Ammonium salts enhance heterogeneous ice nucleation through entropy increase

At atmospheric pressure, pure liquid water can supercool to temperatures below -40⁰C before freezing. However, most naturally-occurring ice nucleation, such as in cloud water droplets, occurs because of a heterogeneous surface in contact with the supercooled water.

Although the process has been studied extensively, it remains challenging to predict ice nucleation. While numerous ice nucleators exist, the mechanisms underlying nucleation remain poorly understood.

Whale conducted a combined theoretical and experimental study on the natural phenomenon of heterogeneous ice nucleation, with specific focus on the anomalous effect of the nucleating temperature increasing in the presence of ammonium salts.

“If presented with the chemical and physical information of a surface, we wouldn’t be able to say the exact temperature at which the surface will nucleate ice,” said author Thomas Whale.

Whale’s investigations imply a commonality in mechanism across a wide range of inorganic ice nucleators: their polar or charged surfaces tend to induce the formation of hydrogen-ordered ice clusters. In addition, the increase in nucleation temperatures caused by ammonium is likely due to an increase in entropy.

This work has important implications for cloud glaciation temperatures, which affect weather and climate. The study of ice nucleation is also significant in cryobiology, where the location and nature of ice formation may determine how well biological material copes with low temperatures.

“The hope is that we may be able to better design surfaces suitable for controlling ice nucleation in these technological contexts,” said Whale.

Source: “Disordering effect of the ammonium cation accounts for anomalous enhancement of heterogeneous ice nucleation,” by Thomas F. Whale, Journal of Chemical Physics (2022). The article can be accessed at https://doi.org/10.1063/5.0084635 .