Molecular dynamics study of ice crystallization and seawater freezing

Abstract

Ice crystallization is a common phenomenon in nature and plays a significant role in various fields such as cloud formation, climate change, aviation, and the food industry.In many applications, the freezing of water in seawater has garnered significant attention due to its relevance to desalination processes. Unlike the freezing of pure water, the process of ice formation in seawater is more complex due to the presence of salt ions, and research on how salt ions interact with water molecules and influence the dynamics of the hydrogen bonding network in water is still limited. In order to provide unique insights into the nanoscopic aspects of ice crystallization, this thesis employs molecular dynamics (MD) simulations to investigate the nanoscale mechanisms of how convex roughness affects the rate of ice nucleation. Additionally, how external shear forces and electric fields influence ion rejection rate during seawater freezing are also explored through MD simulations and experimental methods. In this study, we first examined the impact of convex surface roughness on the process of heterogeneous ice nucleation, (HIN) through MD simulations. We calculated the ice nucleation rate by altering the vertex angle of the sawtooth-structured graphene surface. The study reveals that rough surfaces always suppresses the rate of ice nucleation compared to flat surfaces. As the vertex angle varies, the ice nucleation rate exhibits fluctuations in the order of two magnitudes. Moreover, the findings indicate that both the atomic structure and confinement effects have an impact on both the pace at which ice nucleates and the orientation of ice crystals. When the surface atomic structure is the dominant factor, the ice tends to grow in a direction that matches the surface atomic structure. If the confinement effect is significant, the ice is more likely to grow in directions that align with the spatial structure that best matches the confinement. We also investigated the effect of shear rate on ion rejection rate during seawater freezing through MD simulations, the result shows that there is a linear relationship between the shear rate and ion rejection rate. As the shear rate increases, the ion rejection rate also increases. This phenomenon is attributed to the disruption of hydration bonds by shear forces. Shear forces aid in breaking the hydration bonds formed around ions, thereby enhancing the diffusion coefficient of ions and helping them diffuse into the water. Furthermore, shear forces reduce the energy barrier at the ice-water interface, ultimately promoting the diffusion of ions into the aqueous solution. Our experimental results also confirm the reliability of our MD simulations. Additionally, we investigated the influence of an external electric field on ion rejection rate during seawater freezing through MD simulations. The results reveal that the external electric field can enhance the ion rejection rate, and this effect increases with the strength of the electric field. The electric field influences the interaction between ions and water, as evidenced by changes in the orientation and radial distribution functions of water molecules. Additionally, the external electric field reduces the energy barrier at the liquid-solid interface, thereby increasing the ion rejection rate. Moreover, the results indicate that the external electric field enhances hydrogen bonding among water molecules, which in turn slows down the growth rate of ice. The results of this study provide new insights and approaches for enhancing ion rejection rate during seawater freezing, which will have implications for the development of seawater freezing desalination.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhigang LI (Supervisor) & Baoling HUANG (Supervisor)