Freezing at liquid-vapor interfaces is known as surface freezing. In some materials, like normal alkanes, a frozen layer emerges at the surface at temperatures above their bulk freezing temperature (Tm). This is a thermodynamic phenomenon. Many scientists believe that in water, surface freezing occurs in purely kinetic fashion, as a rate enhancement at the surface at temperatures below Tm, but experiments have not conclusively validated kinetic surface freezing in water. In a “Perspective” article in The Journal of Chemical Physics, researchers Amir Haji-Akbari of Yale University and Pablo G. Debenedetti of Princeton University survey the last fifteen years’ varied published research on surface freezing in water.
Haji-Akbari and Debenedetti use a thermodynamic framework for understanding the surface freezing question. They proceed with discussing influential work, such as that of Azadeh Tabazadeh and colleagues who first proposed kinetic surface-induced ice nucleation. Subsequent research investigated rate measurements for nucleating droplets of different sizes. The authors emphasize the need for more investigations that would further measure ice nucleation rates for < 1 µm diameter droplets.
The authors also describe the work of Ellen Backus and colleagues who used reflection absorption infrared and temperature-programmed desorption spectroscopy to understand bulk versus surface crystallization in amorphous solid water (ASW) and posited that ASW freezing begins near the vapor-glass interface. Subsequent studies by Chunging Yuan and colleagues supported the Backus assessment.
Haji-Akbari and Debenedetti offer ideas on moving forward such as “better ultrafast scattering and electron microscopy techniques” to demonstrate and measure surface-facilitated ice nucleation and new computational ways to “characterize the kinetics of ice nucleation in polarizable models and the effect of polarizability at the surface.” These suggestions are intended to promote further investigation into understanding kinetic surface freezing in water.
Source: “Perspective: Surface freezing in water: A nexus of experiments and simulations,” by Amir Haji-Akbari and Pablo G. Debenedetti, Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4985879.