Freezing Mechanism of Water in Clay Nanopores Using Molecular Dynamics

This study evaluates the freezing mechanism of pure water inside nanopores confined by cohesive soil particles using molecular dynamics (MD). The key contribution of this study is to provide an assessment to better understand the physical and chemical particle interactions among water, ice, and clay surfaces within nanopores under supercooling temperatures. The nanopores were simulated by implementing coarse-grained kaolinite surfaces face-to-face with the milliwatt water model filled in between using LAMMPS (large-scale atomic/molecular massively parallel simulator). The distance between face-to-face kaolinite sheets was controlled to target the desired nanopore size with the implementation of the piston model. The isothermal–isobaric ensemble, NPT, was used as the thermostat with hybrid Lennard–Jones and Stillinger–Weber potentials for the simulations. Simulations proceeded under freezing temperatures from 220 K (about −53°C) to 190 K (about −83°C) to capture the whole evolvement of water crystallization within clay nanopores. CHILL+ algorithm was implemented to detect ice formation and distinguish different types of ice formed within nanopores. Ovito was used as a visualization tool to monitor the progress of crystallization and a descriptive method to observe the interactions within nanopores. The results showed that both hexagonal and cubic ice were formulated almost instantaneously within the clay nanopore at the temperature of 208 K (about −65°C) with 20% more cubic ice than hexagonal ice. Some water remained liquid even as the temperature dropped to 190 K (about −83°C).