Effects of hydroxylated silica surface on molecular-scale water freezing with implications for unsaturated permafrost

In unsaturated frozen soil, a multi-phase complex system, the influence of initial water content on the unfrozen water content remains unclear. Understanding this influence is essential for determination of the soil freezing characteristic curve (SFCC), a constitutive relation in macroscale numerical models for permafrost. This unresolved issue stems from a lack of understanding of the underlying mechanisms, including microscale processes and interfacial effects. Here, we employed molecular dynamics (MD) simulations to investigate the water freezing-thawing process based on hydroxylated silica surface, resembling an unfrozen soil system. The results revealed that the interface between water and silica forms an ordered structure via hydrogen bonding, indicating an attraction of water molecules to the silica surface. The presence of the silica surface depresses the water triple point by 5.0 K. As the water content increases, along with the pore size, the triple point decreases further by nearly 10.0 K for a 10 nm increase in pore size. Through a simple yet physically based model, we identified that a smaller distance from water to silanol groups, corresponding to systems with lower initial water content, leads to a deeper potential well, which subsequently requires more energy for phase change and higher triple point. The deeper potential well also restricts the movement of water molecules, resulting in slower dynamic processes within smaller pores. This implies that the initial water content is important in obtaining the SFCC, since the initial water content can affect the water triple point.