three types of water molecules are classified according to the diffusion and retention behaviors.•“Transmission effect” passes from the water-vapor to the water-clay interface.•Cation hydration controls the specific performance of water films in unsaturated condition.