Pore Size Effect on Melting Point in Saturated Frozen Soil Systems: A Pressure Perspective at Molecular Scale

Abstract

The melting point depression of water confined in porous media with varying pore sizes has been widely observed. Existing models remain insufficient for describing water phase change in smaller pores (i.e., nano pore), where surface effects become significant and water molecules are inhomogeneously distributed near solid surface. This study investigates the pore size effect on melting point from a molecular scale perspective and introduces a local pressure variable, namely lateral pressure. Molecular dynamics (MD) simulations were performed to examine water phase change in a saturated frozen soil system. MD simulations successfully reproduced the trend of decreasing melting point of pore water with decreasing pore size. Further analysis shows that lateral pressure increases as pore size decreases, corresponding to lower melting points. The lateral pressure near the silica–water interface, corresponding to the film water region, is also found to be approximately three orders of magnitude higher than that in the bulk‐water region. A Radial distribution function (RDF) analysis indicates that water molecules in smaller pores exhibit a higher degree of molecular ordering, which contributes to the elevated local pressure. In particular, film water, showing solid‐like ordering, is significant as it can result in errors of up to three orders of magnitude in pressure estimation when the unfrozen water content is high. This study highlights the significance of the local water state/molecular ordering, particularly in film water, in influencing the relationship between pore size and the pore water melting point, with lateral pressure serving as a molecular‐scale descriptor.